Impacts of SO2, Relative Humidity, and Seed Acidity on Secondary Organic Aerosol Formation in the Ozonolysis of Butyl Vinyl Ether

Multiple effects of relative humidity on heterogeneous ozonolysis of cooking organic aerosol proxies from heated peanut oil emissions

The exposure to cooking organic aerosols (COA) is closely related to people's daily lives. Despite extensive investigations into COA's model compounds like oleic acid, the intricacies of heterogeneous ozonolysis of real COA and the effects of ambient conditions like humidity remain elusive. In this work, the ozonolysis of COA proxies from heated peanut oil emissions was investigated using diffuse reflectance infrared Fourier transform (DRIFTS) spectroscopy, and proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). We found that humidity hinders the reaction between ozone and CC double bonds due to the competitive adsorption of water and ozone on COA. Although visible light has little influence on the ozonolysis of COA in the absence of humidity, the ozonolytic CO production is significantly promoted by visible light in the presence of humidity. It may be attributed to the formation of water-derived reactive oxygen species (ROS, mainly HO•) from the photosensitization of polycyclic aromatic hydrocarbons (PAHs) in COA. We also found that humidity can enhance the depolymerization of carboxylic acid dimers and hydrolysis of intrinsic acetals in the COA. Moreover, humidity promotes the release of VOCs during both the dark and light ozonolysis of COA. This work reveals the important roles of humidity-responsive and photo-responsive components in COA during its ozonolysis, and the change in VOC release may guide the control of human VOC exposure in indoor air.

Contrasting molecular characteristics and formation mechanisms of biogenic and anthropogenic secondary organic aerosols at the summit and foot of Mt. Huang, East China

Secondary organic aerosol (SOA) exerts a considerable influence on atmospheric chemistry. However, little information about the vertical distribution of SOA in the alpine setting is available, which limited the simulation of SOA using chemical transport models. Here, a total of 15 biogenic and anthropogenic SOA tracers were measured in PM2.5 aerosols at both the summit (1840 m a.s.l.) and foot (480 m a.s.l.) of Mt. Huang during the winter of 2020 to explore their vertical distribution and formation mechanism. Most of the determined chemical species (e.g., BSOA and ASOA tracers, carbonaceous components, major inorganic ions) and gaseous pollutants at the foot of Mt. Huang were 1.7-3.2 times higher concentrations than those at the summit, suggesting the relatively more significant effect of anthropogenic emissions at the ground level. The ISORROPIA-II model showed that aerosol acidity increases as altitude decreases. Air mass trajectories, potential source contribution function (PSCF), and correlation analysis of BSOA tracers with temperature revealed that SOA at the foot of Mt. Huang was mostly derived from the local oxidation of volatile organic compounds (VOCs), while SOA at the summit was mainly influenced by long-distance transport. The robust correlations of BSOA tracers with anthropogenic pollutants (e.g., NH3, NO2, and SO2) (r = 0.54-0.91, p < 0.05) indicated that anthropogenic emissions could promote BSOA productions in the mountainous background atmosphere. Moreover, most of SOA tracers (r = 0.63-0.96, p < 0.01) and carbonaceous species (r = 0.58-0.81, p < 0.01) were correlated well with levoglucosan in all samples, suggesting that biomass burning played an important role in the mountain troposphere. This work demonstrated that daytime SOA at the summit of Mt. Huang was significantly influenced by the valley breeze in winter. Our results provide new insights into the vertical distributions and provenance of SOA in the free troposphere over East China.

