Heterogeneous reactions of methylglyoxal in acidic media: implications for secondary organic aerosol formation

The role of aldehydes on sulfur based-new particle formation: a theoretical study

Aldehydes play a crucial role in the formation of atmospheric particles, attracting significant attention due to their environmental impact. However, the microscopic mechanisms underlying the formation of aldehyde-involved particles remain uncertain. In this study, through quantum chemical calculations and molecular dynamics (MD) simulations, we investigate the microscopic formation mechanisms of binary and ternary systems composed of three representative aldehydes, two sulfur-based acids, water, and two bases. Our research findings reveal that the most stable structures of acid-aldehyde clusters involve the connection of acids and aldehyde compounds through hydrogen bonds without involving proton transfer reactions, indicating relatively poor cluster stability. However, with the introduction of a third component, the stability of 18 clusters significantly increase. Among these, in ten systems, acids act as catalysts, facilitating reactions between aldehyde compounds and water or alkaline substances to generate glycols and amino alcohols. However, according to MD simulations conducted at 300 K, these acids readily dissociate from the resulting products. In the remaining eight systems, the most stable structural feature involves ion pairs formed by proton transfer reactions between acids and aldehyde compounds. These clusters exhibit remarkable thermodynamic stability. Furthermore, the acidity of the acid, the nature of nucleophilic agents, and the type of aldehyde all play significant roles in cluster stability and reactivity, and they have synergistic effects on the nucleation process. This study offers microscopic insights into the processes of new particle formation involving aldehydes, contributing to a deeper understanding of atmospheric chemistry at the molecular level.

Oligomerization Mechanism of Methylglyoxal Regulated by the Methyl Groups in Reduced Nitrogen Species: Implications for Brown Carbon Formation

Uncertain chemical mechanisms leading to brown carbon (BrC) formation affect the drivers of the radiative effects of aerosols in current climate predictions. Herein, the aqueous-phase reactions of methylglyoxal (MG) and typical reduced nitrogen species (RNSs) are systematically investigated by using combined quantum chemical calculations and laboratory experiments. Imines and diimines are identified from the mixtures of methylamine (MA) and ammonia (AM) with MG, but not from dimethylamine (DA) with the MG mixture under acidic conditions, because deprotonation of DA cationic intermediates is hindered by the amino groups occupied by two methyl groups. It leads to N-heterocycle (NHC) formation in the MG + MA (MGM) and MG + AM (MGA) reaction systems but to N-containing chain oligomer formation in the MG + DA (MGD) reaction system. Distinct product formation is attributed to electrostatic attraction and steric hindrance, which are regulated by the methyl groups of RNSs. The light absorption and adverse effects of NHCs are also strongly related to the methyl groups of RNSs. Our finding reveals that BrC formation is mainly contributed from MG reaction with RNSs with less methyl groups, which have more abundant and broad sources in the urban environments.

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.

Response of aerosol composition to the clean air actions in Baoji city of Fen-Wei River Basin

The implementation of air pollution control measures could alter the compositions of submicron aerosols. Identifying the changes can evaluate the atmospheric responses of the implemented control measures and provide more scientific basis for the formulation of new measures. The Fen-Wei River Basin is the most air polluted region in China, and thereby is a key area for the reduction of emissions. Only limited studies determine the changes in the chemical compositions of submicron aerosols. In this study, Baoji was selected as a representative city in the Fen-Wei River Basin. The compositions of submicron aerosols were determined between 2014 and 2019. Organic fractions were determined through an online instrument (Quadrupole Aerosol Chemical Speciation Monitor, Q-ACSM) and source recognition was performed by the Multilinear Engine (ME-2). The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was also employed to evaluate the contributions of emissions reduction and meteorological conditions to the changes of submicron aerosol compositions. The results indicate that the mass concentrations of submicron aerosols have been substantially decreased after implementation of air pollution control measures. This was mainly attributed to the emission reductions of sulfur dioxide (SO₂) and primary organic aerosol (POA). In addition, the main components that drove the pollution episodes swapped from POA, sulfate, nitrate and less-oxidized organic (LO-OOA) in 2014 to nitrate and more-oxidized OOA (MO-OOA) in 2019. Due to the changes of chemical compositions of both precursors and secondary pollutants, the pollution control measures should be modernized to focus on the emissions of ammonia (NH₃), nitrogen oxides (NO_x) and volatile organic compounds (VOC_s) in this region.

Heterogeneous HONO formation deteriorates the wintertime particulate pollution in the Guanzhong Basin, China

Despite implementation of strict emission mitigation measures since 2013, heavy haze with high levels of secondary aerosols still frequently engulfs the Guanzhong Basin (GZB), China, during wintertime, remarkably impairing visibility and potentially causing severe health issues. Although the observed low ozone (O₃) concentrations do not facilitate the photochemical formation of secondary aerosols, the measured high nitrous acid (HONO) level provides an alternate pathway in the GZB. The impact of heterogeneous HONO sources on the wintertime particulate pollution and atmospheric oxidizing capability (AOC) is evaluated in the GZB. Simulations by the Weather Research and Forecast model coupled with Chemistry (WRF-Chem) reveal that the observed high levels of nitrate and secondary organic aerosols (SOA) are reproduced when both homogeneous and heterogeneous HONO sources are considered. The heterogeneous sources (HET-sources) contribute about 98% of the near-surface HONO concentration in the GZB, increasing the hydroxyl radical (OH) and O₃ concentration by 39.4% and 22.0%, respectively. The average contribution of the HET-sources to SOA, nitrate, ammonium, and sulfate in the GZB is 35.6%, 20.6%, 12.1%, and 6.0% during the particulate pollution episode, respectively, enhancing the mass concentration of fine particulate matters (PM_2.5) by around 12.2%. Our results suggest that decreasing HONO level or the AOC becomes an effective pathway to alleviate the wintertime particulate pollution in the GZB.

A review of secondary organic aerosols formation focusing on organosulfates and organic nitrates

Secondary organic aerosols (SOA) are crucial constitution of fine particulate matter (PM), which are mainly derived from photochemical oxidation products of primary organic matter and volatile organic compounds (VOCs), and can induce terrible impacts to human health, air quality and climate change. As we know, organosulfates (OSs) and organic nitrates (ON) are important contributors for SOA formation, which could be possibly produced through various pathways, resulting in extremely complex formation mechanism of SOA. Although plenty of research has been focused on the origins, spatial distribution and formation mechanisms of SOA, a comprehensive and systematic understanding of SOA formation in the atmosphere remains to be detailed explored, especially the most important OSs and ON dedications. Thus, in this review, we systematically summarize the recent research about origins and formation mechanisms of OSs and ON, and especially focus on their contribution to SOA, so as to have a clearer understanding of the origin, spatial distribution and formation principle of SOA. Importantly, we interpret the complex interaction with coexistence effect of SO_x and NO_x on SOA formation, and emphasize the future insights for SOA research to expect a more comprehensive theory and practice to alleviate SOA burden.

