Organic aerosols in the Earth's atmosphere

Recent advances in mass spectrometry techniques for atmospheric chemistry research on molecular-level

The Earth's atmosphere is composed of an enormous variety of chemical species associated with trace gases and aerosol particles whose composition and chemistry have critical impacts on the Earth's climate, air quality, and human health. Mass spectrometry analysis as a powerful and popular analytical technique has been widely developed and applied in atmospheric chemistry for decades. Mass spectrometry allows for effective detection, identification, and quantification of a broad range of organic and inorganic chemical species with high sensitivity and resolution. In this review, we summarize recently developed mass spectrometry techniques, methods, and applications in atmospheric chemistry research in the past several years on molecular-level. Specifically, new developments of ion-molecule reactors, various soft ionization methods, and unique coupling with separation techniques are highlighted. The new mass spectrometry applications in laboratory studies and field measurements focused on improving the detection limits for traditional and emerging volatile organic compounds, characterizing multiphase highly oxygenated molecules, and monitoring particle bulk and surface compositions.

Contribution of Particulates to Airborne Disease Transmission and Severity: A Review

The emergence of coronavirus disease 2019 (COVID-19) has catalyzed great interest in the spread of airborne pathogens. Airborne infectious diseases are classified into viral, bacterial, and fungal infections. Environmental factors can elevate their transmission and lethality. Air pollution has been reported as the leading environmental cause of disease and premature death worldwide. Notably, ambient particulates of various components and sizes are harmful pollutants. There are two prominent health effects of particles in the atmosphere: (1) particulate matter (PM) penetrates the respiratory tract and adversely affects health, such as heart and respiratory diseases; and (2) bioaerosols of particles act as a medium for the spread of pathogens in the air. Particulates contribute to the occurrence of infectious diseases by increasing vulnerability to infection through inhalation and spreading disease through interactions with airborne pathogens. Here, we focus on the synergistic effects of airborne particulates on infectious disease. We outline the concepts and characteristics of bioaerosols, from their generation to transformation and circulation on Earth. Considering that microorganisms coexist with other particulates as bioaerosols, we investigate studies examining respiratory infections associated with airborne PM. Furthermore, we discuss four factors (meteorological, biological, physical, and chemical) that may impact the influence of PM on the survival of contagious pathogens in the atmosphere. Our review highlights the significant role of particulates in supporting the transmission of infectious aerosols and emphasizes the need for further research in this area.

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

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

Emissions from ships' activities in the anchorage zone: A potential source of sub-micron aerosols in port areas

Busan Port is among the world's top ten most air-polluted ports, but the role of the anchorage zone as a significant contributor to pollution has not been studied. To assess the emission characteristics of sub-micron aerosols, a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed in Busan, South Korea from September 10 to October 6, 2020. The concentration of all AMS-identified species and black carbon were highest when the winds came from the anchorage zone (11.9 µg·m^-3) and lowest with winds from the open ocean (6.64 µg·m^-3). The positive matrix factorization model identified one hydrocarbon-like organic aerosol (HOA) and two oxygenated organic aerosol (OOA) sources. HOAs were highest with winds from Busan Port, while oxidized OOAs were predominant with winds from the anchorage zone (less oxidized) and the open ocean (more oxidized). We calculated the emissions from the anchorage zone using ship activity data and compared them to the total emissions from Busan Port. Our results suggest that emissions from ship activities in the anchorage zone should be considered a significant source of pollution in the Busan Port area, especially given the substantial contributions of gaseous emissions (NOx: 8.78%; volatile organic compounds: 7.52%) and their oxidized moieties as secondary aerosols.

Molecular characteristics of ambient organic aerosols in Shanghai winter before and after the COVID-19 outbreak

During the global pandemic of COVID-19, the world adopted different strategies to avoid the human and economic loss, and so does China. The reduction of human activities during this time period caused reduction in PM emissions. This study adopted a HPLC-Q-TOF-MS to compare the chemical compositions of ambient aerosol samples collected in Shanghai winter before (2018, 2019) and after (2021) the COVID-19 outbreak. The identified compositions were classified into subgroups of CHO, CHN, CHON, CHONS, CHOS and CHN compounds. Results showed that CHO compounds and CHON compounds were dominating the organic compounds in ESI- and ESI+, respectively. The average percentages of CHO- compounds were 57.97 % in 2018, 58.98 % in 2019, and 43.93 % in 2021, respectively. The average percentages of CHON+ compounds were 52.74 % in 2018, 50.34 % in 2019, and 52.02 % in 2021, respectively. The proportion of aliphatic compounds increased gradually during the three years, especially in 2021, indicating that CHO compounds were less affected by aromatic precursors after the COVID-19 outbreak. The contribution of anthropogenic emissions in Shanghai was weakened compared with the previous years. In addition, there was an enhanced emission source containing hydroxyl for CHOS compound formation in 2021. The variations of atmospheric oxidation degree among the three years were not significant.