Geochemical characteristics and socioeconomic associations of carbonaceous aerosols in coal-fueled cities with significant seasonal pollution pattern

Carbonaceous aerosols, comprising organic carbon (OC) and elemental carbon (EC), are critical component of fine particulate matter (PM2.5), with diverse impacts on air quality and human health. This study investigated the concentrations and seasonal patterns of carbonaceous species in PM2.5 during both the heating season (January 2021) and non-heating season (July 2021) in three coal-fueled cities in northern China, as well as the differences in carbonaceous aerosols and their associations with socioeconomic parameters in cities situated on either side of the "Hu Line" in China. The results showed that, owing to intensified coal combustion and unfavorable meteorological conditions, levels of OC, EC, and OC/EC ratios were higher in winter compared to summer. Moreover, the presence of dust (DU) and light pollution (LP) days resulted in elevated OC levels but decreased EC levels. The Char-EC/Soot-EC ratios were highest during LP, followed by CL and DU. A source apportionment analysis demonstrated that coal burning, vehicle exhaust, road dust, and biomass burning were the primary contributors to carbonaceous aerosols, as confirmed by diagnostic ratios, Char-EC/Soot-EC ratios, and PCA analysis. Furthermore, our study found that carbonaceous aerosols concentrations and source apportionment primarily varied with diurnal and seasonal trends and different pollution types. Additionally, at the national scale, population density and urban green space exhibited a positive correlation with OC/EC ratios (p < 0.05), while energy consumption per unit of GDP showed a negative correlation (p < 0.05). The observation that OC/EC ratios were lower in coal-fueled cities than in economy-based cities suggests a more severe pollution scenario. These findings highlight the importance of comprehending of the seasonal variation and chemical characteristics of carbonaceous aerosol for understanding air pollution sources and characteristics, which is essential for both air quality management and human health.

Global PM2.5 and secondary organic aerosols (SOA) levels with sectorial contribution to anthropogenic and biogenic SOA formation

This study estimates global PM_2.5 and anthropogenic and biogenic Secondary Organic Aerosols (a-SOA and b-SOA) and sources contributing to their formation. The global landscape was divided into eleven domains (North America (NAM); South America (SAM); Europe (EUR); North Africa and Middle East (NAF); Equatorial Africa (EAF); South of Africa (SAF); Russia and Central Asia (RUS); Eastern Asia (EAS); South Asia (SAS); Southeast Asia (SEA) and Australia (AUS)) and 46 cities based on varying populations. Three inventories for global emissions were considered: Community Emissions Data System, Model of Emission of Gases and Aerosol, and Global Fire Emissions Database. WRF-Chem model coupled with atmospheric reactions and the secondary organic aerosol model was employed for estimating PM_2.5, a-SOA, and b-SOA for 2018. No city attained WHO's annual PM_2.5 guideline of 5 μg/m³. Delhi, Dhaka, and Kolkata (63-92 μg/m³) in south Asia were the most polluted, and seven cities (mostly in EUR and NAM) met the WHO target IV (10 μg/m³). The highest SOA levels (2-9 μg/m³) were in the cities of SAS and Africa, but with a low SOA contribution to PM_2.5 (3-22%). However, the low levels of SOA (1-3 μg/m³) in EUR and NAM had a higher contribution of SOA to PM_2.5 (20-33%). b-SOA were consistent with the region's vegetation and forest cover. The SOA contribution was dominated by residential emissions in all domains (except in the NAF and AUS) (maximum in SAS). The non-coal industry was the second highest contributor (except in EAF, NAF, and AUS) and EUR had the maximum contribution from agriculture and transport. Globally, residential and industry (non-coal and coal) sectors showed the maximum contribution to SOA, with a-SOA and b-SOA being nearly equal. Ridding of biomass and residential burning of solid fuel is the single most action benefiting the PM_2.5 and SOA concerns.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere

