Coupled partitioning, dilution, and chemical aging of semivolatile organics

Probing the capacity of a spatiotemporal deep learning model for short-term PM2.5 forecasts in a coastal urban area

Accurate forecast of fine particulate matter (PM2.5) is crucial for city air pollution control, yet remains challenging due to the complex urban atmospheric chemical and physical processes. Recently deep learning has been routinely applied for better urban PM2.5 forecasts. However, their capacity to represent the spatiotemporal urban atmospheric processes remains underexplored, especially compared with traditional approaches such as chemistry-transport models (CTMs) and shallow statistical methods other than deep learning. Here we probe such urban-scale representation capacity of a spatiotemporal deep learning (STDL) model for 24-hour short-term PM2.5 forecasts at six urban stations in Rizhao, a coastal city in China. Compared with two operational CTMs and three statistical models, the STDL model shows its superiority with improvements in all five evaluation metrics, notably in root mean square error (RMSE) for forecasts at lead times within 12 h with reductions of 49.8 % and 47.8 % respectively. This demonstrates the STDL model's capacity to represent nonlinear small-scale phenomena such as street-level emissions and urban meteorology that are in general not well represented in either CTMs or shallow statistical models. This gain of small-scale representation in forecast performance decreases at increasing lead times, leading to similar RMSEs to the statistical methods (linear shallow representations) at about 12 h and to the CTMs (mesoscale representations) at 24 h. The STDL model performs especially well in winter, when complex urban physical and chemical processes dominate the frequent severe air pollution, and in moisture conditions fostering hygroscopic growth of particles. The DL-based PM2.5 forecasts align with observed trends under various humidity and wind conditions. Such investigation into the potential and limitations of deep learning representation for urban PM2.5 forecasting could hopefully inspire further fusion of distinct representations from CTMs and deep networks to break the conventional limits of short-term PM2.5 forecasts.

The chemical composition of secondary organic aerosols regulates transcriptomic and metabolomic signaling in an epithelial-endothelial in vitro coculture

BACKGROUND

The formation of secondary organic aerosols (SOA) by atmospheric oxidation reactions substantially contributes to the burden of fine particulate matter (PM2.5), which has been associated with adverse health effects (e.g., cardiovascular diseases). However, the molecular and cellular effects of atmospheric aging on aerosol toxicity have not been fully elucidated, especially in model systems that enable cell-to-cell signaling.

METHODS

In this study, we aimed to elucidate the complexity of atmospheric aerosol toxicology by exposing a coculture model system consisting of an alveolar (A549) and an endothelial (EA.hy926) cell line seeded in a 3D orientation at the air‒liquid interface for 4 h to model aerosols. Simulation of atmospheric aging was performed on volatile biogenic (β-pinene) or anthropogenic (naphthalene) precursors of SOA condensing on soot particles. The similar physical properties for both SOA, but distinct differences in chemical composition (e.g., aromatic compounds, oxidation state, unsaturated carbonyls) enabled to determine specifically induced toxic effects of SOA.

RESULTS

In A549 cells, exposure to naphthalene-derived SOA induced stress-related airway remodeling and an early type I immune response to a greater extent. Transcriptomic analysis of EA.hy926 cells not directly exposed to aerosol and integration with metabolome data indicated generalized systemic effects resulting from the activation of early response genes and the involvement of cardiovascular disease (CVD) -related pathways, such as the intracellular signal transduction pathway (PI3K/AKT) and pathways associated with endothelial dysfunction (iNOS; PDGF). Greater induction following anthropogenic SOA exposure might be causative for the observed secondary genotoxicity.

CONCLUSION

Our findings revealed that the specific effects of SOA on directly exposed epithelial cells are highly dependent on the chemical identity, whereas non directly exposed endothelial cells exhibit more generalized systemic effects with the activation of early stress response genes and the involvement of CVD-related pathways. However, a greater correlation was made between the exposure to the anthropogenic SOA compared to the biogenic SOA. In summary, our study highlights the importance of chemical aerosol composition and the use of cell systems with cell-to-cell interplay on toxicological outcomes.

Overlooked Trace Molecules in Organic Aerosol Revealed by Gas Chromatography-Orbitrap Mass Spectrometry

Molecular characterization of organic aerosol (OA) is crucial for understanding its sources and atmospheric processes. However, the chemical components of OA remain not well constrained. This study used gas chromatography-Orbitrap mass spectrometry (GC-Orbitrap MS) and GC-Quadrupole MS (GC-qMS) to investigate the organic composition in PM2.5 from Xi'an, Northwest China. GC-Orbitrap MS identified 335 organic tracers, including overlooked isomers and low-concentration molecules, approximately 1.6 times more than GC-qMS. The "molecular corridor" assessment shows the superior capability of GC-Orbitrap MS in identifying an expansive range of compounds with higher volatility and oxidation states, such as furanoses/pyranoses, di/hydroxy/ketonic acids, di/poly alcohols, aldehydes/ketones, and amines/amides. Seasonal variations in OA composition reflect diverse sources: increased di/poly alcohols in winter are derived from indoor emissions, furanoses/pyranoses and heterocyclics in spring and summer might be from biogenic emissions and secondary formation, and amides in autumn are probably from biomass burning. Integrating partial least squares discriminant analysis (PLS-DA) and potential source contribution function (PSCF) models, the source similarities and differences are further elucidated, highlighting the role of local emissions and transport from southern cities. This study offers new insights into the OA composition aided by the high mass resolution and sensitivity of GC-Orbitrap MS.

Enhanced Aging of Black Carbon under Recent Clean Air Actions and Future Carbon Neutrality Scenario in China

China has implemented strict emission control measures, but it is unclear how they affect black carbon (BC) aging and light absorption. Here, we use the Community Atmosphere Model version 6 (CAM6) with the four-mode version of the Modal Aerosol Module coupled with machine learning (MAM4-ML) to simulate BC aging during 2011-2018 and 2050/2100 following a carbon neutrality scenario (SSP1-2.6), respectively. During 2011-2018, the mass ratio of coatings to BC (RBC) widely increased (5.4% yr-1) over the east of China. The increased secondary organic aerosol (SOA) coatings dominate (88%) the increased RBC, while the sulfate coatings decrease. The drivers of BC coating changes come from the different magnitudes of emission reductions in secondary aerosol precursors (i.e., volatile organic compounds (VOCs) and SO2) and BC. During 2011-2018, the increased RBC enhances the BC mass absorption cross section (MAC, 0.7% yr-1). In 2050/2100 for SSP1-2.6, emission control leads to further increased RBC (95/145%) and BC MAC (12/17%). For both 2011-2018 and 2050/2100, the enhanced BC MAC partly offsets the declining direct radiative effect (DRE) of BC due to direct emission reduction. As a result, the full impact of direct emission reductions of BC on BC DRE is only 75% for 2011-2018 and 90/94% for 2050/2100.

Cannabinoids detected in exhaled breath condensate after cannabis use

Cannabinoids can be detected in breath after cannabis use, but different breath matrices need to be explored as studies to date with filter-based devices that collect breath aerosols have not demonstrated that breath-based measurements can reliably identify recent cannabis use. Exhaled breath condensate (EBC) is an unexplored aqueous breath matrix that contains condensed volatile compounds and water vapor in addition to aerosols. EBC was collected from participants both before and at two time points (0.7 ± 0.2 h and 1.7 ± 0.3 h) after observed cannabis use. Eleven different cannabinoids were monitored with liquid chromatography tandem mass spectrometry. Five different cannabinoids, including Δ9-tetrahydrocannabinol (THC), were detected in EBC collected from cannabis users. THC was detected in some EBC samples before cannabis use, despite the requested abstinence period. THC was detected in all EBC samples collected at 0.7 h post use and decreased for all participants at 1.7 h. Non-THC cannabinoids were only detected after cannabis use. THC concentrations in EBC samples collected at 0.7 h showed no trend with sample metrics like mass or number of breaths. EBC sampling devices deserve further investigation with respect to modes of cannabis use (e.g, edibles), post use time points, and optimization of cannabinoid recovery.

Oxidation Mechanism and Toxicity Evolution of Linalool, a Typical Indoor Volatile Chemical Product

Linalool, a high-reactivity volatile chemical product (VCP) commonly found in cleaning products and disinfectants, is increasingly recognized as an emerging contaminant, especially in indoor air. Understanding the gas-phase oxidation mechanism of linalool is crucial for assessing its impact on atmospheric chemistry and human health. Using quantum chemical calculations and computational toxicology simulations, we investigated the atmospheric transformation and toxicity evolution of linalool under low and high NO/HO2· levels, representing indoor and outdoor environments. Our findings reveal that linalool can undergo the novel mechanisms involving concerted peroxy (RO2·) and alkoxy radical (RO·) modulated autoxidation, particularly emphasizing the importance of cyclization reactions indoors. This expands the widely known RO2·-dominated H-shift-driven autoxidation and proposes a generalized autoxidation mechanism that leads to the formation of low-volatility secondary organic aerosol (SOA) precursors. Toxicological analysis shows that over half of transformation products (TPs) exhibited higher carcinogenicity and respiratory toxicity compared to linalool. We also propose time-dependent toxic effects of TPs to assess their long-term toxicity. Our results indicate that the strong indoor emission coupled with slow consumption rates lead to significant health risks under an indoor environment. The results highlight complex indoor air chemistry and health concerns regarding persistent toxic products during indoor cleaning, which involves the use of linalool or other VCPs.