Secondary organic aerosol formation from monocyclic aromatic hydrocarbons: insights from laboratory studies

Monocyclic aromatic hydrocarbons (MAHs) are key anthropogenic pollutants and often dominate the volatile organic compound emissions and secondary organic aerosol (SOA) formation especially in the urban atmosphere. To evaluate the environmental impacts of SOA formed from the oxidation of MAHs (aromatic SOA), it is of great importance to elucidate their chemical composition, formation mechanism, and physicochemical properties under various atmospheric conditions. Here we seek to compile a common framework for the current studies on aromatic SOA formation and summarize the knowledge on what has been primarily learned from laboratory studies. This review begins with a brief summary of MAHs' emission characteristics, followed by an overview of atmospheric degradation mechanisms for MAHs as well as gas- and particle-phase reactions involving aromatic SOA formation. SOA formation processes highlighted in this review are complex and depend highly on environmental conditions, posing a substantial challenge for theoretical description of aromatic SOA formation. Therefore, the following issues are further discussed in detail: the response of gas-phase chemistry and aromatic SOA mass yield as well as composition to NO_x levels, particle-phase reactions and molecular characterization of aromatic SOA in the presence of acidic sulfate, and physicochemical processes of SOA formation involving gas- or particle-phase water. Building on this current understanding, available experimental studies on the effects of environmental conditions were explored. A brief description of the atmospheric importance of aromatic SOA including their optical properties and health influences is also presented. Finally, we highlight the current challenges in laboratory studies and outline directions for future aromatic SOA research.

Nitrous acid emission from soil bacteria and related environmental effect over the North China Plain

Soil bacteria could be one of the important sources for ambient HONO. However, the HONO emission from soil bacteria over North China Plain (NCP) with vast croplands has not yet been evaluated. In this study, high-resolution simulations are created to explore the HONO emission from soil bacteria over NCP and related influences on atmospheric chemistry. Ground measurements of critical air pollutants including O₃, HONO, and PM_2.5 compositions are incorporated to constrain the model simulations. Results show that abundant HONO is emitted from soil bacteria over NCP during summertime and the emission rate varies dramatically for different areas (about 0.2 kg km^-2 d^-1 - 2.0 kg km^-2 d^-1). The HONO emission rate presents clear diurnal cycles with peaks of 1.5 kg km^-2 d^-1 in the afternoon and valleys of 0.4 kg km^-2 d^-1 during the early morning hours. The resulting HONO concentration ranges from 0.2 μg m^-3 to 1.4 μg m^-3, which predominates the total HONO concentration in ambient air, particularly in western NCP. The soil bacteria source can significantly alter the diurnal cycles of ambient HONO and OH concentrations over NCP, but only slightly change O₃ and PM_2.5 concentrations via participating photochemistry and secondary aerosol formations. These results highlight the pressing need for the involvement of HONO emission from soil bacteria in modeling studies regarding atmospheric chemistry, particularly in rural areas.

Regional modeling of secondary organic aerosol formation over eastern China: The impact of uptake coefficients of dicarbonyls and semivolatile process of primary organic aerosol

Capturing the secondary organic aerosol (SOA) concentration using the chemical transport model is difficult due to a large knowledge gap of its formation mechanism. Previous studies demonstrated the uptake of dicarbonyls and semivolatile process of primary organic aerosol (POA) emissions are the significant sources of SOA. However, the uptake coefficients of dicarbonyls have large uncertainties and the SOA from the semivolatile process of POA emission remains unclear. We applied the revised reactive uptake parameterization, with "salting effects" for dicarbonyls, and updated approaches for POA to the Community Multiscale Air Quality Modeling System (CMAQ) simulations for October 2014 to study their impacts on modeling the SOA formation over eastern China. We introduce a method of quantifying crystalized or deliquescent aerosols to further improve the parameterization. The revised glyoxal uptake coefficients results in higher glyoxal SOA in the Beijing-Tianjin-Hebei region, where is typically under low relative humidity (RH) and high aerosol pH conditions. It gives lower glyoxal SOA in the Pearl River Delta region, where is typically under high RH and low pH conditions. The updated parameterization gives negligible methylglyoxal SOA due to the low uptake coefficients. The implementation of semivolatile process of POA and the approach for potential SOA from combustion sources will largely decrease the predicted POA and increase the modeled SOA concentrations over eastern China. The increased SOA from POA emissions could improve the model performance for organic carbon and SOA. It slightly improves the performance in PM_2.5 modeling by compensating the reduction of modeled POA. This study indicates the mixed impact of a parameterization considering "salting effects" on modeling the dicarbonyls SOA in key regions of eastern China. It also demonstrates the improved performance by implementing the POA approaches in aerosol modeling using CMAQ. Meanwhile, the uncertainty in the revised reactive uptake parameterization and POA approaches is discussed.

Increasing atmospheric oxidizing capacity weakens emission mitigation effort in Beijing during autumn haze events

Although strict mitigation measures have been implemented since 2013 in Beijing-Tianjin-Hebei (BTH), China, air pollution still frequently occurs. Observations reveal that during pollution episodes in autumn, fine particulate matter (PM_2.5) concentrations have not decreased, and particularly, ozone (O₃) concentrations have increased remarkably from 2013 to 2015 in Beijing. Additionally, a concurrence of O₃ and particulate pollution with high secondary aerosol contributions has been observed frequently, indicating high atmospheric oxidizing capacity (AOC) during particulate pollution. The WRF-Chem model simulations show elevated O₃ concentrations and high fractions of oxygenated secondary aerosols (OSA) in PM_2.5 (0.53-0.73) during the severe pollution period. During daytime there exhibits an AOC-sufficient regime with the persistently high OSA fraction and an AOC-deficient regime with varied OSA fractions, separated by the O₃ level of 80 μg m^-3. Our results suggest that increasing AOC can considerably weaken the emission mitigation effort by enhancing the secondary aerosol formation.

Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueous-phase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information. Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools.

Multigeneration Production of Secondary Organic Aerosol from Toluene Photooxidation

Photooxidation of volatile organic compounds (VOCs) produces secondary organic aerosol (SOA) and light-absorbing brown carbon (BrC) via multiple reaction steps/pathways, reflecting significant chemical complexity relevant to gaseous oxidation and subsequent gas-to-particle conversion. Toluene is an important VOC under urban conditions, but the fundamental chemical mechanism leading to SOA formation remains uncertain. Here, we elucidate multigeneration SOA production from toluene by simultaneously tracking the evolutions of gas-phase oxidation and aerosol formation in a reaction chamber. Large size increase and browning of monodisperse sub-micrometer seed particles occur shortly after initiating oxidation by hydroxyl radical (OH) at 10-90% relative humidity (RH). The evolution in gaseous products and aerosol properties (size/density/optical properties) and chemical speciation of aerosol-phase products indicate that the aerosol growth and browning result from earlier generation products consisting dominantly of dicarbonyl and carboxylic functional groups. While volatile dicarbonyls engage in aqueous reactions to yield nonvolatile oligomers and light-absorbing nitrogen heterocycles/heterochains (in the presence of NH₃) at high RH, organic acids contribute to aerosol carboxylates via ionic dissociation or acid-base reaction in a wide RH range. We conclude that toluene contributes importantly to SOA/BrC formation from dicarbonyls and organic acids because of their prompt and high yields from photooxidation and unique functionalities for participation in aerosol-phase reactions.