Organic peroxides (POs) are organic molecules with one or more peroxide (-O-O-) functional groups. POs are commonly regarded as chemically labile termination products from gas-phase radical chemistry and therefore serve as temporary reservoirs for oxidative radicals (HOx and ROx) in the atmosphere. Owing to their ubiquity, active gas-particle partitioning behavior, and reactivity, POs are key reactive intermediates in atmospheric multiphase processes determining the life cycle (formation, growth, and aging), climate, and health impacts of aerosol. However, there remain substantial gaps in the origin, molecular diversity, and fate of POs due to their complex nature and dynamic behavior. Here, we summarize the current understanding on atmospheric POs, with a focus on their identification and quantification, state-of-the-art analytical developments, molecular-level formation mechanisms, multiphase chemical transformation pathways, as well as environmental and health impacts. We find that interactions with SO2 and transition metal ions are generally the fast PO transformation pathways in atmospheric liquid water, with lifetimes estimated to be minutes to hours, while hydrolysis is particularly important for α-substituted hydroperoxides. Meanwhile, photolysis and thermolysis are likely minor sinks for POs. These multiphase PO transformation pathways are distinctly different from their gas-phase fates, such as photolysis and reaction with OH radicals, which highlights the need to understand the multiphase partitioning of POs. By summarizing the current advances and remaining challenges for the investigation of POs, we propose future research priorities regarding their origin, fate, and impacts in the atmosphere.

Theoretical study on the formation of Criegee intermediates from ozonolysis of pentenal: An example of trans-2-pentenal

In this study, we investigated the reaction mechanism and kinetics of ozone with trans-2-pentenal using density functional theory (DFT) and conventional transition state theory (CTST). At 298 K and 1 atm, the gas-phase reaction mechanisms and kinetic parameters were calculated at the level of CCSD(T)/6-311+G(d,p)//M06-2X/6-311+G(d,p). Both CC and CO bond cycloaddition as well as hydrogen abstraction were found. The calculations indicated that the main reaction path is 1,3-dipole cycloaddition reactions of ozone with CC bond with the relatively lower syn-energy-barrier of 3.35 kcal mol^-1 to form primary ozonide which decomposed to produce a carbonyl oxide called a Criegee intermediate (CI) and an aldehyde. The subsequent reactions of CIs were analysed in detail. It is found that the reaction pathways of the novelty CIs containing an aldehyde group are extremely similar with general CIs when they react with NO, NO₂, SO₂, H₂O, CH₂O and O₂. The condensed Fukui function were calculated to identify the active site of the chosen molecules. At 298 K and 1 atm, the reaction rate coefficient was 9.13 × 10^-18 cm³ molecule^-1 s^-1 with atmospheric lifetime of 1.3 days. The calculated rate constant is in general agreement with the available experimental data. The branching ratios indicated that syn-addition pathways are prior to anti-addition. The atmospheric ratios for CIs formation and the bimolecular reaction rate constants for the Criegee intermediates with the variety of partners were calculated. Our theoretical results are of importance in atmospheric chemistry of unsaturated aldehyde oxidation by ozone.

Dramatic decrease of secondary organic aerosol formation potential in Beijing: Important contribution from reduction of coal combustion emission

Secondary organic aerosol (SOA) formation originating from the emission of anthropogenic volatile organic compounds (VOCs) makes a significant contribution to fine particulate matter (PM_2.5) pollution in urban areas. Investigation on the SOA formation potential (SOAFP) can help us understand the contribution of different sources to SOA formation. To characterize the SOAFP of ambient air from anthropogenic VOCs in the urban area of Beijing, field observation was implemented using a twin oxidation flow reactor (Twin-OFRs) system in the winters of 2016 and 2017. Compared to the winter of 2016, the seasonal-average SOAFP in the winter of 2017 was found to decrease by about 74% (18.6 to 4.9 μg/m³), which is more than that of PM₁ (59%, 48.7 to 20.2 μg/m³), PM_2.5 (61%, 114.4 to 44.8 μg/m³) and CO (57%, 2.1 to 0.9 mg/m³) that mainly comes from the combustion of fossil fuels, suggesting complex affecting factors on SOAFP. The results of wind decomposition mathematical modeling showed that anthropogenic factors and favorable meteorological conditions both contributed significantly to the decrease in SOAFP. The reduction of emissions from scatter coal combustion, which is the key VOCs source for SOAFP, is probably the most important anthropogenic factor affecting SOAFP. In the winter of 2016, the ratio of benzene to toluene is 1.45 that was close to 1.54 representing coal combustion emission; however, it decreased dramatically to 1.05 in the winter of 2017, suggesting considerable reduction of VOC emissions from scatter coal combustion in the latter year due to the coal-to-gas transition in Beijing and surrounding regions. The SOAFP measured in this study considers all ambient VOCs that can react with OH radical, providing another representative method for estimating it. These results could be beneficial to understanding the factors driving SOAFP and its contribution to PM_2.5, especially in regions with high-intensity anthropogenic emissions. Synopsis: This study reported the sharp decline of secondary organic aerosol formation potential (SOAFP) between two consecutive winters in Beijing and analyzed the reasons.