Emerging investigator series: secondary organic aerosol formation from photooxidation of acyclic terpenes in an oxidation flow reactor

One major challenge in predicting secondary organic aerosol (SOA) formation in the atmosphere is incomplete representation of biogenic volatile organic compounds (BVOCs) emitted from plants, particularly those that are emitted as a result of stress - a condition that is becoming more frequent in a rapidly changing climate. One of the most common types of BVOCs emitted by plants in response to environmental stress are acyclic terpenes. In this work, SOA is generated from the photooxidation of acyclic terpenes in an oxidation flow reactor and compared to SOA production from a reference cyclic terpene - α-pinene. The acyclic terpenes used as SOA precursors included β-myrcene, β-ocimene, and linalool. Results showed that oxidation of all acyclic terpenes had lower SOA yields measured after 4 days photochemical age, in comparison to α-pinene. However, there was also evidence that the condensed organic products that formed, while a smaller amount overall, had a higher oligomeric content. In particular, β-ocimene SOA had higher oligomeric content than all the other chemical systems studied. SOA composition data from ultra-high performance liquid chromatography with electrospray ionization mass spectrometry (UHPLC-ESI-MS) was combined with mechanistic modeling using the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) to explore chemical mechanisms that could lead to this oligomer formation. Calculations based on composition data suggested that β-ocimene SOA was more viscous with a higher glass transition temperature than other SOA generated from acyclic terpene oxidation. This was attributed to a higher oligomeric content compared to other SOA systems studied. These results contribute to novel chemical insights about SOA formation from acyclic terpenes and relevant chemistry processes, highlighting the importance of improving underrepresented biogenic SOA formation in chemical transport models.

Volatile Organic Compound Metabolism on Early Earth

Biogenic volatile organic compounds (VOCs) constitute a significant portion of gas-phase metabolites in modern ecosystems and have unique roles in moderating atmospheric oxidative capacity, solar radiation balance, and aerosol formation. It has been theorized that VOCs may account for observed geological and evolutionary phenomena during the Archaean, but the direct contribution of biology to early non-methane VOC cycling remains unexplored. Here, we provide an assessment of all potential VOCs metabolized by the last universal common ancestor (LUCA). We identify enzyme functions linked to LUCA orthologous protein groups across eight literature sources and estimate the volatility of all associated substrates to identify ancient volatile metabolites. We hone in on volatile metabolites with confirmed modern emissions that exist in conserved metabolic pathways and produce a curated list of the most likely LUCA VOCs. We introduce volatile organic metabolites associated with early life and discuss their potential influence on early carbon cycling and atmospheric chemistry.

Using the Multicomponent Aerosol FORmation Model (MAFOR) to Determine Improved VOC Emission Factors in Ship Plumes

International shipping's particulate matter primary emissions have a share in global anthropogenic emissions of between 3% and 4%. Ship emissions of volatile organic compounds (VOCs) can play an important role in the formation of fine particulate matter. Using an aerosol box model for the near-plume scale, this study investigated how the changing VOC emission factor (EF) for ship engines impacts the formation of secondary PM2.5 in ship exhaust plumes that were detected during a measurement campaign. The agreement between measured and modeled particle number size distribution was improved by adjusting VOC emissions, in particular of intermediate-, low-, and extremely low-volatility compounds. The scaling of the VOC emission factor showed that the initial emission factor, based on literature data, had to be multiplied by 3.6 for all VOCs. Information obtained from the box model was integrated into a regional-scale chemistry transport model (CTM) to study the influence of changed VOC ship emissions over the Mediterranean Sea. The regional-scale CTM run with adjusted ship emissions indicated a change in PM2.5 of up to 5% at the main shipping routes and harbor cities in summer. Nevertheless, overall changes due to a change in the VOC EF were rather small, indicating that the size of grid cells in CTMs leads to a fast dilution.

Volatility Basis Set Distributions and Viscosity of Organic Aerosol Mixtures: Insights from Chemical Characterization Using Temperature-Programmed Desorption-Direct Analysis in Real-Time High-Resolution Mass Spectrometry

Quantitative assessment of gas-particle partitioning of individual components within complex atmospheric organic aerosol (OA) mixtures is critical for predicting and comprehending the formation and evolution of OA particles in the atmosphere. This investigation leverages previously documented data obtained through a temperature-programmed desorption-direct analysis in real-time, high-resolution mass spectrometry (TPD-DART-HRMS) platform. This methodology facilitates the bottom-up construction of volatility basis set (VBS) distributions for constituents found in three biogenic secondary organic aerosol (SOA) mixtures produced through the ozonolysis of α-pinene, limonene, and ocimene. The apparent enthalpies (ΔH*, kJ mol-1) and saturation mass concentrations (CT*, μg·m-3) of individual SOA components, determined as a function of temperature (T, K), facilitated an assessment of changes in VBS distributions and gas-particle partitioning with respect to T and atmospheric total organic mass loadings (tOM, μg·m-3). The VBS distributions reveal distinct differences in volatilities among monomers, dimers, and trimers, categorized into separate volatility bins. At the ambient temperature of T = 298 K, only monomers efficiently partition between gas and particle phases across a broad range of atmospherically relevant tOM values of 1-100 μg·m-3. Partitioning of dimers and trimers becomes notable only at T > 360 K and T > 420 K, respectively. The viscosity of SOA mixtures is assessed using a bottom-up calculation approach, incorporating the input of elemental formulas, ΔH*, CT*, and particle-phase mass fractions of the SOA components. Through this approach, we are able to accurately estimate the variations in SOA viscosity that result from the evaporation of its components. These variations are, in turn, influenced by atmospherically relevant changes in tOM and T. Comparison of the calculated SOA viscosity and diffusivity values with literature reported experimental results shows close agreement, thereby validating the employed calculation approach. These findings underscore the significant potential for TPD-DART-HRMS measurements in enabling the untargeted analysis of organic molecules within OA mixtures. This approach facilitates quantitative assessment of their gas-particle partitioning and allows for the estimation of their viscosity and condensed-phase diffusion, thereby contributing valuable insights to atmospheric models.

Elucidating the unexpected importance of intermediate-volatility organic compounds (IVOCs) from refueling procedure

Evaporative emissions release organic compounds comparable to gasoline exhaust in China. However, the measurement of intermediate volatility organic compounds (IVOCs) is lacking in studies focusing on gasoline evaporation. This study sampled organics from a real-world refueling procedure and analyzed the organic compounds using comprehensive two-dimensional gas chromatography coupled with a mass spectrometer (GC×GC-MS). The non-target analysis detected and quantified 279 organics containing 93 volatile organic compounds (VOCs, 64.9 ± 7.4 % in mass concentration), 182 IVOCs (34.9 ± 7.4 %), and 4 semivolatile organic compounds (SVOCs, 0.2 %). The refueling emission profile was distinct from that of gasoline exhaust. The b-alkanes in the B12 volatility bin are the most abundant IVOC species (1.9 ± 1.4 μg m-3) in refueling. A non-negligible contribution of 17.5 % to the ozone formation potential (OFP) from IVOCs was found. Although IVOCs are less in concentration, secondary organic aerosol potential (SOAP) from IVOCs (58.1 %) even exceeds SOAP from VOCs (41.6 %), mainly from b-alkane in the IVOC range. At the molecular level, the proportion of cyclic compounds in SOAP (12.1 %) indeed goes above its mass concentration (3.1 %), mainly contributed by cyclohexanes and cycloheptanes. As a result, the concentrations and SOAP of cyclic compounds (>50 %) could be overestimated in previous studies. Our study found an unexpected contribution of IVOCs from refueling procedures to both ozone and SOA formation, providing new insights into secondary pollution control policy.

Response of biogenic secondary organic aerosol formation to anthropogenic NOx emission mitigation

This study investigates the effects of anthropogenic nitrogen oxide (NOx) mitigation reduction on secondary organic aerosol (SOA) formation from monoterpene and sesquiterpene precursors across Europe, using the three-dimensional (3-D) Chemical Transport Model (CTM) CHIMERE. Two SOA mechanisms of varying complexity are employed: the GENOA-generated Biogenic Mechanism (GBM) and the Hydrophobic/Hydrophilic Organic mechanism (H2O). GBM is a condensed SOA mechanism generated by automatic reduction from near-explicit chemical mechanisms (i.e., the Master Chemical Mechanism - MCM and the peroxy radical autoxidation mechanism - PRAM) using the GENerator of Reduced Organic Aerosol Mechanisms version 2.0 (GENOA v2.0). Conversely, the H2O mechanism is developed primarily based on experimental data, with simplified chemical pathways and SOA formation yields reflecting those from chamber experiments. In the 3-D simulations conducted for the summer of 2018 over Europe, the implementation of GBM significantly improved the model's performance in comparison to simulations using the H2O mechanism, yielding results more consistent with measured aerosol concentrations extracted from the EBAS database. In response to NOx emission mitigation, simulated SOA concentrations increase with GBM but decrease when using the H2O mechanism, unless a highly oxygenated molecules (HOMs) formation scheme is incorporated. The SOA composition becomes more oxidized and concentrations elevate after NOx reduction, particularly in simulations using GBM. These higher concentrations are likely due to enhanced reaction rates of organic peroxy radicals (RO2) with HO2, resulting in more oxidized products from monoterpene degradation that favors HOM formation. The results suggest that detailed SOA mechanisms including autoxidation are necessary for accurate predictions of SOA concentrations in 3-D modeling.