Synergistic effect of glutaric acid and ammonia/amine/amide on their hydrates in the clustering: A theoretical study

The formation of molecular clusters makes influence on the atmosphere. The clusters of glutaric acid (GA) and common ammonia (A), amine (methylamine MA, dimethylamine DMA) and representative amide (urea U) along with water molecule were systematically studied theoretically. GA-A-nW (n = 1, 2), GA-MA-nW (n = 1, 2), GA-DMA-1W and GA-U-nW (n = 1-6) are predicted to be feasible thermodynamically with the hydrogen bonds as interaction force. GA and urea promote the clustering synergistically, and ammonia, methylamine, dimethylamine promote the clustering of small GA hydrates (n = 1-2), while inhibit that of large GA hydrates (n = 3-6). The results of humidity show that un-hydrate or mono-hydrate is the main form of GA-mbase-nW (m = 0, 1; n = 1-6) under relative humidity of 20%, 50% and 80%. The global minima remain dominant over the temperature range of 220-320 K. GA contributes more to the Rayleigh scattering properties than sulfuric acid. More importantly, the local minima can undergo isomerization to form the global minima crossing a free energy barrier ranging from 6.66 to 11.78 kcal mol^-1. This study indicates that GA and base molecules play a synergistic role to promote the formation of clusters. We hope it can provide more insights on interesting clustering in theory.

Mitigating NOX emissions does not help alleviate wintertime particulate pollution in Beijing-Tianjin-Hebei, China

Stringent mitigation measures have reduced wintertime fine particulate matter (PM_2.5) concentrations by 42.2% from 2013 to 2018 in the Beijing-Tianjin-Hebei (BTH) region, but severe PM pollution still frequently engulfs the region. The observed nitrate aerosols have not exhibited a significant decreasing trend and constituted a major fraction (about 20%) of the total PM_2.5, although the surface-measured NO₂ concentration has decreased by over 20%. The contributions of nitrogen oxides (NO_X) emissions mitigation to the nitrate and PM_2.5 concentrations and how to alleviate nitrate aerosols efficiently under the current situation still remains elusive. The WRF-Chem model simulations of a persistent and heavy PM pollution episode in January 2019 in the BTH reveal that NO_X emissions mitigation does not help lower wintertime nitrate and PM_2.5 concentrations under current conditions in the BTH. A 50% reduction in NO_X emissions only decreases nitrate mass by 10.3% but increases PM_2.5 concentrations by 3.2%, because the substantial O₃ increase induced by NO_X mitigation offsets the HNO₃ loss and enhances sulfate and secondary organic aerosols formation. Our results are further consolidated by the occurrence of severe PM pollution in the BTH during the COVID-19 outbreak, with a significant reduction in NO₂ concentration. Mitigation of NH₃ emissions constitutes the priority measure to effectively lower the nitrate and PM_2.5 concentrations in the BTH under current conditions, with 35.5% and 12.7% decrease, respectively, when NH₃ emissions are reduced by 50%.

Unexpected Oligomerization of Small α-Dicarbonyls for Secondary Organic Aerosol and Brown Carbon Formation

Large amounts of small α-dicarbonyls (glyoxal and methylglyoxal) are produced in the atmosphere from photochemical oxidation of biogenic isoprene and anthropogenic aromatics, but the fundamental mechanisms leading to secondary organic aerosol (SOA) and brown carbon (BrC) formation remain elusive. Methylglyoxal is commonly believed to be less reactive than glyoxal because of unreactive methyl substitution, and available laboratory measurements showed negligible aerosol growth from methylglyoxal. Herein, we present experimental results to demonstrate striking oligomerization of small α-dicarbonyls leading to SOA and BrC formation on sub-micrometer aerosols. Significantly more efficient growth and browning of aerosols occur upon exposure to methylglyoxal than glyoxal under atmospherically relevant concentrations and in the absence/presence of gas-phase ammonia and formaldehyde, and nonvolatile oligomers and light-absorbing nitrogen-heterocycles are identified as the dominant particle-phase products. The distinct aerosol growth and light absorption are attributed to carbenium ion-mediated nucleophilic addition, interfacial electric field-induced attraction, and synergetic oligomerization involving organic/inorganic species, leading to surface- or volume-limited reactions that are dependent on the reactivity and gaseous concentrations. Our findings resolve an outstanding discrepancy concerning the multiphase chemistry of small α-dicarbonyls and unravel a new avenue for SOA and BrC formation from atmospherically abundant, ubiquitous carbonyls and ammonia/ammonium sulfate.

Atmospheric Chemistry of Allylic Radicals from Isoprene: A Successive Cyclization-Driven Autoxidation Mechanism

The atmospheric chemistry of isoprene has broad implications for regional air quality and the global climate. Allylic radicals, taking 13-17% yield in the isoprene oxidation by ^•Cl, can contribute as much as 3.6-4.9% to all possible formed intermediates in local regions at daytime. Considering the large quantity of isoprene emission, the chemistry of the allylic radicals is therefore highly desirable. Here, we investigated the atmospheric oxidation mechanism of the allylic radicals using quantum chemical calculations and kinetics modeling. The results indicate that the allylic radicals can barrierlessly combine with O₂ to form peroxy radicals (RO₂^•). Under ≤100 ppt NO and ≤50 ppt HO₂^• conditions, the formed RO₂^• mainly undergo two times "successive cyclization and O₂ addition" to finally form the product fragments 2-alkoxy-acetaldehyde (C₂H₃O₂^•) and 3-hydroperoxy-2-oxopropanal (C₃H₄O₄). The presented reaction illustrates a novel successive cyclization-driven autoxidation mechanism. The formed 3-hydroperoxy-2-oxopropanal product is a new isomer of the atmospheric C₃H₄O₄ family and a potential aqueous-phase secondary organic aerosol precursor. Under >100 ppt NO condition, NO can mediate the cyclization-driven autoxidation process to form C₅H₇NO₃, C₅H₇NO₇, and alkoxy radical-related products. The proposed novel autoxidation mechanism advances our current understanding of the atmospheric chemistry of both isoprene and RO₂^•.

Explosive Secondary Aerosol Formation during Severe Haze in the North China Plain

Severe haze events with exceedingly high-levels of fine aerosols occur frequently over the past decades in the North China Plain (NCP), exerting profound impacts on human health, weather, and climate. The development of effective mitigation policies requires a comprehensive understanding of the haze formation mechanisms, including identification and quantification of the sources, formation, and transformation of the aerosol species. Haze evolution in this region exhibits distinct physical and chemical characteristics from clean to polluted periods, as evident from increasing stagnation and relative humidity, but decreasing solar radiation as well as explosive secondary aerosol formation. The latter is attributed to highly elevated concentrations of aerosol precursor gases and is reflected by rapid increases in the particle number and mass concentrations, both corresponding to nonequilibrium chemical processes. Considerable new knowledge has been acquired to understand the processes regulating haze formation, particularly in light of the progress in elucidating the aerosol formation mechanisms. This review synthesizes recent advances in understanding secondary aerosol formation, by highlighting several critical chemical/physical processes, that is, new particle formation and aerosol growth driven by photochemistry and aqueous chemistry as well as the interaction between aerosols and atmospheric stability. Current challenges and future research priorities are also discussed.

New particle formation (NPF) events in China urban clusters given by sever composite pollution background

New Particle Formation (NPF) refers to transformation of gaseous precursors in the atmosphere due to nucleation and subsequent growth process through physicochemical interaction. It has generated a lot of interest due to its profound impact on global and regional environment, climate and human health. We reviewed the studies on NPF in three city clusters of China: the North China Plain, the Yangtze River Delta and the Pearl River Delta obtained through experiment simulations (e.g., chamber simulation, flow-tube simulation, etc.), field observations, and numerical simulations. Due to its atmospheric background pollution and strong oxidation capacities resulting in high source rate of precursors, China's atmosphere possesses challenges different from those evaluated in previous studies on cleaning sites and other developing countries. Hence, NPF events can simultaneously exhibit high condensable sink, formation rate and growth rate. In addition, the high intensity of anthropogenic emissions in urban China has led to greater diversity of pollutant species involved in NPF nucleation and subsequent growth, compared to the dominant role of biogenic precursors at cleaning sites. Differences in geographical location and industrial structure also lead to significant distinctions in NPF characteristics of the three city clusters. Consequently, the lack of understanding of nucleation mechanism of complexly polluted background sites makes the global and regional climate models with submodels based on clean background have enormous uncertainty when applied to urban China. The establishment of a mature research ecosystem including field observations, laboratory simulations and numerical simulations is the key to the breakthrough of NPF research in China.