Relative Humidity Changes the Role of SO2 in Biogenic Secondary Organic Aerosol Formation

SO₂ influences secondary organic aerosol (SOA) and organosulfates (OSs) formation but mechanisms remain elusive. This study focuses on this topic by investigating biogenic γ-terpinene ozonolysis under various SO₂ and relative humidity (RH) conditions. With a constant SO₂ concentration (∼110 ppb), the increase in RH transformed SO₂ sinks from stabilized Criegee intermediates (sCIs) to peroxides in aerosol particles. The associated changes in particle acidity and liquid water content may collectively first lead to decreased and then increased SOA yield with increasing RH, with the turning point appearing at ∼30% RH. The abundance of most OSs formed under 45% RH was more than 5 times higher than that of OSs formed under 10% RH, possibly due to interactions of dissolved SO₂ with hydroperoxides (ROOH) in SOA. ROOHs formed from the autoxidation processes of alkylperoxy radicals were proposed to be precursors for highly oxidized OSs (HOOSs) that decreased SOA volatility and showed a certain abundance in ambient aerosols. This study highlights that high RH potentially enhances the contribution of SO₂ to OSs formation, and particularly, HOOSs formation during monoterpene ozonolysis in the atmosphere.

Effect of relative humidity on SOA formation from aromatic hydrocarbons: Implications from the evolution of gas- and particle-phase species

Relative humidity (RH) plays a significant role in secondary organic aerosol (SOA) formation, but the mechanisms remain uncertain. Using a 30 m³ indoor smog chamber, the influences of RH on SOA formation from two conventional anthropogenic aromatics (toluene and m-xylene) were investigated from the perspective of both the gas- and particle- phases based on the analysis of multi-generation gas-phase products and the chemical composition of SOA, which clearly distinguishes from many previous works mainly focused on the particle-phase. Compared to experiments with RH of 2.0%, SOA yields increased by 11.1%-133.4% and 4.0%-64.5% with higher RH (30.0%-90.0%) for toluene and m-xylene, respectively. The maximum SOA concentration always appeared at 50.0% RH, which is consistent with the change trend of SOA concentration with RH in the summertime field observation. The most plausible reason is that the highest gas-phase OH concentration was observed at 50.0% RH, when the increases in gas-phase OH formation and OH uptake to aerosols and chamber walls with increasing RH reached a balance. The maximum OH concentration was accompanied by a notable decay of second-generation products and formation of third-generation products at 50.0% RH. With further increasing RH, more second-generation products with insufficient oxidation degree will be partitioned into the aerosol phase, and the aqueous-phase oxidation process will also be promoted due to the enhanced uptake of OH. These processes concurrently caused the O/C and oxidation state of carbon (OSc) to first increase and then slightly decrease. This work revealed the complex influence of RH on SOA formation from aromatic VOCs through affecting the OH concentration, partitioning of advanced gas-phase oxidation products as well as aqueous-phase oxidation processes. Quantitative studies to elucidate the role of RH in the partitioning of oxidation products should be conducted to further clarify the mechanism of the influence of RH on SOA formation.