Kinetics of oligomer-forming reactions involving the major functional groups present in atmospheric secondary organic aerosol particles

Atmospheric organic aerosol particles impact climate as well as human and environmental health. Secondary organic aerosol (SOA), which is formed by the gas-to-particle partitioning of products of the oxidation of volatile organic compounds (VOCs) emitted from biogenic or anthropogenic sources, contributes a large fraction of this material. In the particle phase, these products can undergo accretion reactions to form oligomers that impact the formation, composition, and chemical-physical properties of aerosols. While these reactions are known to occur in the atmosphere, data and models describing their kinetics and equilibria are sparse. Here, reactions of compounds containing potentially reactive hydroperoxide, hydroxyl, carboxyl, aldehyde, and ketone groups were investigated in single and phase-separated organic/aqueous mixtures in the absence and presence of a sulfuric acid catalyst. Compounds containing these groups and a nonreactive UV-absorbing nitrate group were synthesized and their reactions and products were monitored and characterized using high-performance liquid chromatography with UV detection (HPLC-UV), electrospray ionization-mass spectrometry (ESI-MS), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Reactions were observed between hydroperoxides and aldehydes to form peroxyhemiacetals, and between carboxylic acids and alcohols to form esters, and their rate and equilibrium constants were determined. No reactions were observed in other mixtures, indicating that under the conditions of these experiments only a few reaction pathways form oligomers. Reactions were also conducted with probe compounds and SOA formed in an environmental chamber reaction of α-pinene with O₃. Whereas in a previous study we observed a rapid hydroperoxide reaction in this SOA, among the other compounds studied here only alcohols reacted. These results provide insight into the types of accretion reactions that are likely to occur in atmospheric aerosols, and the rate and equilibrium constants can be used to better model SOA chemistry.

Aqueous processing of water-soluble organic compounds in the eastern United States during winter

Aqueous multi-phase processes are significant contributors to organic aerosol (OA) mass in the atmosphere. This study characterizes the formation of water-soluble organic matter during the winter in the eastern United States through simultaneous measurements of water-soluble organic carbon in the gas and particle phases (WSOC_g and WSOC_p, respectively). The formation of secondary WSOC_p occurred primarily through two pathways: (1) absorptive partitioning of oxygenated organics to the bulk OA and (2) aqueous phase processes. WSOC_p formation through the former pathway was evident through the relationship between the fraction of total WSOC in the particle phase (F_p) and the total OA concentration. Conversely, evidence for nighttime aqueous WSOC_p formation was based upon the strong enhancement in F_p with increasing relative humidity, indicating the uptake of WSOC_g to aerosol liquid water (ALW). The F_p-RH relationship was only observed for temperatures between 0-10 °C, suggesting conditions for aqueous multi-phase processes were enhanced during these times. Temperature exhibited an inverse relationship with ALW and a proportional relationship with aerosol potassium. ALW and biomass burning precursors were both abundant in the 0-10 °C temperature range, facilitating aqueous WSOC_p formation. To assess the impact of particle drying on the WSOC_p concentrations, the particle measurements alternated between ambient and dried channels. No change was observed in the concentration of particles before and after drying, indicating that the WSOC_p formed through the uptake of WSOC_g into OA and ALW remained in the condensed phase upon particle drying at all temperature ranges. This work contributes to our understanding of sources, pathways, and factors affecting aqueous aerosol formation in the winter.