Introductory lecture: air quality in megacities

Urbanization is an ongoing global phenomenon as more and more people are moving from rural to urban areas for better employment opportunities and a higher standard of living, leading to the growth of megacities, broadly defined as urban agglomeration with more than 10 million inhabitants. Intense activities in megacities induce high levels of air pollutants in the atmosphere that harm human health, cause regional haze and acid deposition, damage crops, influence air quality in regions far from the megacity sources, and contribute to climate change. Since the Great London Smog and the first recognized episode of Los Angeles photochemical smog seventy years ago, substantial progress has been made in improving the scientific understanding of air pollution and in developing emissions reduction technologies. However, much remains to be understood about the complex processes of atmospheric oxidation mechanisms; the formation and evolution of secondary particles, especially those containing organic species; and the influence of emerging emissions sources and changing climate on air quality and health. While air quality has substantially improved in megacities in developed regions and some in the developing regions, many still suffer from severe air pollution. Strong regional and international collaboration in data collection and assessment will be beneficial in strengthening the capacity. This article provides an overview of the sources of emissions in megacities, atmospheric physicochemical processes, air quality trends and management in a few megacities, and the impacts on health and climate. The challenges and opportunities facing megacities due to lockdown during the COVID-19 pandemic is also discussed.

Increasing wintertime ozone levels and secondary aerosol formation in the Guanzhong basin, central China

The observed near-surface ozone (O₃) concentration has been remarkably increasing during recent years in winter in the Guanzhong basin, central China, showing a continuous enhancement of the atmospheric oxidizing capacity (AOC). The impact of such a change in the AOC on secondary aerosol formation, however, has not yet been assessed. In this study, we simulate the formation of O₃ and airborne particles in the atmosphere using the WRF-Chem model, in which the AOC is calculated quantitatively, to understand the responses of secondary aerosols to the AOC increase. Meteorological observations, air pollutants including O₃, NO₂, SO₂, CO, and PM_2.5 concentrations at ambient monitoring sites, and the main compositions of submicron particulates measured using ACSM are used to constrain the model simulation. The model result shows that the population hourly and postmeridian O_x (=O₃ + NO₂) concentrations are good indicators for the wintertime AOC in the basin, suggested by the significantly positive correlations between them. Sensitivity experiments present that the AOC changes may exert important influences on fine particle (PM_2.5) concentration with an average rate of 1.94 (μg m^-3)/(10⁶ cm^-3 s^-1) for Δ(PM_2.5)/Δ(AOC), which is mostly caused by the mass changes in secondary organic aerosol (43%) and nitrate aerosol (40%) and less attributed to the ammonium (11%) and sulfate (6%) components.

Effects of NOx and SO2 on the secondary organic aerosol formation from the photooxidation of 1,3,5-trimethylbenzene: A new source of organosulfates

1,3,5-Trimethylbeneze (TMB) is an important constituent of anthropogenic volatile organic compounds that contributes to the formation of secondary organic aerosol (SOA). A series of chamber experiments were performed to probe the effects of NO_x and SO₂ on SOA formation from TMB photooxidation. The molecular composition of TMB SOA was investigated by ultra-high performance liquid chromatography/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS). We found that the SOA yield increases notably with elevated NO_x concentrations under low-NO_x condition ([TMB]₀/[NO_x]₀ > 10 ppbC ppb^-1), while an opposite trend is observed in high-NO_x experiments ([TMB]₀/[NO_x]₀ < 10 ppbC ppb^-1). The increase in SOA yield in low-NO_x regime is attributed to the increase of NO_x-induced OH concentrations. The formation of low-volatility species might be suppressed, thereby leading to a lower SOA yield in high-NO_x conditions. Moreover, SOA formation was promoted in experiment with SO₂ addition. Multifunctional products containing carbonyl, acid, alcohol, and nitrate functional groups were characterized in TMB/NO_x photooxidation, whereas several organosulfates (OSs) and nitrooxy organosulfates were identified in TMB/NO_x/SO₂ photooxidation based on HR-Q-TOFMS analysis. The formation mechanism relevant to the detected compounds in SOA were proposed. Based on our measurements, the photooxidation of TMB in the presence of SO₂ may be a new source of OSs in the atmosphere. The results presented here also deepen the understanding of SOA formation under relatively complex polluted environments.

Carbenium ion-mediated oligomerization of methylglyoxal for secondary organic aerosol formation

Secondary organic aerosol (SOA) from photochemical oxidation of volatile organic compounds represents one of the most dominant constituents of fine particulate matter in the troposphere, with profound implications for air quality and climate. However, the fundamental chemical mechanisms leading to SOA formation remain highly uncertain. Here, we show oligomer formation from methylglyoxal with the carbenium ions as the key intermediate using quantum chemical calculations. This cationic oligomerization is demonstrated to proceed via barrierless pathways and occurs at fast rates on weakly acidic aqueous aerosols and/or cloud droplets under typical tropospheric conditions. In contrast to a previously proposed hydration mechanism, out results reveal that the carbenium ion-mediated oligomerization of methylglyoxal provides a major SOA source from anthropogenic and biogenic emissions.

Secondary organic aerosol (SOA) represents a major constituent of tropospheric fine particulate matter, with profound implications for human health and climate. However, the chemical mechanisms leading to SOA formation remain uncertain, and atmospheric models consistently underpredict the global SOA budget. Small α-dicarbonyls, such as methylglyoxal, are ubiquitous in the atmosphere because of their significant production from photooxidation of aromatic hydrocarbons from traffic and industrial sources as well as from biogenic isoprene. Current experimental and theoretical results on the roles of methylglyoxal in SOA formation are conflicting. Using quantum chemical calculations, we show cationic oligomerization of methylglyoxal in aqueous media. Initial protonation and hydration of methylglyoxal lead to formation of diols/tetrol, and subsequent protonation and dehydration of diols/tetrol yield carbenium ions, which represent the key intermediates for formation and propagation of oligomerization. On the other hand, our results reveal that the previously proposed oligomerization via hydration for methylglyoxal is kinetically and thermodynamically implausible. The carbenium ion-mediated mechanism occurs barrierlessly on weakly acidic aerosols and cloud/fog droplets and likely provides a key pathway for SOA formation from biogenic and anthropogenic emissions.