The Chemical Landscape of Leaf Surfaces and Its Interaction with the Atmosphere

Atmospheric chemists have historically treated leaves as inert surfaces that merely emit volatile hydrocarbons. However, a growing body of evidence suggests that leaves are ubiquitous substrates for multiphase reactions-implying the presence of chemicals on their surfaces. This Review provides an overview of the chemistry and reactivity of the leaf surface's "chemical landscape", the dynamic ensemble of compounds covering plant leaves. We classified chemicals as endogenous (originating from the plant and its biome) or exogenous (delivered from the environment), highlighting the biological, geographical, and meteorological factors driving their contributions. Based on available data, we predicted ≫2 μg cm-2 of organics on a typical leaf, leading to a global estimate of ≫3 Tg for multiphase reactions. Our work also highlighted three major knowledge gaps: (i) the overlooked role of ambient water in enabling the leaching of endogenous substances and mediating aqueous chemistry; (ii) the importance of phyllosphere biofilms in shaping leaf surface chemistry and reactivity; (iii) the paucity of studies on the multiphase reactivity of atmospheric oxidants with leaf-adsorbed chemicals. Although biased toward available data, we hope this Review will spark a renewed interest in the leaf surface's chemical landscape and encourage multidisciplinary collaborations to move the field forward.

Molecular-Level Insights into the Relationship between Volatility of Organic Aerosol Constituents and PM2.5 Air Pollution Levels: A Study with Ultrahigh-Resolution Mass Spectrometry

Volatility of organic aerosols (OAs) significantly influences new particle formation and the occurrence of particulate air pollution. However, the relationship between the volatility of OA and the level of particulate air pollution (i.e., particulate matter concentration) is not well understood. In this study, we compared the chemical composition (identified by an ultrahigh-resolution Orbitrap mass spectrometer) and volatility (estimated based on a predeveloped parametrization method) of OAs in urban PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) samples from seven German and Chinese cities, where the PM2.5 concentration ranged from a light (14 μg m-3) to heavy (319 μg m-3) pollution level. A large fraction (71-98%) of compounds in PM2.5 samples were attributable to intermediate-volatility organic compounds (IVOCs) and semivolatile organic compounds (SVOCs). The fraction of low-volatility organic compounds (LVOCs) and extremely low-volatility organic compounds (ELVOCs) decreased from clean (28%) to heavily polluted urban regions (2%), while that of IVOCs increased from 34 to 62%. We found that the average peak area-weighted volatility of organic compounds in different cities showed a logarithmic correlation with the average PM2.5 concentration, indicating that the volatility of urban OAs increases with the increase of air pollution level. Our results provide new insights into the relationship between OA volatility and PM pollution levels and deepen the understanding of urban air pollutant evolution.

Characteristics of typical intermediate and semi volatile organic compounds in Shanghai during China International Import Expo event

To ensure good air quality during the China International Import Expo (CIIE) event, stringent emission-reduction measures were implemented in Shanghai. To assess the efficacy of these measures, this study measured typical categories of intermediate/semi volatile organic compounds (I/SVOCs), including alkanes (C10-C26 n-alkanes and pristane), EPA-priority polycyclic aromatic hydrocarbons (PAHs), alkylnaphthalenes, benzothiazole (BTH) and chlorobenzenes (CBs), at an urban site of Shanghai before and during two CIIE events (2019 and 2020; non-CIIE versus CIIE). The average concentrations of alkanes and PAHs during both 2019 and 2020 CIIE events decreased by approximately 41% and 17%, respectively, compared to non-CIIE periods. However, the decline in BTH and CBs was only observed during CIIE-2019. Secondary organic aerosol (SOA) formation from alkanes, PAHs and BTH was evaluated under atmospheric conditions, revealing considerable SOA contributions from dimethylnaphthalenes and BTH. Positive matrix factorization (PMF) analysis further revealed that life-related sources, such as cooking and residential emissions, make a noticeable contribution (21.6%) in addition to the commonly concerned gasoline-vehicle sources (31.5%), diesel-related emissions (20.8%), industrial emissions (18.6%) and ship emissions (7.5%). These findings provide valuable insights into the efficacy of the implemented measures in reducing atmospheric I/SVOCs levels. Moreover, our results highlight the significance of exploring additional individual species of I/SVOCs and life-related sources for further research and policy development.

Correlation gas chromatography and two-dimensional volatility basis methods to predict gas-particle partitioning for e-cigarette aerosols

E-cigarette aerosols contain a complex mixture of harmful and potentially harmful chemicals. Once released into the environment, they evolve and become new sources of indoor air pollutants that could pose a significant threat to both users and non-users. However, current understanding of the physicochemical properties of e-cigarette aerosol constituents that govern gas-particle partitioning in the atmosphere is limited, making it difficult to estimate the health risks associated with exposure. Here, we used correlation gas chromatography (C-GC) and two-dimensional volatility basis set (2D-VBS) methods to determine the vapor pressures and volatility for commonly reported toxic and irritating e-cigarette aerosol constituents. The vapor pressures of target compounds at 298 K were estimated from the Antoine-type linear relationship between the vapor pressure of reference standards and their retention times. Our C-GC results showed an overall positive correlation (R = 0.84) with estimates using the EPI (Estimation Programs Interface) Suite. The volatility calculated by 2D-VBS correlates well with the calculated vapor pressure from both C-GC (R = 0.82) and EPI Suite (R = 0.85). The volatility distribution also indicated fresh e-cigarette aerosol constituents are mainly more volatile organic compounds. Our case study revealed that low-vapor-pressure compounds (e.g., σ-dodecalactone, γ-decalactone, and maltol) become enriched in the e-cigarette aerosols within 2 hours following vaping emissions. Overall, these findings demonstrate the applicability of the C-GC and 2D-VBS methods for determining the physiochemical properties of e-cigarette aerosol constituents, which can aid in assessing the dynamic chemical composition of e-cigarette aerosols and exposures to vaping emissions in indoor environments.

Nonlinear effect of NOx concentration decrease on secondary aerosol formation in the Beijing-Tianjin-Hebei region: Evidence from smog chamber experiments and field observations

During the COVID-19 lockdown in the Beijing-Tianjin-Hebei (BTH) region in China, large decrease in nitrogen oxides (NOx) emissions, especially in the transportation sector, could not avoid the occurrence of heavy PM2.5 pollution where nitrate dominated the PM2.5 mass increase. To experimentally reveal the effect of NOx control on the formation of PM2.5 secondary components (nitrate in particular), photochemical simulation experiments of mixed volatile organic compounds (VOCs) under various NOx concentrations with smog chamber were performed. The proportions of gaseous precursors in the control experiment were comparable to ambient conditions typically observed in the BTH region. Under relatively constant VOCs concentrations, when the initial NOx concentration was reduced to 40% of that in the control experiment (labelled as NOx,0), the particle mass concentration was not significantly reduced, but when the initial NOx concentration decreased to 20 % of NOx,0, the mass concentration of particles as well as nitrate and organics showed a sudden decrease. A "critical point" where the mass concentration of secondary aerosol started to decline as the initial NOx concentration decreased, located at 0.2-0.4 NOx,0 (or 0.18-0.44 NO2,0) in smog chamber experiments. The oxidation capacity and solar radiation intensity played key roles in the mass concentration and compositions of the formed particles. In field observations in the BTH region in the autumn and winter seasons, the "critical point" exist at 0.15-0.34 NO2,0, which coincided mostly with the laboratory simulation results. Our results suggest that a reduction of NOx emission by >60% could lead to significant reductions of secondary aerosol formation, which can be an effective way to further alleviate PM2.5 pollution in the BTH region.

Development and performance evaluation of online monitors for near real-time measurement of total and water-soluble organic carbon in fine and coarse ambient PM

In this study, we developed two online monitors for total organic carbon (TOC) and water-soluble organic carbon (WSOC) measurements in fine (dp < 2.5μm) and coarse (2.5μm < dp < 10μm) particulate matter (PM), respectively. Their performance has been evaluated in laboratory and field tests to demonstrate the feasibility of using these monitors to measure near real-time concentrations, with consideration of their potential for being employed in long-term measurements. The fine PM collection setup was equipped with a versatile aerosol concentration enrichment system (VACES) connected to an aerosol-into-liquid-sampler (AILS), whereas two virtual impactors (VIs) in tandem with a modified BioSampler were used to collect coarse PM. These particle collection setups were in tandem with a Sievers M9 TOC analyzer to read TOC and WSOC concentrations in aqueous samples hourly. The average hourly TOC concentration measured by our developed monitors in fine and coarse PM were 5.17 ± 2.41 and 0.92 ± 0.29 μg/m3, respectively. In addition, our TOC readings showed good agreement and were comparable with those quantified using Sunset Lab EC/OC analyzer operating in parallel as a reference. Furthermore, we conducted field tests to produce diurnal profiles of fine PM-bound WSOC, which can show the effects of ambient temperature on maximum values in the nighttime chemistry of the winter, as well as on increased photochemical activities in afternoon peaks during the summer. According to our experimental campaign, WSOC mean values during the study period (3.07 μg/m3 for the winter and 2.7 μg/m3 for the summer) were in a comparable range with those of earlier studies in Los Angeles. Overall, our results corroborate the performance of our developed monitors in near real-time measurements of TOC and WSOC, which can be employed for future source apportionment studies in Los Angeles and other areas, aiding in understanding the health impacts of different pollution sources.