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

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

A retrospect of ozone formation mechanisms during the COVID-19 lockdown: The potential role of isoprene

Wuhan took strict measures to prevent the spread of COVID-19 from January 26 to April 7 in 2020. The lockdown reduced the concentrations of atmospheric pollutants, except ozone (O₃). To investigate the increase in O₃ during the lockdown, trace gas pollutants were collected. The initial concentrations of volatile organic compounds (VOCs) were calculated based on a photochemical ratio method, and the ozone formation potential (OFP) was obtained using the initial and measured VOC concentrations. The O₃ formation regime was NO_X-limited based on the VOCs/NO_X diurnal ratios during the lockdown period. The reduced nitric oxide (NO) concentrations and lower wind speed (WS) could explain the night-time O₃ accumulation. The initial total VOCs (TVOCs) during the lockdown were 47.6 ± 2.9 ppbv, and alkenes contributed 48.1%. The photochemical loss amounts of alkenes were an order of magnitude higher than those of alkenes in the same period in 2019 and increased from 16.6 to 28.0 ppbv in the daytime. The higher initial alkene concentrations sustained higher OFP during the lockdown, reaching between 252.4 and 504.4 ppbv. The initial isoprene contributed approximately 35.0-55.0% to the total OFP and had a positive correlation with the increasing O₃ concentrations. Approximately 75.5% of the temperatures were concentrated in the range of 5 and 20 °C, which were higher than those in 2019. In addition to stronger solar radiation, the higher temperatures induced higher isoprene emission rates, partially accounting for the higher isoprene concentrations. Lower isoprene-emitting trees should be considered for future urban vegetation to control O₃ episodes.

Oxidation of Phenolic Aldehydes by Ozone and Hydroxyl Radicals at the Air-Solid Interface

Biomass burning emissions contain abundant phenolic aldehydes (e.g., syringaldehyde, vanillin, and 4-hydroxybenaldehyde) that are oxidized during atmospheric transport, altering the physicochemical properties of particulates. Herein, the oxidative processing of thin films made of syringaldehyde, vanillin, and 4-hydroxybenaldehyde is studied at the air-solid interface under a variable O3(g) molar ratio (410 ppbv-800 ppmv) and relative humidity (0-90%). Experiments monitored the absorption changes of C=C, C=O, and -COOH vibration changes during the oxidation of thin films by transmission Fourier transform infrared spectroscopy (FTIR). Selected spectroscopic features of aromatic ring cleavage by O3(g) revealed the production of carboxylic acids. Instead, monitoring O-H stretching provided a comparison of a hydroxylation channel from in situ produced hydroxyl radical. The overall oxidation reactivity trend syringaldehyde > vanillin > 4-hydroxybenzladehyde can be explained based on the additional electron density from methoxide substituents to the ring. The reactive uptake coefficient of O3(g) increases for higher relative humidity, e.g., for syringaldehyde by 18 and 215 times at 74% and 90% relative humidity (RH), respectively, as compared to dry conditions. A Langmuir-Hinshelwood mechanism fits well the kinetics of oxidation under a variable O3(g) molar ratio at 74% RH, providing useful information that should be included in atmospheric chemistry models.

Molecular composition and light-absorbing properties of organic aerosols from west-coast of tropical India

Tropical coastal regions may provide a unique feature to study the photooxidation of various organic aerosols and their climatic effects because of high humid atmosphere and intense solar radiation. However, knowledge about organic molecular composition and its light absorption properties remains concealed, particularly over tropical Indian regions. The present study is an investigation on water-soluble dicarboxylic acids, ω-oxoacids, pyruvic acid, α-dicarbonyls, brown carbon (BrC), and other chemical species in PM_1.1 collected at a coastal urban location (Kochi) on the west coast of tropical India under distinct air masses. Molecular distribution of dicarboxylic acids was characterized by the predominance of oxalic acid (C₂) in all the air masses followed by adipic (C₆) or terephthalic (tPh) and phthalic (Ph) acids. On average, total diacids-C accounted for 5.03 ± 1.01 % of TC. Total diacid concentration showed strong linear relationships with organic (OC), elemental carbon (EC), and non-sea-salt potassium (nss-K⁺). Except for the northwest (NW) air mass period, the concentration of C₂ diacid and its ratios (C₂/total diacids, C₂/ωC₂, C₂/Gly) showed a strong linear relationship with nss-SO₄^2-. By combining all these results together with Pearson correlation analysis, the present study demonstrates that organic aerosols over the study region were predominantly produced by aqueous-phase oxidation of precursor compounds derived from biomass burning and combustion-related emissions. The mass absorption coefficient of BrC (b_{abs-BrC-365nm}) was strongly correlated with nss-K⁺, implying that biomass burning emissions are major sources of BrC. The absorption angstrom exponent (AÅE) values of water (methanol) extracts ranged from 3.20 to 3.83 (3.05-4.55) during the entire sampling period, indicating the substantial contribution of BrC chromophores to light absorption over the region. On average, BrC absorbs 10.6 ± 6.4 % and 22.4 ± 5.75 % of solar radiation compared to BC in water and methanol extracts, respectively, suggesting that BrC is a significant aerosol climate forcing agent over the west coast of tropical India.