Adsorption and isomerization of glyoxal and methylglyoxal at the air/hydroxylated silica surface

We present results from molecular dynamics simulations coupled with enhanced sampling techniques on the adsorption and isomerization of glyoxal (GL) and methylglyoxal (MG) at the air/hydroxylated silica (α-Quartz) interface. GL and MG are two organic compounds present in the atmosphere as oxidation products of both biogenic and anthropogenic precursors. By adsorption and hydration on liquid droplets or wetted dust particles, they can enable aerosol growth in the atmosphere. Moreover, thanks to the different polar characters of their trans and cis conformers, GL and MG have been suggested as possible molecular switches capable of responding to changes in solvent polarity. Here, we show that the hydroxylated silica surface does not significantly catalyze the trans-to-cis isomerization, but it stabilizes the cis-isomers, indicating a higher interfacial cis/trans relative concentration compared to the gas phase. Moreover, adsorbed GL prefers to lie parallel on the silica surface, while adsorbed MG shows a tilted orientation. In particular, we report the aldehyde group pointing upward (downward) to the gas phase (to the silica surface) in trans-MG (cis-MG). These results will help in the rationalization of upcoming experimental and modeling work on the adsorption of ketonic compounds on dust aerosols, while it clarifies the catalytic role of the solid substrate surface in promoting conformational changes.

Hydration, Solvation, and Isomerization of Methylglyoxal at the Air/Water Interface: New Mechanistic Pathways

Aqueous-phase processing of methylglyoxal (MG) has been suggested to play a key role in the formation of secondary organic aerosols and catalyze particle growth in the atmosphere. However, the details of these processes remain speculative owing to the lack of a complete description of the physicochemical behavior of MG on atmospheric aerosols. Here, the solvation and hydrolysis of MG at the air/liquid water interface is studied via classical and first-principles molecular dynamics simulations combined with free-energy methods. Our results reveal that the polarity of the water solvent catalyzed the trans-to-cis isomerization of MG at the air/liquid water interface relative to the gas phase. Despite the presence of a hydrophobic group, MG often solvates with both the ketone and methyl groups parallel to the water interface. Analysis of the instantaneous water surface reveals that when MG is in the trans state, the methyl group repels interfacial water to maintain the planarity of the molecule, indicating that lateral and temporal inhomogeneities of interfacial environments are important for fully characterizing the solvation of MG. The counterintuitive behavior of the hydrophobic group is ascribed to a tendency to maximize the number of hydrogen bonds between MG and interfacial water while minimizing the torsional free energy. This drives MG hydration, and our simulations indicate that the formation of MG diol is catalyzed at the air/liquid water interface compared to the gas phase and occurs through nucleophilic attack of water on the carbonyl carbon.

Severe haze in northern China: A synergy of anthropogenic emissions and atmospheric processes

Regional severe haze represents an enormous environmental problem in China, influencing air quality, human health, ecosystem, weather, and climate. These extremes are characterized by exceedingly high concentrations of fine particulate matter (smaller than 2.5 µm, or PM_2.5) and occur with extensive temporal (on a daily, weekly, to monthly timescale) and spatial (over a million square kilometers) coverage. Although significant advances have been made in field measurements, model simulations, and laboratory experiments for fine PM over recent years, the causes for severe haze formation have not yet to be systematically/comprehensively evaluated. This review provides a synthetic synopsis of recent advances in understanding the fundamental mechanisms of severe haze formation in northern China, focusing on emission sources, chemical formation and transformation, and meteorological and climatic conditions. In particular, we highlight the synergetic effects from the interactions between anthropogenic emissions and atmospheric processes. Current challenges and future research directions to improve the understanding of severe haze pollution as well as plausible regulatory implications on a scientific basis are also discussed.

Formation and Optical Properties of Brown Carbon from Small α-Dicarbonyls and Amines

Brown Carbon (BrC) aerosols scatter and absorb solar radiation, directly affecting the Earth's radiative budget. However, considerable uncertainty exists concerning the chemical mechanism leading to BrC formation and their optical properties. In this work, BrC particles were prepared from mixtures of small α-dicarbonyls (glyoxal and methylglyoxal) and amines (methylamine, dimethylamine, and trimethylamine). The absorption and scattering of BrC particles were measured using a photoacoustic extinctometer (405 and 532 nm), and the chemical composition of the α-dicarbonyl-amine mixtures was analyzed using orbitrap-mass spectrometry and thermal desorption-ion drift-chemical ionization mass spectrometry. The single scattering albedo for methylglyoxal-amine mixtures is smaller than that of glyoxal-amine mixtures and increases with the methyl substitution of amines. The mass absorption cross-section for methylglyoxal-amine mixtures is two times higher at 405 nm wavelength than that at 532 nm wavelength. The derived refractive indexes at the 405 nm wavelength are 1.40-1.64 for the real part and 0.002-0.195 for the imaginary part. Composition analysis in the α-dicarbonyl-amine mixtures reveals N-heterocycles as the dominant products, which are formed via multiple steps involving nucleophilic attack, steric hindrance, and dipole-dipole interaction between α-dicarbonyls and amines. BrC aerosols, if formed from the particle-phase reaction of methylglyoxal with methylamine, likely contribute to atmospheric warming.

Wintertime nitrate formation during haze days in the Guanzhong basin, China: A case study

In this study, the formation of nitrate aerosol from 16 to 24 December 2015 in the Guanzhong basin, China is simulated using the WRF-Chem model. The predicted near-surface O₃, NO₂, and fine particulate matters (PM_2.5) in the basin and inorganic aerosols and nitrous acid (HONO) in Xi'an are generally in good agreement with the observations. Sensitivity studies show that the heterogeneous HONO sources play an appreciable role in the nitrate formation in the basin, contributing 9.2% of nitrate mass concentrations during heavy haze days. Nitrate formation is also affected by sulfate due to their competition for ammonia, particularly in urban areas. A 50% decrease in SO₂ emissions enhances the nitrate concentration by 6.2% during heavy haze days on average in the basin, and a 50% increase in SO₂ emission reduces the nitrate concentration by 9.7%. The roles of HONO and sulfate competition in nitrate formation are strongly modulated by ammonia. Agricultural emissions predominate the nitrate level in the basin (93.5%), but the non-agricultural sources cannot substantially influence nitrate formation (3.7%-14.6%). Reducing agricultural emission is an effective control strategy to mitigate nitrate pollution in the basin.

A systematic ab initio optimization of monohydrates of HCl•HNO3•H2SO4 aggregates

Hydrates of HCl, HNO₃ and H₂SO₄ involved in polar stratospheric clouds capture the attention of researchers due to the mixtures composed with them. The molecular aggregates generated with these strong acids show different behaviors, geometries and nucleation reactions at atmospheric temperatures. Here is presented a systematic ab initio optimization study of monohydrates of HCl•HNO₃•H₂SO₄ using the Density Functional Theory, by means of geometry optimizations carried out with B3LYP hybrid method and aug-cc-pVTZ basis set, a high level of theory, within Gaussian 09 program. This systematic optimization procedure consists to situate systematically the H₂O molecule around the cluster in study, on the favorable positions to develop higher quantity of hydrogen bonds as possible, in order to obtain major quantity of different electronic structures of these monohydrates. Applying this systematic optimization methodology over previously optimized complexes of HCl, HNO₃ and H₂SO₄, the present theoretical approach provides thirty-two different optimized electronic structures of monohydrates that were yielded from seven initial groups of (HCl•HNO₃•H₂SO₄)-complex, placing the H₂O in eight positions around them. Moreover, their Infrared spectra have been predicted for all (HCl•HNO₃•H₂SO₄)-monohydrates achieved. Likewise, It is shown the outcomes of the electronic energies, relative Gibbs free energies, Infrared spectra, the wavenumbers of hydrogen bonds, inter-monomeric parameters, electronic structures of (HCl•HNO₃•H₂SO₄)-monohydrates. These monohydrates could be considered precursors of the atmospheric heterogeneous nucleation reactions. These results can be useful to experimentalists of Catalysis, Astrophysics, Corrosion of metals and ceramics, aromatic compounds reactions, even environmental pollution and industrial smog.