Gas-to-Particle Partitioning of Products from Ozonolysis of Δ3-Carene and the Effect of Temperature and Relative Humidity

Formation of oxidized products from Δ3-carene (C10H16) ozonolysis and their gas-to-particle partitioning at three temperatures (0, 10, and 20 °C) under dry conditions (<2% RH) and also at 10 °C under humid (78% RH) conditions were studied using a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) combined with a filter inlet for gases and aerosols (FIGAERO). The Δ3-carene ozonolysis products detected by the FIGAERO-ToF-CIMS were dominated by semivolatile organic compounds (SVOCs). The main effect of increasing temperature or RH on the product distribution was an increase in fragmentation of monomer compounds (from C10 to C7 compounds), potentially via alkoxy scission losing a C3 group. The equilibrium partitioning coefficient estimated according to equilibrium partitioning theory shows that the measured SVOC products distribute more into the SOA phase as the temperature decreases from 20 to 10 and 0 °C and for most products as the RH increases from <2 to 78%. The temperature dependency of the saturation vapor pressure (above an assumed liquid state), derived from the partitioning method, also allows for a direct way to obtain enthalpy of vaporization for the detected species without accessibility of authentic standards of the pure substances. This method can provide physical properties, beneficial for, e.g., atmospheric modeling, of complex multifunctional oxidation products.

Oligomer formation from cross-reaction of Criegee intermediates in the styrene-isoprene-O3 mixed system

Alkene ozonolysis can produce stabilized Criegee intermediates (SCIs), which play a key role in oligomers' formation. Though styrene and isoprene coexist in the ambient atmosphere as important anthropogenic and biogenic secondary organic aerosol (SOA) precursors, respectively, their cross-reactions have not received attention. This study investigated the interactions of SCIs from styrene and isoprene ozonolysis for the first time. The high-resolution Orbitrap mass spectrometer was used to determine the unique ion mass spectra of the isoprene-styrene-O3 mixture. The results show that the signal intensities of new ions account for >8.4% of total ions in the mass spectra of the styrene-isoprene-O3 mixed system. Styrene and isoprene ozonolysis can produce characteristic C7-SCI and C4-SCI, respectively. C7-SCI and C4-SCI can be involved in the cross-reactions, and the results of tandem mass spectra directly confirmed both C7-SCI and C4-SCI as chain units. The O/C and H/C ratios of cross-products are in the range of 0.38-1.07 and 1.00-1.50, respectively, which are consistent with cross-reaction products. Adding a C7-SCI unit reduces the oligomer's volatility by 1.3-1.4 orders of magnitude lower than adding a C4-SCI unit. Thus, C4-SCI can compete with C7-SCI to react with styrene-derived RO2/RC(O)OH to produce more volatile cross-products, while the less volatile cross-products can be formed when isoprene-derived RO2/RC(O)OH reacted with C7-SCI instead of C4-SCI. The SOA yield of the mixed system is lower than that of the single styrene-O3 system but higher than that of the single isoprene-O3 system. Ambient particles were also collected, and 5 possible SCI-related cross-products were identified. This study illustrates the effects of SCI-related cross-reactions on SOA components and physicochemical properties, providing a basis for future research on SCI-related cross-reactions that frequently occur in the ambient atmosphere.

Modeling the Formation of Organic Compounds across Full Volatility Ranges and Their Contribution to Nanoparticle Growth in a Polluted Atmosphere

Nanoparticle growth influences atmospheric particles' climatic effects, and it is largely driven by low-volatility organic vapors. However, the magnitude and mechanism of organics' contribution to nanoparticle growth in polluted environments remain unclear because current observations and models cannot capture organics across full volatility ranges or track their formation chemistry. Here, we develop a mechanistic model that characterizes the full volatility spectrum of organic vapors and their contributions to nanoparticle growth by coupling advanced organic oxidation modeling and kinetic gas-particle partitioning. The model is applied to Nanjing, a typical polluted city, and it effectively captures the volatility distribution of low-volatility organics (with saturation vapor concentrations <0.3 μg/m3), thus accurately reproducing growth rates (GRs), with a 4.91% normalized mean bias. Simulations indicate that as particles grow from 4 to 40 nm, the relative fractions of GRs attributable to organics increase from 59 to 86%, with the remaining contribution from H2SO4 and its clusters. Aromatics contribute much to condensable organic vapors (∼37%), especially low-volatility vapors (∼61%), thus contributing the most to GRs (32-46%) as 4-40 nm particles grow. Alkanes also contribute 19-35% of GRs, while biogenic volatile organic compounds contribute minimally (<13%). Our model helps assess the climatic impacts of particles and predict future changes.

Automating methods for estimating metabolite volatility

The volatility of metabolites can influence their biological roles and inform optimal methods for their detection. Yet, volatility information is not readily available for the large number of described metabolites, limiting the exploration of volatility as a fundamental trait of metabolites. Here, we adapted methods to estimate vapor pressure from the functional group composition of individual molecules (SIMPOL.1) to predict the gas-phase partitioning of compounds in different environments. We implemented these methods in a new open pipeline called volcalc that uses chemoinformatic tools to automate these volatility estimates for all metabolites in an extensive and continuously updated pathway database: the Kyoto Encyclopedia of Genes and Genomes (KEGG) that connects metabolites, organisms, and reactions. We first benchmark the automated pipeline against a manually curated data set and show that the same category of volatility (e.g., nonvolatile, low, moderate, high) is predicted for 93% of compounds. We then demonstrate how volcalc might be used to generate and test hypotheses about the role of volatility in biological systems and organisms. Specifically, we estimate that 3.4 and 26.6% of compounds in KEGG have high volatility depending on the environment (soil vs. clean atmosphere, respectively) and that a core set of volatiles is shared among all domains of life (30%) with the largest proportion of kingdom-specific volatiles identified in bacteria. With volcalc, we lay a foundation for uncovering the role of the volatilome using an approach that is easily integrated with other bioinformatic pipelines and can be continually refined to consider additional dimensions to volatility. The volcalc package is an accessible tool to help design and test hypotheses on volatile metabolites and their unique roles in biological systems.

Modeling of secondary organic aerosols (SOA) based on two commonly used air quality models in China: Consistent S/IVOCs contribution but large differences in SOA aging

Secondary organic aerosol (SOA) is an important component of atmospheric fine particulate matter (PM2.5), with contributions from anthropogenic and biogenic volatile organic compounds (AVOC and BVOC) and semi- (SVOC) and intermediate volatility organic compounds (IVOC). Policymakers need to know which SOA precursors are important but accurate simulation of SOA magnitude and contributions remain uncertain. Findings from existing SOA modeling studies have many inconsistencies due to differing emission inventory methodologies/assumptions, air quality model (AQM) algorithms, and other aspects of study methodologies. To address some of the inconsistencies, we investigated the role of different AQM SOA algorithms by applying two commonly used models, CAMx and CMAQ, with consistent emission inventories to simulate SOA concentrations and contributions for July and November 2018 in China. Both models have a volatility basis set (VBS) SOA algorithm but with different parameters and treatments of SOA photochemical aging. SOA generated from BVOC (i.e., BSOA) is found to be more important in southern China. In contrast, SOA generated from anthropogenic precursors is more prevalent in the North China Plain (NCP), Yangtze River Delta (YRD), Sichuan Basin and Central China. Both models indicate negligible SOA formation from SVOC emissions compared to other precursors. In July, when BVOC emissions are abundant, SOA is predominantly contributed by BSOA (except for NCP), followed by IVOC-SOA (i.e., SOA produced from IVOC) and ASOA (i.e., SOA produced from anthropogenic VOC). In contrast, in November, IVOC became the leading SOA contributor for all selected regions except PRD, illustrating the important contribution of IVOC emissions to SOA formation. While both models generally agree in terms of the spatial distributions and seasonal variations of different SOA components, CMAQ tends to predict higher BSOA, while CAMx generates higher ASOA concentrations. As a result, CMAQ results suggest that BSOA concentration is always higher than ASOA in November, while CAMx emphasizes the importance of ASOA. Utilizing a conceptual model, we found that different treatment of SOA aging between the two models is a major cause of differences in simulated ASOA concentrations. The step-wise SOA aging scheme implemented in the CAMx VBS (based on gas-phase reactions with OH radical and similar to other models) exhibits a strong enhancement effect on simulated ASOA concentrations, and this effect increases with the ambient organic aerosol (OA) concentrations. The CMAQ aerosol module implements a different SOA aging scheme that represents particle-phase oligomerization and has smaller impacts on total OA. Different structures and/or parameters of the SOA aging schemes are being used in current models, which could greatly affect model simulations of OA in ways that are difficult to anticipate. Our results indicate that future control policies should aim at reducing IVOC emissions as well as traditional VOC emissions. In addition, aging schemes are the major driver in CMAQ vs. CAMx treatments of ASOA and their resulting predicted mass. More sophisticated measurement data (e.g., with resolved OA components) and/or chamber experiments (e.g., investigating how aging influences SOA yields) are needed to better characterize SOA aging and constrain model parameterizations.

Alkylperoxy radicals are responsible for the formation of oxygenated primary organic aerosol

Organic aerosol (OA) is an air pollutant ubiquitous in urban atmospheres. Urban OA is usually apportioned into primary OA (POA), mostly emitted by mobile sources, and secondary OA (SOA), which forms in the atmosphere due to oxidation of gas-phase precursors from anthropogenic and biogenic sources. By performing coordinated measurements in the particle phase and the gas phase, we show that the alkylperoxy radical chemistry that is responsible for low-temperature ignition also leads to the formation of oxygenated POA (OxyPOA). OxyPOA is distinct from POA emitted during high-temperature ignition and is chemically similar to SOA. We present evidence for the prevalence of OxyPOA in emissions of a spark-ignition engine and a next-generation advanced compression-ignition engine, highlighting the importance of understanding OxyPOA for predicting urban air pollution patterns in current and future atmospheres.