Chemistry of Secondary Organic Aerosol Formation from Reactions of Monoterpenes with OH Radicals in the Presence of NOx

The oxidation of volatile organic compounds (VOCs), which are emitted to the atmosphere from natural and anthropogenic sources, leads to the formation of ozone and secondary organic aerosol (SOA) particles that impact air quality and climate. In the study reported here, we investigated the products of the reactions of five biogenic monoterpenes with OH radicals (an important daytime oxidant) under conditions that mimic the chemistry that occurs in polluted air, and developed mechanisms to explain their formation. Experiments were conducted in an environmental chamber, and information on the identity of gas-phase molecular products was obtained using online mass spectrometry, while liquid chromatography and two methods of functional group analysis were used to characterize the SOA composition. The gas-phase products of the reactions were similar to those formed in our previous studies of the reactions of these monoterpenes with NO₃ radicals (an important nighttime oxidant), in that they all contained various combinations of nitrate, carbonyl, hydroxyl, ester, and ether groups. But in spite of this, less SOA was formed in OH/NO_x reactions and it was composed of monomers, while SOA formed in NO₃ radical reactions consisted of acetal and hemiacetal oligomers formed by particle-phase accretion reactions. In addition, it appeared that some monomers underwent particle-phase hydrolysis, whereas oligomers did not. These differences are due primarily to the arrangement of hydroxyl, carbonyl, nitrate, and ether groups in the monomers, which can in turn be explained by differences in OH and NO₃ radical reaction mechanisms. The results provide insight into the impact of VOC structure on the amount and composition of SOA formed by atmospheric oxidation, which influence important aerosol properties such as volatility and hygroscopicity.

Effect of NO2 on nocturnal chemistry of isoprene: Gaseous oxygenated products and secondary organic aerosol formation

As one of the most abundant non-methane hydrocarbon in the atmosphere, isoprene has attracted lots of attention on its oxidation processes and environmental effects. However, less is known about the nocturnal chemistry of isoprene with multiple oxidants coexisting in the atmosphere. Besides, though highly oxygenated molecules (HOMs) have recently been recognized to contribute to secondary organic aerosol (SOA) formation, the specific contribution of measured HOMs on SOA formation in isoprene oxidation has not been well established. In this study, the oxidation of isoprene was simulated under dark and various NO₂/O₃ conditions. Plenty of oxidation products were identified by combining two state-of-the-art time-of-flight mass spectrometers, and more species with high C and N numbers and low volatilities were detected under high NO₂ conditions. The nocturnal oxidation of isoprene was found to be governed by synergic effects of multiple oxidants, including O₃, NO₃•, and •OH at the same time, and the oxidation proportions changed with NO₂. NO₂ promoted the formation of most N-containing products especially N2 products, because of the decisive role of NO₃• on their formation. Nevertheless, some products such as C₅H₁₀O_3-5, C₅H₁₁NO₆, and C₁₀H₁₆N₂O_10,11 showed a better correlation with HO₂NO₂ rather than NO₂/O₃, indicating the importance of HO₂• chemistry on the oxidation products formation. Though the concentration of measured oxygenated products was dominated by volatile and semi-volatile organic compounds, the low- and extremely low-volatile organic compounds contributed over 97 % to the SOA formation potential. However, challenges still exist in accurately simulating SOA formation from the measured oxygenated molecules to match the measurement, and further comprehensive characterization of oxidation products in both gas and aerosol phases at the molecular level is needed.