OH-Initiated Oxidation of Acetylacetone: Implications for Ozone and Secondary Organic Aerosol Formation

Acetylacetone (AcAc) is a common atmospheric oxygenated volatile organic compound due to broad industrial applications, but its atmospheric oxidation mechanism is not fully understood. We investigate the mechanism, kinetics, and atmospheric fate of the OH-initiated oxidation for the enolic and ketonic isomers of AcAc using quantum chemical and kinetic rate calculations. OH addition to enol-AcAc is more favorable than addition to keto-AcAc, with the total rate constant of 1.69 × 10^-13 exp(1935/T) cm³ molecule^-1 s^-1 over the temperature range of 200-310 K. For the reaction of the enol-AcAc with OH, the activation energies of H-abstraction are at least 4 kcal mol^-1 higher than those of OH-addition, and the rate constants for OH-addition are by 2-3 orders of magnitude higher than those for H-abstraction. Oxidation of AcAc is predicted to yield significant amounts of acetic acid and methylglyoxal, larger than those are currently recognized. A lifetime of less than a few hours for AcAc is estimated throughout the tropospheric conditions. In addition, we present field measurements in Beijing and Nanjing, China, showing significant concentrations of AcAc in the two urban locations. Our results reveal that the OH-initiated oxidation of AcAc contributes importantly to ozone and SOA formation under polluted environments.

Investigating missing sources of glyoxal over China using a regional air quality model (RAMS-CMAQ)

Currently, modeling studies tend to significantly underestimate observed space-based glyoxal (CHOCHO) vertical column densities (VCDs), implying the existence of missing sources of glyoxal. Several recent studies suggest that the emissions of aromatic compounds and molar yields of glyoxal in the chemical mechanisms may both be underestimated, which can affect the simulated glyoxal concentrations. In this study, the influences of these two factors on glyoxal amounts over China were investigated using the RAMS-CMAQ modeling system for January and July 2014. Four sensitivity simulations were performed, and the results were compared to satellite observations. These results demonstrated significant impacts on glyoxal concentrations from these two factors. In case 1, where the emissions of aromatic compounds were increased three-fold, improvements to glyoxal VCDs were seen in high anthropogenic emissions regions. In case 2, where molar yields of glyoxal from isoprene were increased five-fold, the resulted concentrations in July were 3-5-fold higher, achieving closer agreement between the modeled and measured glyoxal VCDs. The combined changes from both cases 1 and 2 were applied in case 3, and the model succeeded in further reducing the underestimations of glyoxal VCDs. However, the results over most of the regions with pronounced anthropogenic emissions were still underestimated. So the molar yields of glyoxal from anthropogenic precursors were considered in case 4. With these additional mole yield changes (a two-fold increase), the improved concentrations agreed better with the measurements in regions of the lower reaches of the Yangtze River and Yellow River in January but not in July.

Aerosol and boundary-layer interactions and impact on air quality

Air quality is concerned with pollutants in both the gas phase and solid or liquid phases. The latter are referred to as aerosols, which are multifaceted agents affecting air quality, weather and climate through many mechanisms. Unlike gas pollutants, aerosols interact strongly with meteorological variables with the strongest interactions taking place in the planetary boundary layer (PBL). The PBL hosting the bulk of aerosols in the lower atmosphere is affected by aerosol radiative effects. Both aerosol scattering and absorption reduce the amount of solar radiation reaching the ground and thus reduce the sensible heat fluxes that drive the diurnal evolution of the PBL. Moreover, aerosols can increase atmospheric stability by inducing a temperature inversion as a result of both scattering and absorption of solar radiation, which suppresses dispersion of pollutants and leads to further increases in aerosol concentration in the lower PBL. Such positive feedback is especially strong during severe pollution events. Knowledge of the PBL is thus crucial for understanding the interactions between air pollution and meteorology. A key question is how the diurnal evolution of the PBL interacts with aerosols, especially in vertical directions, and affects air quality. We review the major advances in aerosol measurements, PBL processes and their interactions with each other through complex feedback mechanisms, and highlight the priorities for future studies.

Reassessing the atmospheric oxidation mechanism of toluene

Photochemical oxidation of aromatic hydrocarbons leads to tropospheric ozone and secondary organic aerosol (SOA) formation, with profound implications for air quality, human health, and climate. Toluene is the most abundant aromatic compound under urban environments, but its detailed chemical oxidation mechanism remains uncertain. From combined laboratory experiments and quantum chemical calculations, we show a toluene oxidation mechanism that is different from the one adopted in current atmospheric models. Our experimental work indicates a larger-than-expected branching ratio for cresols, but a negligible formation of ring-opening products (e.g., methylglyoxal). Quantum chemical calculations also demonstrate that cresols are much more stable than their corresponding peroxy radicals, and, for the most favorable OH (ortho) addition, the pathway of H extraction by O₂ to form the cresol proceeds with a smaller barrier than O₂ addition to form the peroxy radical. Our results reveal that phenolic (rather than peroxy radical) formation represents the dominant pathway for toluene oxidation, highlighting the necessity to reassess its role in ozone and SOA formation in the atmosphere.

Heavy pollution episodes, transport pathways and potential sources of PM2.5 during the winter of 2013 in Chengdu (China)

Air mass concentration data from 8 environmental quality monitoring sites and meteorological data of Chengdu from 1 December 2013 to 28 February 2014 were used in this study. Chengdu suffered five continuous heavy pollutions during this winter due to the basin terrain and the meteorological conditions of low wind speed, low precipitation and high relative humidity. Analysing the hourly resolution time series of pollutants' concentrations, variation of PM_2.5 in the urban area followed a growing "saw-tooth cycle" pattern during the heavy pollution, with a daily cycle of bimodal distribution. The massive letting-off of fireworks within a short period of time on the Eve of the Lunar New Year under the unfavourable diffusion conditions resulted in an extreme pollution event. The sharply rising Longmen-Qionglai Mountains to the west of the Sichuan Basin not only acted as a huge barrier to block the air mass from the east but also favoured the formation of a local circulation. The cluster analysis of back trajectories revealed that up to 77% of them came from the inner part of the Basin. Combining the concentration data of PM_2.5 with air mass back trajectories, a potential source contribution function (PSCF) model and a concentration-weighted trajectory (CWT) method were used to evaluate the transport pathways and sources over PM_2.5 of Chengdu, revealing that the main potential sources of PM_2.5 were located in southeast cities and the western margin of the Sichuan Basin. The result provided advice for the government to take measures in improving air quality.

Enhanced Light Scattering of Secondary Organic Aerosols by Multiphase Reactions

Secondary organic aerosol (SOA) plays a pivotal role in visibility and radiative forcing, both of which are intrinsically linked to the refractive index (RI). While previous studies have focused on the RI of SOA from traditional formation processes, the effect of multiphase reactions on the RI has not been considered. Here, we investigate the effects of multiphase processes on the RI and light-extinction of m-xylene-derived SOA, a common type of anthropogenic SOA. We find that multiphase reactions in the presence of liquid water lead to the formation of oligomers from intermediate products such as glyoxal and methylglyoxal, resulting in a large enhancement in the RI and light-scattering of this SOA. These reactions will result in increases in light-scattering efficiency and direct radiative forcing of approximately 20%-90%. These findings improve our understanding of SOA optical properties and have significant implications for evaluating the impacts of SOA on the rapid formation of regional haze, global radiative balance, and climate change.