Model Framework for Predicting Semivolatile Organic Material Emissions Indoors from Organic Aerosol Measurements: Applications to HOMEChem Stir-Frying

Cooking activities emit myriad low-volatility, semivolatile, and highly volatile organic compounds that together form particles that can accumulate to large indoor concentrations. Absorptive partitioning thermodynamics governs the particle-phase organic aerosol concentration mainly via temperature and sorbing mass impacts. Cooking activities can increase the organic sorbing mass by 1-2 orders of magnitude, increasing particle-phase concentrations and affecting emission rate calculations. Although recent studies have begun to probe the volatility characteristics of indoor cooking particles, parametrizations of cooking particle mass emissions have largely neglected these thermodynamic considerations. Here, we present an improved thermodynamics-based model framework for estimating condensable organic material emission rates from a time series of observed concentrations, given that adequate measurements or assumptions can be made about the volatility of the emitted species. We demonstrate the performance of this methodology by applying data from stir-frying experiments performed during the House Observations of Microbial and Environmental Chemistry (HOMEChem) campaign to a two-zone box model representing the UTest House. Preliminary estimates of organic mass emitted on a per-stir-fry basis for three types of organic aerosol factors are presented. Our analysis highlights that using traditional nonvolatile particle models and emission characterizations for some organic aerosol emitting activities can incorrectly attribute concentration changes to emissions rather than thermodynamic effects.

Intermediate-volatility aromatic hydrocarbons from the rubber products industry in China

As key components of intermediate-volatility organic compounds (IVOCs), intermediate-volatility aromatic hydrocarbons (IAHs) are important precursors of ozone and secondary organic aerosol (SOA). Rubber products (RP) industry has significant influence on ozone and SOA formation, yet few studies are available to characterize their emissions of IAHs. Here we conducted measurements of IAHs emitted from rubber products (RP) factories in China. Tens of C10-C12 IAH species were identified with C10H14-AH (such as tetramethyl benzene) and naphthalene (C10H8) as the dominant species, accounting for 57.0 % - 100.0 % of total IAHs emissions. On average, IAHs showed higher concentrations (1.1 × 102-1.2 × 103 μg m-3) in mixing, extrusion, painting, crushing, and grinding processes than those (8.2-14 μg m-3) in vulcanization and gumming processes as well as warehouse. Moreover, IAHs concentrations were 1.3-1.7 times of volatile aromatic hydrocarbons (VAHs; C6-C9 aromatics) in the emissions from mixing, extrusion, crushing and grinding processes. The average IAHs to volatile organic compounds (VOCs) ratios also showed relatively higher values (0.1-0.7) in these processes, which were significantly higher than those of 0.01-0.03 observed in other industries, and even comparable to the IVOCs to VOCs ratio of 0.2 used for estimating solvent-related emission. The ozone and SOA formation potential values of IAHs were 1.1-2.6 times and 0.9-3.9 times those of VAHs, respectively, and were 0.5-1.0 times and 0.9-1.9 times those of total VOCs in emissions of mixing, extrusion, crushing, and grinding processes of the RP industry. The total emission of IAHs was estimated to be 115.8 Gg from the RP industry in China, which could account for 64.5 % of total IAH emissions from all industrial sectors. This study further suggests that the RP industry might be an important emission source of IAHs with substantially higher ozone and SOA formation potentials.

Impact of Biomass Burning Organic Aerosol Volatility on Smoke Concentrations Downwind of Fires

Biomass burning particulate matter (BBPM) affects regional air quality and global climate, with impacts expected to continue to grow over the coming years. We show that studies of North American fires have a systematic altitude dependence in measured BBPM normalized excess mixing ratio (NEMR; ΔPM/ΔCO), with airborne and high-altitude studies showing a factor of 2 higher NEMR than ground-based measurements. We report direct airborne measurements of BBPM volatility that partially explain the difference in the BBPM NEMR observed across platforms. We find that when heated to 40-45 °C in an airborne thermal denuder, 19% of lofted smoke PM1 evaporates. Thermal denuder measurements are consistent with evaporation observed when a single smoke plume was sampled across a range of temperatures as the plume descended from 4 to 2 km altitude. We also demonstrate that chemical aging of smoke and differences in PM emission factors can not fully explain the platform-dependent differences. When the measured PM volatility is applied to output from the High Resolution Rapid Refresh Smoke regional model, we predict a lower PM NEMR at the surface compared to the lofted smoke measured by aircraft. These results emphasize the significant role that gas-particle partitioning plays in determining the air quality impacts of wildfire smoke.

Global Anthropogenic Emissions of Full-Volatility Organic Compounds

Traditional global emission inventories classify primary organic emissions into nonvolatile organic carbon and volatile organic compounds (VOCs), excluding intermediate-volatility and semivolatile organic compounds (IVOCs and SVOCs, respectively), which are important precursors of secondary organic aerosols. This study establishes the first global anthropogenic full-volatility organic emission inventory with chemically speciated or volatility-binned emission factors. The emissions of extremely low/low-volatility organic compounds (xLVOCs), SVOCs, IVOCs, and VOCs in 2015 were 13.2, 10.1, 23.3, and 120.5 Mt, respectively. The full-volatility framework fills a gap of 18.5 Mt I/S/xLVOCs compared with the traditional framework. Volatile chemical products (VCPs), domestic combustion, and on-road transportation sources were dominant contributors to full-volatility emissions, accounting for 30, 30, and 12%, respectively. The VCP and on-road transportation sectors were the main contributors to IVOCs and VOCs. The key emitting regions included Africa, India, Southeast Asia, China, Europe, and the United States, among which China, Europe, and the United States emitted higher proportions of IVOCs and VOCs owing to the use of cleaner fuel in domestic combustion and more intense emissions from VCPs and on-road transportation activities. The findings contribute to a better understanding of the impact of organic emissions on global air pollution and climate change.

Impacts of haze and nitrogen oxide alleviation on summertime ozone formation: A modeling study over the Yangtze River Delta, China

The strict emission control measures have profoundly changed the air pollution in the Yangtze River Delta (YRD) region, China. However, the impacts of decreasing fine particulates (PM2.5) and nitrogen oxide (NOx) on summer ozone (O3) formation still remain disputable. We perform simulations in the 2018 summer over the YRD using the WRF-Chem model that considers the aerosol radiative forcing (ARF) and HO2 heterogeneous loss on aerosol surface. The model reasonably reproduces the measured spatiotemporal surface O3 and PM2.5 concentrations and aerosol compositions. Model sensitivity experiments show that the NOx mitigation during recent years changes daytime O3 formation in summer from the transition regime to the NOx-sensitive regime in the YRD. The decreasing NOx emission generally weakens O3 formation and lowers ambient O3 levels in summer during recent years, except for some urban centers of megacities. While, the haze alleviation characterized by a decline in ambient PM2.5 concentration in the past years largely counteracts the daytime O3 decrease caused by NOx mitigation, largely contributing to the persistently high levels of summertime O3. The counteracting effect is dominantly attributed to the attenuated ARF and minorly contributed by the suppressed HO2 uptake and heterogeneous loss on aerosol surface. These results highlight that the repeated O3 pollution in the YRD is closely associated with NOx and haze alleviation and more efforts must be taken to achieve lower O3 levels.

Nonequilibrium Behavior in Isoprene Secondary Organic Aerosol

Recent studies have shown that instantaneous gas-particle equilibrium partitioning assumptions fail to predict SOA formation, even at high relative humidity (∼85%), and photochemical aging seems to be one driving factor. In this study, we probe the minimum aging time scale required to observe nonequilibrium partitioning of semivolatile organic compounds (SVOCs) between the gas and aerosol phase at ∼50% RH. Seed isoprene SOA is generated by photo-oxidation in the presence of effloresced ammonium sulfate seeds at <1 ppbv NOx, aged photochemically or in the dark for 0.3-6 h, and subsequently exposed to fresh isoprene SVOCs. Our results show that the equilibrium partitioning assumption is accurate for fresh isoprene SOA but breaks down after isoprene SOA has been aged for as short as 20 min even in the dark. Modeling results show that a semisolid SOA phase state is necessary to reproduce the observed particle size distribution evolution. The observed nonequilibrium partitioning behavior and inferred semisolid phase state are corroborated by offline mass spectrometric analysis on the bulk aerosol particles showing the formation of organosulfates and oligomers. The unexpected short time scale for the phase transition within isoprene SOA has important implications for the growth of atmospheric ultrafine particles to climate-relevant sizes.

Combined Smog Chamber/Oxidation Flow Reactor Study on Aging of Secondary Organic Aerosol from Photooxidation of Aromatic Hydrocarbons

Secondary organic aerosol (SOA) is a significant component of atmospheric fine particulate matter (PM2.5), and their physicochemical properties can be significantly changed in the aging process. In this study, we used a combination consisting of a smog chamber (SC) and oxidation flow reactor (OFR) to investigate the continuous aging process of gas-phase organic intermediates and SOA formed from the photooxidation of toluene, a typical aromatic hydrocarbon. Our results showed that as the OH exposure increased from 2.6 × 1011 to 6.3 × 1011 molecules cm-3 s (equivalent aging time of 2.01-4.85 days), the SOA mass concentration (2.9 ± 0.05-28.7 ± 0.6 μg cm-3) and corrected SOA yield (0.073-0.26) were significantly enhanced. As the aging process proceeds, organic acids and multiple oxygen-containing oxidation products are continuously produced from the photochemical aging process of gas-phase organic intermediates (mainly semi-volatile and intermediate volatility species, S/IVOCs). The multigeneration oxidation products then partition to the aerosol phase, while functionalization of SOA rather than fragmentation dominated in the photochemical aging process, resulting in much higher SOA yield after aging compared to that in the SC. Our study indicates that SOA yields as a function of OH exposure should be considered in air quality models to improve SOA simulation, and thus accurately assess the impact on SOA properties and regional air quality.