Surface Oxidation of Phenolic Aldehydes: Fragmentation, Functionalization, and Coupling Reactions

Substantial amounts of phenolic aldehydes, represented by the structures of syringaldehyde, vanillin, and 4-hydroxybenzaldehyde, are emitted to the atmosphere during biomass burning. The oxidative transformation of phenolic aldehydes during atmospheric transport has the potential to modify the physicochemical properties of particulates, which play a vital role in Earth's climate and human health. Herein, thin solid films made of syringaldehyde, vanillin, and 4-hydroxybenzaldehyde are oxidized in contact with O3(g) under a relative humidity of 74% representative of average global conditions. New physical insights into the surface reactions are achieved by analyzing isopropanol-extracted films before and during oxidation by multiple techniques. Changes in electronic transitions at 220, 310, and 350-400 nm registered by UV-vis spectroscopy show that the oxidized films have enhanced mass absorption coefficients at λ > 300 nm. Electrospray ionization (ESI) mass spectrometry (MS) and ion chromatography with conductivity and MS detection of extracted oxidized films confirm aromatic ring cleavage of syringaldehyde and vanillin by O3(g) with the production of carboxylic acids. Carboxylic acids were observed as anions ([M - H]-) at m/z 45 (formic acid), 73 (glyoxylic acid), 75 (glycolic acid), 89 (oxalic acid), 115 (maleic acid), 117 (mesoxalic acid), 119 (tartronic acid), and 129 (maleic acid monomethyl ester), while other polyfunctional products were registered by ultrahigh-pressure liquid chromatography with UV-vis and MS detection. In situ production of hydroxyl radicals is confirmed by demethoxylation products and ipso attack at the C1 ring position holding the -C(H)═O group. The order of reactivity increased with the number of methoxy substituents that donate electron density to the aromatic ring. Combined oxidation mechanisms for the three compounds are proposed based on all of the experimental observations and explain the contribution of aged biomass burning material to secondary organic aerosol formation.

Secondary organic aerosol formation at an urban background site on the coastline of South China: Precursors and aging processes

Understanding the formation mechanisms of secondary organic aerosols (SOA) is an arduous task in atmospheric chemistry. In November 2018, a sampling campaign was conducted at an urban background site in Hong Kong for characterization of secondary air pollution. A high-resolution time-of-flight aerosol mass spectrometer was used to monitor the compositions of non-refractory submicron particulate matters (NR-PM₁), and multiple online instruments provided us with comprehensive auxiliary data. Organic aerosol (OA) constituted the largest fraction (43.8%) of NR-PM₁, and 86.5% of the organics was contributed by the oxygenated OA (OOA, secondary components). Formation mechanisms of a dominant and more variable component of the less-oxidized OOA (labelled as LO-OOA1 in this study) and the more-oxidized OOA (MO-OOA) were explored. Based on the multilinear regression with molecular markers of OA (e.g., hydroxybenzonic acids and 2,3-dihydroxy-4-oxopentanoic acid), we presumed that anthropogenic organic compounds, especially aromatics, were the most likely precursors of LO-OOA1. MO-OOA correlated well with odd oxygen (O_x), and its concentration responded positively to the increase of liquid water content (LWC) in NR-PM₁, indicating that the formation of MO-OOA involved photochemical oxidation and aqueous processes. It exhibited the best correlation with malic acid which can be formed through the oxidation of various precursors. Moreover, it was plausible that LO-OOA1 was further oxidized to MO-OOA through aqueous processes, as indicated by the consistent diurnal variations of MO-OOA to LO-OOA1 ratio and LWC. This study highlights the important roles of anthropogenic emissions and aqueous processes in SOA formation in coastal areas downwind of cities.

Isoprene-Chlorine Oxidation in the Presence of NOx and Implications for Urban Atmospheric Chemistry

Fine particulate matter (PM_2.5) is a key indicator of urban air quality. Secondary organic aerosol (SOA) contributes substantially to the PM_2.5 concentration. Discrepancies between modeling and field measurements of SOA indicate missing sources and formation mechanisms. Recent studies report elevated concentrations of reactive chlorine species in inland and urban regions, which increase the oxidative capacity of the atmosphere and serve as sources for SOA and particulate chlorides. Chlorine-initiated oxidation of isoprene, the most abundant nonmethane hydrocarbon, is known to produce SOA under pristine conditions, but the effects of anthropogenic influences in the form of nitrogen oxides (NO_x) remain unexplored. Here, we investigate chlorine-isoprene reactions under low- and high-NO_x conditions inside an environmental chamber. Organic chlorides including C₅H₁₁ClO₃, C₅H₉ClO₃, and C₅H₉ClO₄ are observed as major gas- and particle-phase products. Modeling and experimental results show that the secondary OH-isoprene chemistry is significantly enhanced under high-NO_x conditions, accounting for up to 40% of all isoprene oxidized and leading to the suppression of organic chloride formation. Chlorine-initiated oxidation of isoprene could serve as a source for multifunctional (chlorinated) organic oxidation products and SOA in both pristine and anthropogenically influenced environments.