Persistent sulfate formation from London Fog to Chinese haze

Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO₂ by NO₂ is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH₃ neutralization or under cloud conditions. Under polluted environments, this SO₂ oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH₃ and NO₂ control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.

Observational constraints on glyoxal production from isoprene oxidation and its contribution to organic aerosol over the Southeast United States

We use a 0-D photochemical box model and a 3-D global chemistry-climate model, combined with observations from the NOAA Southeast Nexus (SENEX) aircraft campaign, to understand the sources and sinks of glyoxal over the Southeast United States. Box model simulations suggest a large difference in glyoxal production among three isoprene oxidation mechanisms (AM3ST, AM3B, and MCM v3.3.1). These mechanisms are then implemented into a 3-D global chemistry-climate model. Comparison with field observations shows that the average vertical profile of glyoxal is best reproduced by AM3ST with an effective reactive uptake coefficient γ_glyx of 2 × 10^-3, and AM3B without heterogeneous loss of glyoxal. The two mechanisms lead to 0-0.8 μg m^-3 secondary organic aerosol (SOA) from glyoxal in the boundary layer of the Southeast U.S. in summer. We consider this to be the lower limit for the contribution of glyoxal to SOA, as other sources of glyoxal other than isoprene are not included in our model. In addition, we find that AM3B shows better agreement on both formaldehyde and the correlation between glyoxal and formaldehyde (R_GF = [GLYX]/[HCHO]), resulting from the suppression of δ-isoprene peroxy radicals (δ-ISOPO₂). We also find that MCM v3.3.1 may underestimate glyoxal production from isoprene oxidation, in part due to an underestimated yield from the reaction of IEPOX peroxy radicals (IEPOXOO) with HO₂. Our work highlights that the gas-phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.

Optical and Physicochemical Properties of Brown Carbon Aerosol: Light Scattering, FTIR Extinction Spectroscopy, and Hygroscopic Growth

A great deal of attention has been paid to brown carbon aerosol in the troposphere because it can both scatter and absorb solar radiation, thus affecting the Earth's climate. However, knowledge of the optical and chemical properties of brown carbon aerosol is still limited. In this study, we have investigated different aspects of the optical properties of brown carbon aerosol that have not been previously explored. These properties include extinction spectroscopy in the mid-infrared region and light scattering at two different visible wavelengths, 532 and 402 nm. A proxy for atmospheric brown carbon aerosol was formed from the aqueous reaction of ammonium sulfate with methylglyoxal. The different optical properties were measured as a function of reaction time for a period of up to 19 days. UV/vis absorption experiments of bulk solutions showed that the optical absorption of aqueous brown carbon solution significantly increases as a function of reaction time in the spectral range from 200 to 700 nm. The analysis of the light scattering data, however, showed no significant differences between ammonium sulfate and brown carbon aerosol particles in the measured scattering phase functions, linear polarization profiles, or the derived real parts of the refractive indices at either 532 or 402 nm, even for the longest reaction times with greatest visible extinction. The light scattering experiments are relatively insensitive to the imaginary part of the refractive index, and it was only possible to place an upper limit of k ≤ 0.01 on the imaginary index values. These results suggest that after the reaction with methylglyoxal the single scattering albedo of ammonium sulfate aerosol is significantly reduced but that the light scattering properties including the scattering asymmetry parameter, which is a measure of the relative amount of forward-to-backward scattering, remain essentially unchanged from that of unprocessed ammonium sulfate. The optical extinction properties in the mid-IR range (800 to 7000 cm(-1)) also showed no significant changes in either the real or the imaginary parts of the refractive indices for brown carbon aerosol particles when compared to ammonium sulfate. Therefore, changes in the optical properties of ammonium sulfate in the mid-IR spectral range due to reaction with methylglyoxal appear to be insignificant. In addition to these measurements, we have characterized additional physicochemical properties of the brown carbon aerosol particles including hygroscopic growth using a tandem-differential mobility analyzer. Compared to ammonium sulfate, brown carbon aerosol particles are found to have lower deliquescence relative humidity (DRH), efflorescence relative humidity (ERH), and hygroscopic growth at the same relative humidities. Overall, our study provides new details of the optical and physicochemical properties of a class of secondary organic aerosol which may have important implications for atmospheric chemistry and climate.

Hygroscopic Characteristics of Alkylaminium Carboxylate Aerosols

The hygroscopic growth factor (HGF) and cloud condensation nuclei (CCN) activity for a series of alkylaminium carboxylate aerosols have been measured using a hygroscopicity tandem differential mobility analyzer coupled to a condensation particle counter and a CCN counter. The particles, consisting of the mixtures of mono- (acetic, propanoic, p-toluic, and cis-pinonic acid) and dicarboxylic (oxalic, succinic, malic, adipic, and azelaic acid) acid with alkylamine (mono-, di-, and trimethylamines), represent those commonly found under diverse environmental conditions. The hygroscopicity parameter (κ) of the alkylaminium carboxylate aerosols was derived from the HGF and CCN results and theoretically calculated. The HGF at 90% RH is in the range of 1.3 to 1.8 for alkylaminium monocarboxylates and 1.1 to 2.2 for alkylaminium dicarboxylates, dependent on the molecular functionality (i.e., the carboxylic or OH functional group in organic acids and methyl substitution in alkylamines). The κ value for all alkylaminium carboxylates is in the range of 0.06-1.37 derived from the HGF measurements at 90% RH, 0.05-0.49 derived from the CCN measurements, and 0.22-0.66 theoretically calculated. The measured hygroscopicity of the alkylaminium carboxylates increases with decreasing acid to base ratio. The deliquescence point is apparent for several of the alkylaminium dicarboxylates but not for the alkylaminium monocarboxylates. Our results reveal that alkylaminium carboxylate aerosols exhibit distinct hygroscopic and deliquescent characteristics that are dependent on their molecular functionality, hence regulating their impacts on human health, air quality, and direct and indirect radiative forcing on climate.

Interfacial Tensions of Aged Organic Aerosol Particle Mimics Using a Biphasic Microfluidic Platform

Secondary organic aerosol (SOA) particles are a major component of atmospheric particulate matter, yet their formation processes and ambient properties are not well understood. These complex particles often contain multiple interfaces due to internal aqueous- and organic-phase partitioning. Aerosol interfaces can profoundly affect the fate of condensable organic compounds emitted into the atmosphere by altering the way in which ambient organic vapors interact with suspended particles. To accurately predict the evolution of SOA in the atmosphere, we must improve our understanding of aerosol interfaces. In this work, biphasic microscale flows are used to measure interfacial tension of reacting methylglyoxal, formaldehyde, and ammonium sulfate aqueous mixtures with a surrounding oil phase. Our experiments show a suppression of interfacial tension as a function of organic content that remains constant with reaction time for methylglyoxal-ammonium sulfate systems. We also reveal an unexpected time dependence of interfacial tension over a period of 48 h for ternary solutions of both methylglyoxal and formaldehyde in aqueous ammonium sulfate, indicating a more complicated behavior of surface activity where there is competition among dissolved organics. From these interfacial tension measurements, the morphology of aged atmospheric aerosols with internal liquid-liquid phase separation is inferred.