Characterization and dark oxidation of the emissions of a pellet stove

Pellet combustion in residential heating stoves has increased globally during the last decade. Despite their high combustion efficiency, the widespread use of pellet stoves is expected to adversely impact air quality. The atmospheric aging of pellet emissions has received even less attention, focusing mainly on daytime conditions, while the degree to which pellet emissions undergo night-time aging as well as the role of relative humidity remain poorly understood. In this study, environmental simulation chamber experiments were performed to characterize the fresh and aged organic aerosol (OA) emitted by a pellet stove. The fresh pellet stove PM1 (particulate matter with an aerodynamic diameter less than 1 μm) emissions consisted mainly of OA (93 ± 4%, mean ± standard deviation) and black carbon (5 ± 3%). The primary OA (POA) oxygen-to-carbon ratio (O : C) was 0.58 ± 0.04, higher than that of fresh logwood emissions. The fresh OA at a concentration of 70 μg m-3 (after dilution and equilibration in the chamber) consisted of semi-volatile (68%), low and extremely low volatility (16%) and intermediate-volatility (16%) compounds. The oxidation of pellet emissions under dark conditions was investigated by injecting nitrogen dioxide (NO2) and ozone (O3) into the chamber, at different (10-80%) relative humidity (RH) levels. In all dark aging experiments secondary organic aerosol (SOA) formation was observed, increasing the OA levels after a few hours of exposure to NO3 radicals. The change in the aerosol composition and the extent of oxidation depended on RH. For low RH, the SOA mass formed was up to 30% of the initial OA, accompanied by a moderate change in both O : C levels (7-8% increase) and the OA spectrum. Aging under higher RH conditions (60-80%) led to a more oxygenated aerosol (increase in O : C of 11-18%), but only a minor (1-10%) increase in OA mass. The increase in O : C at high RH indicates the importance of heterogeneous aqueous reactions in this system, that oxidize the original OA with a relatively small net change in the OA mass. These results show that the OA in pellet emissions can chemically evolve under low photochemical activity (e.g. the wintertime period) with important enhancement in SOA mass under certain conditions.

Volatility Parametrization of Low-Volatile Components of Ambient Organic Aerosols Based on Molecular Formulas

Evaluating the volatility of organic compounds based solely on their molecular formulas would avoid tough demands in deriving molecular structures. Here, we deployed an iodide-adduct Long Time-of-Flight Chemical Ionization Mass Spectrometry (LToF-CIMS) combined with a Filter Inlet for Gases and AEROsols (FIGAERO) to investigate molecular formulas and thermograms of organic compounds on ambient particulate samples collected in the summer of 2021 in a suburban site of Shanghai and to estimate saturation vapor pressures of low- and semivolatile components of ambient organic aerosols. Then, a hierarchical cluster analysis and a subsequent classification of obtained clusters by similarity calculation were applied to the measured data set of molecular formulas and saturation vapor pressures of organic aerosols with at least a 2/3 appearance frequency, together with a similar data set collected at a rural site in the Beijing-Tianjin-Hebei region during the winter of 2018 (Ren et al., 2018), to classify all compounds into multiple groups. For each group of compounds, parametrizations between volatility and elemental composition were derived, and then relationships between each group of parameters and the mean O:C were established to achieve a volatility-molecular formula parametrization with the O:C as a key input. Statistical comparison of estimated volatilities of low-volatile organic compounds shows a much better performance of our parametrization than previous molecular formula-based ones.

Shortwave absorption by wildfire smoke dominated by dark brown carbon

Wildfires emit large amounts of black carbon and light-absorbing organic carbon, known as brown carbon, into the atmosphere. These particles perturb Earth's radiation budget through absorption of incoming shortwave radiation. It is generally thought that brown carbon loses its absorptivity after emission in the atmosphere due to sunlight-driven photochemical bleaching. Consequently, the atmospheric warming effect exerted by brown carbon remains highly variable and poorly represented in climate models compared with that of the relatively nonreactive black carbon. Given that wildfires are predicted to increase globally in the coming decades, it is increasingly important to quantify these radiative impacts. Here we present measurements of ensemble-scale and particle-scale shortwave absorption in smoke plumes from wildfires in the western United States. We find that a type of dark brown carbon contributes three-quarters of the short visible light absorption and half of the long visible light absorption. This strongly absorbing organic aerosol species is water insoluble, resists daytime photobleaching and increases in absorptivity with night-time atmospheric processing. Our findings suggest that parameterizations of brown carbon in climate models need to be revised to improve the estimation of smoke aerosol radiative forcing and associated warming.

Real-World Emission Characteristics of Full-Volatility Organics Originating from Nonroad Agricultural Machinery during Agricultural Activities

Nonroad agricultural machinery (NRAM) emissions constitute a significant source of air pollution in China. Full-volatility organics originating from 19 machines under 6 agricultural activities were measured synchronously. The diesel-based emission factors (EFs) for full-volatility organics were 4.71 ± 2.78 g/kg fuel (average ± standard deviation), including 91.58 ± 8.42% volatile organic compounds (VOCs), 7.94 ± 8.16% intermediate-volatility organic compounds (IVOCs), 0.28 ± 0.20% semivolatile organic compounds (SVOCs), and 0.20 ± 0.16% low-volatility organic compounds (LVOCs). Full-volatility organic EFs were significantly reduced by stricter emission standards and were the highest under pesticide spraying activity. Our results also demonstrated that combustion efficiency was a potential factor influencing full-volatility organic emissions. Gas-particle partitioning in full-volatility organics could be affected by multiple factors. Furthermore, the estimated secondary organic aerosol formation potential based on measured full-volatility organics was 143.79 ± 216.80 mg/kg fuel and could be primarily attributed to higher-volatility-interval IVOCs (bin₁₂-bin₁₆ contributed 52.81 ± 11.58%). Finally, the estimated emissions of full-volatility organics from NRAM in China (2021) were 94.23 Gg. This study provides first-hand data on full-volatility organic EFs originating from NRAM to facilitate the improvement of emission inventories and atmospheric chemistry models.

Volatility of Secondary Organic Aerosol from β-Caryophyllene Ozonolysis over a Wide Tropospheric Temperature Range

We investigated secondary organic aerosol (SOA) from β-caryophyllene oxidation generated over a wide tropospheric temperature range (213-313 K) from ozonolysis. Positive matrix factorization (PMF) was used to deconvolute the desorption data (thermograms) of SOA products detected by a chemical ionization mass spectrometer (FIGAERO-CIMS). A nonmonotonic dependence of particle volatility (saturation concentration at 298 K, C298K*) on formation temperature (213-313 K) was observed, primarily due to temperature-dependent formation pathways of β-caryophyllene oxidation products. The PMF analysis grouped detected ions into 11 compound groups (factors) with characteristic volatility. These compound groups act as indicators for the underlying SOA formation mechanisms. Their different temperature responses revealed that the relevant chemical pathways (e.g., autoxidation, oligomer formation, and isomer formation) had distinct optimal temperatures between 213 and 313 K, significantly beyond the effect of temperature-dependent partitioning. Furthermore, PMF-resolved volatility groups were compared with volatility basis set (VBS) distributions based on different vapor pressure estimation methods. The variation of the volatilities predicted by different methods is affected by highly oxygenated molecules, isomers, and thermal decomposition of oligomers with long carbon chains. This work distinguishes multiple isomers and identifies compound groups of varying volatilities, providing new insights into the temperature-dependent formation mechanisms of β-caryophyllene-derived SOA particles.

Real time measurements of the secondary organic aerosol formation and aging from ambient air using an oxidation flow reactor in seoul during winter

Herein, the formation and aging processes of organic aerosol (OA) in urban Seoul, Korea, during winter were investigated using a high-resolution aerosol mass spectrometer (HR-ToF-AMS) and an oxidation flow reactor (OFR). The results demonstrated that the highest secondary OA (SOA) production (ΔOA = 3.44 μg m^-3 with a relative OA enhancement ratio (EROA) = 1.40) occurred at ∼2 eq. days of OH exposure. Particularly, higher SOA production was observed under the following atmospheric conditions: high relative humidity (RH) (>70%) and high PM₁ mass concentration (>50 μg m^-3), demonstrating that oxidation capacity, heterogeneous and aqueous phase reactions are important for further oxidation. Additionally, increased SOA formation occurs under both higher hydrocarbon-like OA and more oxidized OOA conditions. Further oxidation of both freshly emitted and aged and/or transported OA can be a remarkable further source of SOA in winter in Seoul and further downwind areas. In particular, the high mass concentration of MO-OOA in high total PM₁ would be an important indication that SOA formation could be accelerated by a heterogeneous reaction, necessitating additional investigations on the haze formation process.