Oxidation Flow Reactor Results in a Chinese Megacity Emphasize the Important Contribution of S/IVOCs to Ambient SOA Formation

Oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) formation potential of ambient air in Guangzhou, China was investigated using a field-deployed oxidation flow reactor (OFR). The OFR was used to mimic hours to weeks of atmospheric exposure to hydroxyl (OH) radicals within the 2-3 min residence time. A comprehensive investigation on the variation of VOCs and OVOCs as a function of OH exposure is shown. Substantial formation of organic acids and nitrogen-containing OVOC species were observed. Maximum SOA formation in the OFR was observed following 1-4 equiv days' OH exposure. SOA produced from known/measured VOC/IVOC precursors such as single-ring aromatics and long-chain alkanes can account for 52-75% of measured SOA under low NO_x and 26-60% under high NO_x conditions based on laboratory SOA yield parametrizations. To our knowledge, this is the first time that the contribution (8-20%) of long-chain (C₈-C₂₀) alkane oxidation to OFR SOA formation was quantified from direct measurement. By additionally estimating contribution from unmeasured semivolatile and intermediate volatility compounds (S/IVOCs) that are committed with C₈-C₂₀ alkanes, 64-100% of the SOA formation observed in the OFR can be explained, signifying the important contribution of S/IVOCs such as large cyclic alkanes to ambient SOA.

Organic aerosol compositions and source estimation by molecular tracers in Dushanbe, Tajikistan

To elucidate the molecular composition and sources of organic aerosols in Central Asia, carbonaceous compounds, major ions, and 15 organic molecular tracers of total suspended particulates (TSP) were analyzed from September 2018 to August 2019 in Dushanbe, Tajikistan. Extremely high TSP concentrations (annual mean ± std: 211 ± 131 μg m^-3) were observed, particularly during summer (seasonal mean ± std: 333 ± 183 μg m^-3). Organic carbon (OC: 11.9 ± 7.0 μg m^-3) and elemental carbon (EC: 5.1 ± 2.2 μg m^-3) exhibited distinct seasonal variations from TSP, with the highest values occurring in winter. A high concentration of Ca²⁺ was observed (11.9 ± 9.2 μg m^-3), accounting for 50.8% of the total ions and reflecting the considerable influence of dust on aerosols. Among the measured organic molecular tracers, levoglucosan was the predominant compound (632 ± 770 ng m^-3), and its concentration correlated significantly with OC and EC during the study period. These findings highlight biomass burning (BB) as an important contributor to the particulate air pollution in Dushanbe. High ratios of levoglucosan to mannosan, and syringic acid to vanillic acid suggest that mixed hardwood and herbaceous plants were the main burning materials in the area, with softwood being a minor one. According to the diagnostic tracer ratio, OC derived from BB constituted a large fraction of the primary OC (POC) in ambient aerosols, accounting for an annual mean of nearly 30% and reaching 63% in winter. The annual contribution of fungal spores to POC was 10%, with a maximum of 16% in spring. Measurements of plant debris, accounting for 3% of POC, divulged that these have the same variation as fungal spores.

Evolution of organic carbon during COVID-19 lockdown period: Possible contribution of nocturnal chemistry

Carbonaceous aerosol is one of the main components of atmospheric particulate matter, which is of great significance due to its role in climate change, earth's radiation balance, visibility, and human health. In this work, carbonaceous aerosols were measured in Shijiazhuang and Beijing using the OC/EC analyzer from December 1, 2019 to March 15, 2020, which covered the Coronavirus Disease 2019 (COVID-19) pandemic. The observed results show that the gas-phase pollutants, such as NO, NO₂, and aerosol-phase pollutants (Primary Organic Compounds, POC) from anthropogenic emissions, were significantly reduced during the lockdown period due to limited human activities in North China Plain (NCP). However, the atmospheric oxidation capacity (Ox/CO) shows a significantly increase during the lockdown period. Meanwhile, additional sources of nighttime Secondary Organic Carbon (SOC), Secondary Organic Aerosol (SOA), and b_{abs, BrC}(370 nm) are observed and ascribed to the nocturnal chemistry related to NO₃ radical. The Potential Source Contribution Function (PSCF) analysis indicates that the southeast areas of the NCP region contributed more to the SOC during the lockdown period than the normal period. Our results highlight the importance of regional nocturnal chemistry in SOA formation.