Anthropogenic-Biogenic Interactions at Night: Enhanced Formation of Secondary Aerosols and Particulate Nitrogen- and Sulfur-Containing Organics from β-Pinene Oxidation

Mixing of anthropogenic gaseous pollutants and biogenic volatile organic compounds impacts the formation of secondary aerosols, but still in an unclear manner. The present study explores secondary aerosol formation via the interactions between β-pinene, O₃, NO₂, SO₂, and NH₃ under dark conditions. Results showed that aerosol yield can be largely enhanced by more than 330% by NO₂ or SO₂ but slightly enhanced by NH₃ by 39% when the ratio of inorganic gases to β-pinene ranged from 0 to 1.3. Joint effects of NO₂ and SO₂ and SO₂ and NH₃ existed as aerosol yields increased with NO₂ but decreased with NH₃ when SO₂ was kept constant. Infrared spectra showed nitrogen-containing aerosol components derived from NO₂ and NH₃ and sulfur-containing species derived from SO₂. Several particulate organic nitrates (MW 215, 229, 231, 245), organosulfates (MW 250, 264, 280, 282, 284), and nitrooxy organosulfates (MW 295, 311, 325, 327, and 343) were identified using high-resolution orbitrap mass spectrometry in NO₂ and SO₂ experiments, and their formation mechanism is discussed. Most of these nitrogen- and sulfur-containing species have been reported in ambient particles. Our results suggest that the complex interactions among β-pinene, O₃, NO₂, SO₂, and NH₃ during the night might serve as a potential pathway for the formation of particulate nitrogen- and sulfur-containing organics, especially in polluted regions with both anthropogenic and biogenic influences.

Quantitative relationship between the structures and properties of VOCs and SOA formation on the surfaces of acidic aerosol particles

In this research, all the efforts, based on a series of molecular dynamics simulations on the interfacial process between VOC-contaminated air and acidic sulfate, were made to find how the structures and properties of VOCs are related to the formation of SOAs. The experimental fractional aerosol coefficients (FACs) were used to quantify the SOA formation and 14 VOC species were chosen based on the atmosphere inventory and the FAC magnitude. Finally, the quantitative relationship (QR) was found through the FAC as a function of the two variables the total valid interactions (Tg) and the diffusion coefficient (D), with R square 0.94. Meanwhile, the effect of water was explored and the QR was proved to be rational and reliable. The QR not only explained the SOA formation capacity of VOCs, but could also predict the SOA formation of new molecules.

Impacts of Mixed Gaseous and Particulate Pollutants on Secondary Particle Formation during Ozonolysis of Butyl Vinyl Ether

To clarify how coexisting atmospheric pollutants affect secondary organic aerosol (SOA) formation, we investigated the effects of mixed gaseous pollutants (CO and SO₂) and mixed organic-inorganic (MOI) particles on SOA formation during n-butyl vinyl ether (BVE) ozonolysis. Higher CO levels (90 ppm) were found to significantly change the chemical composition of SOA (prompting monomers while reducing oligomer formation) without causing much change in the overall SOA mass. Based on the positive matrix factorization (PMF) analysis, heterogeneous chemical conversions between preformed and newly formed SOA were the major pathways of SOA formation in the presence of MOI particles. Furthermore, MOI particles had an enhancing effect on SOA formation at 1% relative humidity (RH) but a negligible effect at higher RH (10 and 55%). The enhancing effect was attributed to the formation of multifunctional products resulting from high functionalization of preformed and newly formed SOA. The negligible effect observed was ascribed to the cleavage of unstable oligomers as a result of the reversible oligomerization of preformed and newly formed SOA. Even so, MOI particles could still affect the composition of newly formed SOA. These results highlight the need to account for the significant effect of mixed gaseous and particulate pollutants on both SOA constituents and their evolution.