Characterization of products formed from the oxidation of toluene and m-xylene with varying NOx and OH exposure

Aromatic volatile organic compounds (VOCs) are an important precursor of secondary organic aerosol (SOA) in the urban environment. SOA formed from the oxidation of anthropogenic VOCs can be substantially more abundant than biogenic SOA and has been shown to account for a significant fraction of fine particulate matter in urban areas. A potential aerosol mass (PAM) chamber was used to investigate the oxidised products from the photo-oxidation of m-xylene and toluene. The experiments were carried out with OH radical as oxidant in both high- and low-NO_x conditions and the resultant aerosol samples were collected using quartz filters and analysed by GC × GC-TOFMS. Results show the oxidation products derived from both precursors included ring-retaining and -opening compounds (unsaturated aldehydes, unsaturated ketones and organic acids) with a high number of ring-opening compounds observed from toluene oxidation. Glyoxal and methyl glyoxal were the major ring-cleavage products from both oxidation systems, indicating that a bicyclic route plays an important role in their formation. SOA yields were higher for both precursors under high-NO_x (toluene: 0.111; m-xylene: 0.124) than at low-NO_x (toluene: 0.089; m-xylene: 0.052), likely linked to higher OH concentrations during low-NO_x experiments which may lead to higher degree of fragmentation. DHOPA (2,3-dihydroxy-4-oxo-pentanoic acid), a known tracer of toluene oxidation, was observed in both oxidation systems. The mass fraction of DHOPA in SOA from toluene oxidation was about double the value reported previously, but it should not be regarded as a tracer solely for oxidation of toluene as m-xylene oxidation gave a similar relative yield.

Synergistic Effects of SO2 and NH3 Coexistence on SOA Formation from Gasoline Evaporative Emissions

Vehicular evaporative emissions make an increasing contribution to anthropogenic sources of volatile organic compounds (VOCs), thus contributing to secondary organic aerosol (SOA) formation. However, few studies have been conducted on SOA formation from vehicle evaporative VOCs under complex pollution conditions with the coexistence of NO_x, SO₂, and NH₃. In this study, the synergistic effects of SO₂ and NH₃ on SOA formation from gasoline evaporative VOCs with NO_x were examined using a 30 m³ smog chamber with the aid of a series of mass spectrometers. Compared with the systems involving SO₂ or NH₃ alone, SO₂ and NH₃ coexistence had a greater promotion effect on SOA formation, which was larger than the cumulative effect of the two promotions alone. Meanwhile, contrasting effects of SO₂ on the oxidation state (OSc) of SOA in the presence or absence of NH₃ were observed, and SO₂ could further increase the OSc with the coexistence of NH₃. The latter was attributed to the synergistic effects of SO₂ and NH₃ coexistence on SOA formation, wherein N-S-O adducts can be formed from the reaction of SO₂ with N-heterocycles generated in the presence of NH₃. Our study contributes to the understanding of SOA formation from vehicle evaporative VOCs under highly complex pollution conditions and its atmospheric implications.

Volatility distribution of primary organic aerosol emissions from household crop waste combustion in China

Biomass-burning emissions are a significant source of primary organic aerosol (POA). Volatility is one of the most important physical properties of organic aerosol (OA). Dilution and thermodenuder (TD) measurements were used to investigate the volatility of POA from household crop waste combustion in China. Between 10% and 30% of the POA desorbed when diluted from 20:1 to 120:1, while 10%-40% of POA evaporated in the TD when heated to 150 °C. Thus, a considerable proportion of the POA emissions were volatile. A dynamic mass transfer model was applied to derived volatility distributions of POA based on TD data. A best fit volatility distributions for POA and associated mass accommodation coefficients (α), and the enthalpy of vaporization (ΔH_vap) were presented. The emissions factors and volatility distribution of POA emission from household crop waste combustion in this study can be used to improve emission inventories and simulate gas-particle partitioning of organic aerosol in chemical transport models.

From the HOMEChem frying pan to the outdoor atmosphere: chemical composition, volatility distributions and fate of cooking aerosol

Cooking organic aerosol (COA) is frequently observed in urban field studies. Like other forms of organic aerosol, cooking emissions partition between gas and particle phases; a quantitative understanding of the species volatility governing this partitioning is essential to model the transport and fate of COA. However, few cooking-specific volatility measurements are available, and COA is often assumed to be semi-volatile. We use measurements from a thermodenuder coupled to an aerosol chemical speciation monitor during the HOMEChem study to investigate the chemical components and volatility of near-source COA. We found that fresh emissions of COA have three chemical components: a biomass burning-like component (COA_BBOA), a lower volatility component associated with cooking oil (COA_oil-2), and a higher volatility component associated with cooking oil (COA_oil-1). We provide characteristic mass spectra and volatility profiles for these components. We develop a model to describe the partitioning of these emissions as they dilute through the house and outdoor atmosphere. We show that the total emissions from cooking can be misclassified in air quality studies that use semi-volatile emissions as a proxy for cooking aerosol, due to the presence of substantial mass in lower volatility bins of COA not generally represented in models. Primary emissions of COA can thus be not only primary sources of urban aerosol pollution, but also sources of semi-volatile organic compounds that undergo secondary chemistry in the atmosphere and contribute to ozone formation and secondary organic aerosol.

Source characterization of volatile organic compounds in urban Beijing and its links to secondary organic aerosol formation

Volatile organic compounds (VOCs) are precursors for ozone and secondary organic aerosol (SOA) formation, thereby playing a vital role in atmospheric chemistry and urban air quality. To characterize the relationship between VOCs and SOA, organics both in gas and particulate phases were concurrently measured in urban Beijing. The VOCs and organic aerosol (OA) were apportioned into factors with different oxidation levels by applying the factorization analysis on their detailed mass spectra. Six factors of VOCs were identified, including four primary VOCs (PVOC) factors and two secondary VOCs (SVOC) factors. The PVOC factors dominated the total VOCs when the air mass originated in the cleaner northern areas, while SVOC factors dominated for polluted southern air masses. The normalized concentrations of PVOC and primary OA factors showed consistent diurnal variations regardless of air mass directions, owing to the relatively stable local emissions during the experimental period. This contrasted with the secondary factors due to more complex transformation processes. The traffic-related VOCs and solid fuel combustion VOCs negatively correlated with SOA, implying that they may have contributed to the SOA formation through photooxidation. The VOCs in lower oxidation levels were found to have poor correlations with the less oxidized SOA, whereas they correlated strongly to the more oxidized SOA. This implied that the less oxidized SOA may be in a transition state, where its production and loss rates were balanced. These served as products of VOCs oxidation and reactants of more oxidized SOA formation, playing important roles on the VOC to SOA transformation. The identified VOC emission sources and their photochemical production of SOA should be considered in air quality policy planning.

Vapors Are Lost to Walls, Not to Particles on the Wall: Artifact-Corrected Parameters from Chamber Experiments and Implications for Global Secondary Organic Aerosol

Atmospheric models of secondary organic aerosol (OA) (SOA) typically rely on parameters derived from environmental chambers. Chambers are subject to experimental artifacts, including losses of (1) particles to the walls (PWL), (2) vapors to the particles on the wall (V2PWL), and (3) vapors to the wall directly (VWL). We present a method for deriving artifact-corrected SOA parameters and translating these to volatility basis set (VBS) parameters for use in chemical transport models (CTMs). Our process involves combining a box model that accounts for chamber artifacts (Statistical Oxidation Model with a TwO-Moment Aerosol Sectional model (SOM-TOMAS)) with a pseudo-atmospheric simulation to develop VBS parameters that are fit across a range of OA mass concentrations. We found that VWL led to the highest percentage change in chamber SOA mass yields (high NO_x: 36-680%; low NO_x: 55-250%), followed by PWL (high NO_x: 8-39%; low NO_x: 10-37%), while the effects of V2PWL are negligible. In contrast to earlier work that assumed that V2PWL was a meaningful loss pathway, we show that V2PWL is an unimportant SOA loss pathway and can be ignored when analyzing chamber data. Using our updated VBS parameters, we found that not accounting for VWL may lead surface-level OA to be underestimated by 24% (0.25 μg m^-3) as a global average or up to 130% (9.0 μg m^-3) in regions of high biogenic or anthropogenic activity. Finally, we found that accurately accounting for PWL and VWL improves model-measurement agreement for fine mode aerosol mass concentrations (PM_2.5) in the GEOS-Chem model.

Response of organic aerosol characteristics to emission reduction in Yangtze River Delta region

Organic aerosol (OA) is a major component of atmospheric particulate matter (PM) with complex composition and formation processes influenced by various factors. Emission reduction can alter both precursors and oxidants which further affects secondary OA formation. Here we provide an observational analysis of secondary OA (SOA) variation properties in Yangtze River Delta (YRD) of eastern China in response to large scale of emission reduction during Chinese New Year (CNY) holidays from 2015 to 2020, and the COVID-19 pandemic period from January to March, 2020. We found a 17% increase of SOA proportion during the COVID lockdown. The relative enrichment of SOA is also found during multi-year CNY holidays with dramatic reduction of anthropogenic emissions. Two types of oxygenated OA (OOA) influenced by mixed emissions and SOA formation were found to be the dominant components during the lockdown in YRD region. Our results highlight that these emission-reduction-induced changes in organic aerosol need to be considered in the future to optimize air pollution control measures.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at 10.1007/s11783-023-1714-0 and is accessible for authorized users.