Air quality changes in cities during the COVID-19 lockdown: A critical review

In response to the rapid spread of coronavirus disease-2019 (COVID-19) within and across countries and the need to protect public health, governments worldwide introduced unprecedented measures such as restricted road and air travel and reduced human mobility in 2020. The curtailment of personal travel and economic activity provided a unique opportunity for researchers to assess the interplay between anthropogenic emissions of primary air pollutants, their physical transport, chemical transformation, ultimate fate and potential health impacts. In general, reductions in the atmospheric levels of outdoor air pollutants such as particulate matter (PM), nitrogen dioxide (NO2), carbon monoxide (CO), sulfur dioxide (SO2), and volatile organic compounds (VOCs) were observed in many countries during the lockdowns. However, the levels of ozone (O3), a secondary air pollutant linked to asthma and respiratory ailments, and secondary PM were frequently reported to remain unchanged or even increase. An increase in O3 can enhance the formation of secondary PM2.5, especially secondary organic aerosols, through the atmospheric oxidation of VOCs. Given that the gaseous precursors of O3 (VOCs and NOx) are also involved in the formation of secondary PM2.5, an integrated control strategy should focus on reducing the emission of the common precursors for the co-mitigation of PM2.5 and O3 with an emphasis on their complex photochemical interactions. Compared to outdoor air quality, comprehensive investigations of indoor air quality (IAQ) are relatively sparse. People spend more than 80% of their time indoors with exposure to air pollutants of both outdoor and indoor origins. Consequently, an integrated assessment of exposure to air pollutants in both outdoor and indoor microenvironments is needed for effective urban air quality management and for mitigation of health risk. To provide further insights into air quality, we do a critical review of scientific articles, published from January 2020 to December 2020 across the globe. Finally, we discuss policy implications of our review in the context of global air quality improvement.

Effects of Anthropogenic and Biogenic Volatile Organic Compounds on Los Angeles Air Quality

Assessing the role of volatile organic compounds (VOCs) in production of ozone and secondary organic aerosol (SOA) is especially important in light of ongoing policy goals. Here, we estimated the ozone formation potential (OFP) and SOA formation potential (SOAP) of anthropogenic and biogenic VOC emissions to evaluate (1) anthropogenic VOCs and associated sectors that dominate OFP and SOAP and (2) the potential impacts of enhanced biogenic VOCs from urban greening programs on air quality in Los Angeles county. In the present-day scenario, ethylene had the largest OFP followed by m & p-xylene, toluene, propylene, and formaldehyde. The top five contributors to SOAP were toluene, mineral spirits, benzene, heptadecane, and hexadecane. Mobile and solvent sources were the dominant VOC sources for both OFP and SOAP. The potential increases in biogenic VOC emissions due to future urban greening had significant effects on urban air quality that offset the benefits of reducing anthropogenic VOC emissions. This study demonstrates that urban greening programs in Los Angeles county, and likely other cities as well, need to account for both anthropogenic and biogenic VOC contributions to secondary pollution, and greening cities should consider using vegetation types with low VOC emissions to avoid further degradation to urban air quality.

Seasonal and temporal variations of ambient aerosols in a deciduous broadleaf forest from northern Japan: Contributions of biomass burning and biological particles

Total suspended particulate (TSP) samples were collected in a deciduous broadleaf forest in Sapporo, Hokkaido, Japan, from January to December 2010 to understand the molecular composition and abundance of sugar compounds (SCs) in atmospheric aerosols. We analyzed the samples for anhydrosugars, primary sugars, and sugar alcohols using a gas chromatograph-mass spectrometer. The annual mean concentrations of total SCs ranged from 16.1 to 1748 ng m^-3 (avg. 311 ng m^-3) with maxima in spring (avg. 484 ng m^-3) and minima in winter (avg. 28.2 ng m^-3). Primary sugars and sugar alcohols followed the seasonal pattern of total SCs. High levels of anhydrosugars in winter (avg. 22.9 ng m^-3) suggest a contribution of biomass burning from domestic heating due to lower ambient temperature. The high levels of arabitol and mannitol in spring followed by summer and autumn denote the contribution from multiple sources, i.e., growing vegetation and fungal spores in Sapporo forest. We observed an enhanced contribution of bioaerosols emitted from plant blossoms in spring and leaf decomposition in autumn. The identical seasonal trends of glucose and trehalose implied their similar sources in forest aerosols. Conversely, the highest concentration of sucrose in spring was due to the pollen emissions by blooming plants. Positive matrix factorization (PMF) analyses of the SCs suggested that organic aerosols in the deciduous forest are associated with the emissions from multiple sources, including vegetation, microbes, pollens, and wintertime biomass burning. The PMF analysis also suggested that vegetation is the primary carbon source in the forest atmosphere. The diagnostic mass ratios of levoglucosan to mannosan demonstrated the dominance of softwood burning. We noted that the meteorological parameters substantially affect the emission sources and seasonal concentrations of SCs in the deciduous forest.