Hydration of oxalic acid–ammonia complex: atmospheric implication and Rayleigh-scattering properties

A previous study of the binary system (H₂C₂O₄)(NH₃)_n (n = 1–6) suggested that an oxalic acid–ammonia complex may participate in atmospheric aerosol formations.

Significant Contributions of Isoprene to Summertime Secondary Organic Aerosol in Eastern United States

A modified SAPRC-11 (S11) photochemical mechanism with more detailed treatment of isoprene oxidation chemistry and additional secondary organic aerosol (SOA) formation through surface-controlled reactive uptake of dicarbonyls, isoprene epoxydiol and methacrylic acid epoxide was incorporated in the Community Multiscale Air Quality Model (CMAQ) to quantitatively determine contributions of isoprene to summertime ambient SOA concentrations in the eastern United States. The modified model utilizes a precursor-origin resolved approach to determine secondary glyoxal and methylglyoxal produced by oxidation of isoprene and other major volatile organic compounds (VOCs). Predicted OC concentrations show good agreement with field measurements without significant bias (MFB ∼ 0.07 and MFE ∼ 0.50), and predicted SOA reproduces observed day-to-day and diurnal variation of Oxygenated Organic Aerosol (OOA) determined by an aerosol mass spectrometer (AMS) at two locations in Houston, Texas. On average, isoprene SOA accounts for 55.5% of total predicted near-surface SOA in the eastern U.S., followed by aromatic compounds (13.2%), sesquiterpenes (13.0%) and monoterpenes (10.9%). Aerosol surface uptake of isoprene-generated glyoxal, methylglyoxal and epoxydiol accounts for approximately 83% of total isoprene SOA or more than 45% of total SOA. A domain wide reduction of NOx emissions by 40% leads to a slight decrease of domain average SOA by 3.6% and isoprene SOA by approximately 2.6%. Although most of the isoprene SOA component concentrations are decreased, SOA from isoprene epoxydiol is increased by ∼16%.

Hydration of a sulfuric acid–oxalic acid complex: acid dissociation and its atmospheric implication

We have investigated structural characteristics and thermodynamics of the hydration of a sulfuric acid–oxalic acid complex using density functional theory to gain insight into the ternary nucleation and its atmospheric implication.

Elucidating severe urban haze formation in China

As the world's second largest economy, China has experienced severe haze pollution, with fine particulate matter (PM) recently reaching unprecedentedly high levels across many cities, and an understanding of the PM formation mechanism is critical in the development of efficient mediation policies to minimize its regional to global impacts. We demonstrate a periodic cycle of PM episodes in Beijing that is governed by meteorological conditions and characterized by two distinct aerosol formation processes of nucleation and growth, but with a small contribution from primary emissions and regional transport of particles. Nucleation consistently precedes a polluted period, producing a high number concentration of nano-sized particles under clean conditions. Accumulation of the particle mass concentration exceeding several hundred micrograms per cubic meter is accompanied by a continuous size growth from the nucleation-mode particles over multiple days to yield numerous larger particles, distinctive from the aerosol formation typically observed in other regions worldwide. The particle compositions in Beijing, on the other hand, exhibit a similarity to those commonly measured in many global areas, consistent with the chemical constituents dominated by secondary aerosol formation. Our results highlight that regulatory controls of gaseous emissions for volatile organic compounds and nitrogen oxides from local transportation and sulfur dioxide from regional industrial sources represent the key steps to reduce the urban PM level in China.

Heterogeneous chemistry of glyoxal on acidic solutions. An oligomerization pathway for secondary organic aerosol formation

The heterogeneous chemistry of glyoxal on sulfuric acid surfaces has been investigated at various acid concentrations and temperatures, utilizing a low-pressure fast flow laminar reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS). The uptake coefficient (γ) of glyoxal ranges from (1.2 ± 0.06) × 10(-2) to (2.5 ± 0.01) × 10(-3) for 60-93 wt % H2SO4 at 253-273 K. The effective Henry's Law constant (H*) ranges from (98.9 ± 4.9) × 10(5) to (1.6 ± 0.1) × 10(5) M atm(-1) for 60-93 wt % at 263-273 K. Both the uptake coefficient and Henry's Law constant increase with decreasing acid concentration and temperature. Our results reveal a reaction mechanism of hydration followed by oligomerization for glyoxal on acidic media, indicating an efficient aqueous reaction of glyoxal on hygroscopic particles leading to secondary organic aerosol formation.

Seasonal characteristics of oxalic acid and related SOA in the free troposphere of Mt. Hua, central China: implications for sources and formation mechanisms

PM10 aerosols from the summit of Mt. Hua (2060 m a.s.l) in central China during the winter and summer of 2009 were analyzed for dicarboxylic acids, ketocarboxylic acids and α-dicarbonyls. Molecular composition of dicarboxylic acids (C2-C11) in the free tropospheric aerosols reveals that oxalic acid (C2, 399 ± 261 ng m(-3) in winter and 522 ± 261 ng m(-3) in summer) is the most abundant species in both seasons, followed by malonic (C3) and succinic (C4) acids, being consistent with that on ground levels. Most of the diacids are more abundant in summer than in winter, but adipic (C6) and phthalic (Ph) acids are twice lower in summer, suggesting more significant impact of anthropogenic pollution on the wintertime alpine atmosphere. Moreover, glyoxal (Gly) and methylglyoxal (mGly) are also lower in summer (12 ± 6.1 ng m(-3)) than in winter (22 ± 13 ng m(-3)). As both dicarbonyls are a major precursor of C2, their seasonal variation patterns, which are opposite to those of the diacids, indicate that the mountain troposphere is more oxidative in summer. C2 showed strong linear correlations with levoglucosan in winter and oxidation products of isoprene and monoterpene in summer. PCA analysis further suggested that the wintertime C2 and related SOA in the Mt. Hua troposphere mostly originate from photochemical oxidations of anthropogenic pollutants emitted from biofuel and coal combustion in lowland regions. On contrast, the summertime C2 and related SOA mostly originate from further oxidation of the mountainous isoprene and monoterpene oxidation products. The AIM model calculation results showed that oxalic acid concentration well correlated with particle acidity (R(2)=0.60) but not correlated with particle liquid water content, indicating that particle acidity favors the organic acid formation because aqueous-phase C2 production is the primary mechanism of C2 formation in ambient aerosols and is driven by acid-catalyzed oxidation.

Organic matrix effects on the formation of light-absorbing compounds from α-dicarbonyls in aqueous salt solution

Aqueous-phase reactions of organic compounds are of general importance in environmental systems. Reactions of α-dicarbonyl compounds in the aqueous phase of atmospheric aerosols can impact their climate-relevant physical properties including hygroscopicity and absorption of light. Less-reactive water-soluble organic compounds may contribute an organic matrix component to the aqueous environment, potentially impacting the reaction kinetics. In this work we demonstrate the effects of organic matrices on the self-reactions of glyoxal (Gly) and methylglyoxal (mGly) in aqueous solutions containing ammonium sulfate. At an organic-to-sulfate mass ratio of 2 : 1, carbohydrate-like matrices resembling oxidized organic aerosol material reduce the rate of formation of light-absorbing products by up to an order of magnitude. The greatest decreases in the reaction rates were observed for organic matrices with smaller, more linear molecular structures. Initial UV-Vis spectra, product studies, relative rate data, acidity changes, and viscosity measurements suggest that shifts in carbonyl equilibria, due in part to (hemi)acetal formation with the matrix, reduce the rate of formation of light-absorbing imidazole and oligomer species.