Development of Volatility Distributions for Organic Matter in Biomass Burning Emissions

The volatility distribution of organic emissions from biomass burning and other combustion sources can determine their atmospheric evolution due to partitioning/aging. The gap between measurements and models predicting secondary organic aerosol has been partially attributed to the absence of semi- and intermediate volatility organic compounds (S/I-VOC) in models and measurements. However, S/I-VOCs emitted from these sources and typically quantified using the volatility basis framework (VBS) are not well understood. For example, the amount and composition of S/I-VOCs and their variability across different biomass burning sources such as residential woodstoves, open field burns, and laboratory simulated open burning are uncertain. To address this, a novel filter-in-tube sorbent tube sampling method collected S/I-VOC samples from biomass burning experiments for a range of fuels and combustion conditions. Filter-in-tube samples were analyzed using thermal desorption-gas chromatography-mass spectrometry (TD/GC/MS) for compounds across a wide range of volatilities (saturation concentrations; -2 ≤ logC* ≤ 6). The S/I-VOC measurements were used to calculate volatility distributions for each emissions source. The distributions were broadly consistent across the sources with IVOCs accounting for 75% - 90% of the total captured organic matter, while SVOCs and LVOCs were responsible for 6% - 13% and 1% - 12%, respectively. The distributions and predicted partitioning were generally consistent with literature. Particulate matter emission factors spanned two orders of magnitude across the sources. This work highlights the potential of inferring gas-particle partitioning behavior of biomass burning emissions using filter-in-tube sorbent samples analyzed offline. This simplifies both sampling and analysis of S/I-VOCs for studies focused on capturing the full range of organics emitted.

Non-linear effects of secondary organic aerosol formation and properties in multi-precursor systems

Secondary organic aerosol (SOA) contributes significantly to ambient fine particulate matter that affects climate and human health. Monoterpenes represent an important class of biogenic volatile organic compounds (VOCs) and their oxidation by nitrate radicals poses a substantial source of SOA globally. Here, we investigate the formation and properties of SOA from nitrate radical oxidation of two common monoterpenes, α-pinene and limonene. When two monoterpenes are oxidized simultaneously, we observe a ~50% enhancement in the formation of SOA from α-pinene and a ~20% reduction in limonene SOA formation. The change in SOA yields is accompanied by pronounced changes in aerosol chemical composition and volatility. These non-linear effects are not observed in a sequential oxidation experiment. Our results highlight that unlike currently assumed in atmospheric models, the interaction of products formed from individual VOCs should be accounted for to accurately describe SOA formation and its climate and health impacts.

Emission, transport, deposition, chemical and radiative impacts of mineral dust during severe dust storm periods in March 2021 over East Asia

A Regional Air Quality Model System (named RAQMS) coupled with a developed dust model driven by WRF was applied to synthetically investigate the emission, transport, deposition, budget, and chemical and radiative effects of mineral dust during the severe dust storm periods of 10-31 March 2021. Model results were validated against a variety of ground, vertical and satellite observations, which demonstrated a generally good model ability in reproducing meteorological variables, particulate matter and compositions, and aerosol optical properties. The first dust storm (DS1), which was the severest one since 2010 was originated from the Gobi Desert in southern Mongolia on 14 March, with the dust emission flux reaching 2785 μg m^-2 s^-1 and the maximum dust concentration exceeding 18,000 μg m^-3 in the dust deflation region. This dust storm resulted in remarkably high hourly PM₁₀ observations up to 7506 μg m^-3, 1887 μg m^-3, and 2704 μg m^-3 in Beijing, Tianjin, and Shijiazhuang on 15 March, respectively, and led to a maximum decrease in surface shortwave radiation up to 313.4 W m^-2 (72 %) in Beijing. The second dust storm (DS2) broke out in the deserts of eastern Mongolia, with lower dust emission than the first one. The extinction of shortwave radiation by dust aerosols led to a reduction in photolysis rate and consequently decreases in O₃ and secondary aerosol concentrations over the North China Plain (NCP), whereas total sulfate and nitrate concentrations consistently increased due to heterogeneous reactions on dust surfaces over the middle reaches of the Yellow River and the NCP region during DS1. Sulfate and nitrate formation through heterogeneous reactions were enhanced in the dust backflow on 16-17 March by approximately 18 % and 24 % on average in the NCP. Heterogeneous reactions and photolysis rate reduction by mineral dust jointly led to average changes in sulfate, nitrate, ammonium, and secondary organic aerosol (SOA) concentrations by 13.0 %, 13.5 %, -12.3 %, and -4.4 %, respectively, in the NCP region during DS1, larger than the changes in the Yangtze River Delta (YRD). The maximum dry deposition settled in the 7-11 μm size range in downwind land and ocean areas, while wet deposition peaked in the 4.7-7 μm size range in the entire domain. Wet deposition was approximately twice the dry deposition over mainland China except for dust source regions. During 10-31 March, the total dust emission, dry and wet depositions were estimated to be 31.4 Tg, 13.78 Tg and 4.75 Tg, respectively, with remaining 12.87 Tg of dust aerosols (41 % of the dust emission) suspending in the atmosphere or transporting to other continents and oceans.

Suppression of the phenolic SOA formation in the presence of electrolytic inorganic seed

Phenolic compounds are largely attributed to wildfire gases and rapidly react with atmospheric oxidants to form persistent phenoxy free radicals, which influence atmospheric chemistry and secondary organic aerosol (SOA) formation. In this study, phenol or o-cresol was photochemically oxidized under various conditions (NO_x levels, humidity, and seed conditions) in an outdoor photochemical reactor. Unexpectedly, SOA growth of both phenols was suppressed in the presence of salted aqueous aerosol compared to non-seed SOA. This discovery is different from the typical SOA formation of aromatic or biogenic hydrocarbons, which show noticeably higher SOA yields via organic aqueous reactions. Phenol, o-cresol, and their phenolic products (e.g., catechols) are absorbed in aqueous aerosol and form phenoxy radicals via heterogeneous reactions under sunlight. The resulting phenoxy radicals are redistributed between the gas and particle phases. Gaseous phenoxy radicals quickly react with ozone to form phenyl peroxide radicals and regenerated through a NO_x cycle to retard phenol oxidation and SOA formation. The explicit oxidation mechanisms of phenol or o-cresol in the absence of aqueous phase were derived including the Master Chemical Mechanism (MCM v3.3.1) and the path for peroxy radical adducts originating from the addition of an OH radical to phenols to form low volatility products (e.g., multi-hydroxy aromatics). The resulting gas mechanisms of phenol or o-cresol were, then, applied to the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model to predict SOA formation via multiphase partitioning of organics and aerosol-phase oligomerization. The model well simulated chamber-generated phenolic SOA in absence of wet-inorganic seed, but significantly overestimated SOA mass in presence of wet seed. This study suggests that heterogeneous chemistry to form phenoxy radicals needs to be included to improve SOA prediction from phenols. The suppression of atmospheric oxidation due to phenoxy radicals in wet inorganic aerosol can explain the low SOA formation during wildfire episodes.

Phase state of organic aerosols may limit temperature-driven thermodynamic repartitioning following outdoor-to-indoor transport

Ambient aerosols often experience temperature and humidity gradients following outdoor-to-indoor transport, causing organic aerosols (OA) to either gain or lose mass via gas-particle repartitioning. Recent models have sought to quantify these effects using equilibrium partitioning thermodynamics. However, evidence suggests some indoor OA may possess glassy or semisolid phase states with higher viscosities than liquid OA. Characteristic partitioning timescales of higher-viscosity particles are significantly longer than for liquid particles, which may either fully or partially inhibit repartitioning. For outdoor OA experiencing a temperature change during transport indoors, the ultimate repartitioning state depends on the relationship between the gas-particle partitioning rate coefficient (k_gp) of semivolatile organics and the indoor particle loss rate coefficient (l_p). That is, thermodynamic equilibrium partitioning may occur when semivolatile k_gp ≫ l_p, no repartitioning when semivolatile k_gp ≪ l_p, and partial repartitioning when their magnitudes are similar. Longer indoor particle lifetimes, higher particle number, and larger particle sizes all raise k_gp (driving repartitioning towards equilibrium). For simulated U.S. residences, equilibrium condensation was likely reached in humid climate zones during warm meteorological conditions. In colder regions, the degree of evaporative repartitioning depended on whether organics could repartition before the particle phase state adjusts to indoor conditions, which is uncertain. When an appreciable temperature gradient exists, this study not only confirmed that all outdoor-originating OA that is liquid indoors will reach thermodynamic equilibrium, but also concluded that a plurality (46% for this domain) of such OA that is semisolid may also achieve thermodynamic equilibrium during its indoor lifetime.

Rate of atmospheric brown carbon whitening governed by environmental conditions

Biomass burning organic aerosol (BBOA) in the atmosphere contains many compounds that absorb solar radiation, called brown carbon (BrC). While BBOA is in the atmosphere, BrC can undergo reactions with oxidants such as ozone which decrease absorbance, or whiten. The effect of temperature and relative humidity (RH) on whitening has not been well constrained, leading to uncertainties when predicting the direct radiative effect of BrC on climate. Using an aerosol flow-tube reactor, we show that the whitening of BBOA by oxidation with ozone is strongly dependent on RH and temperature. Using a poke-flow technique, we show that the viscosity of BBOA also depends strongly on these conditions. The measured whitening rate of BrC is described well with the viscosity data, assuming that the whitening is due to oxidation occurring in the bulk of the BBOA, within a thin shell beneath the surface. Using our combined datasets, we developed a kinetic model of this whitening process, and we show that the lifetime of BrC is 1 d or less below ∼1 km in altitude in the atmosphere but is often much longer than 1 d above this altitude. Including this altitude dependence of the whitening rate in a chemical transport model causes a large change in the predicted warming effect of BBOA on climate. Overall, the results illustrate that RH and temperature need to be considered to understand the role of BBOA in the atmosphere.