Secondary Ion Chemistry Mediated by Ozone and Acidic Organic Molecules in Iodide-Adduct Chemical Ionization Mass Spectrometry

Iodide-adduct chemical ionization mass spectrometry (I-CIMS) is a widely used technique in the atmospheric chemistry community to detect oxygenated volatile organic compounds (OVOCs) in real time. In this work, we report the occurrence of secondary ion chemistry from interactions between a strong oxygen donor (such as O₃ and peracids) and acidic OVOCs (such as carboxylic acids and organic hydroperoxides) in the ion-molecule reaction (IMR) region of I-CIMS. Such interactions can lead to acidic organic molecules (HA or HB) clustering with [IO]^- (e.g., [HA + IO]^-) and dimer adducts ([A + B + I]^-), in addition to the well-known iodide clusters ([HA + I]^-). This ion chemistry was probed using common chemical standards as well as the gas-phase oxidation products of α-pinene and isoprene in a flowtube reactor. The results show that secondary ion chemistry can lead to misinterpretations of molecular compositions and distributions of the gas-phase products and an overestimation of the elemental O/C ratio overall. Nevertheless, the varying degrees of signal change in response to the secondary ion chemistry might be a clue to inform OVOCs' functionalities. Specifically, in the α-pinene ozonolysis system, the extents of ion signal reduction in the presence of additional acids in the IMR suggest that C₉H₁₄O₄ produced in the gas phase is a peracid, rather than the often-assumed pinic acid. Thus, we suggest that the potential application of the secondary ion chemistry to inform organic functionalities is promising, which could help better understand the molecular compositions of gas-phase OVOCs and the reaction mechanisms therein.

Secondary Organic Aerosol Formation of Fleet Vehicle Emissions in China: Potential Seasonality of Spatial Distributions

Vehicle emissions are an important source of urban particular matter. To investigate the secondary organic aerosol (SOA) formation potential of real-world vehicle emissions, we exposed on-road air in Beijing to hydroxyl radicals generated in an oxidation flow reactor (OFR) under high-NO_x conditions on-board a mobile laboratory and characterized SOA and their precursors with a suite of state-of-the-art instrumentation. The OFR produced 10-170 μg m^-3 of SOA with a maximum SOA formation potential of 39-50 μg m^-3 ppmv^-1 CO that occurred following an integrated OH exposure of (1.3-2.0) × 10¹¹ molecules cm^-3 s. The results indicate relatively shorter photochemical ages for maximum SOA production than previous OFR results obtained under low-NO_x conditions. Such timescales represent the balance of functionalization and fragmentation, possibly resulting in different spatial distributions of SOA in different seasons as the oxidant level changes. The detected precursors may explain as much as 13% of the observed SOA with the remaining plausibly contributed by the oxidation of undetected intermediate-volatility organic compounds. Extrapolation of the results suggests an annual SOA production rate of 0.78 Tg yr^-1 from mobile gasoline sources in China, highlighting the importance of effective regulation of gaseous vehicular precursors to improve air quality in the future.

Highly Resolved Composition during Diesel Evaporation with Modeled Ozone and Secondary Aerosol Formation: Insights into Pollutant Formation from Evaporative Intermediate Volatility Organic Compound Sources

As stricter regulations continue to reduce vehicular emissions, other emission sources such as evaporative emissions from road building and volatile consumer products have become more important in overall pollutant forming emissions in many urban areas. Emission regulations have historically targeted volatile organic compounds (VOCs) to reduce ozone, but intermediate volatility organic compounds (IVOCs) also contribute to ozone formation and the formation of secondary organic aerosol (SOA) that often dominates fine particulate matter. Emission rates and pollutant formation from IVOCs are not well constrained in current inventories and models. This study uses diesel fuel as a representative IVOC mixture in evaporation tests performed in a wind tunnel under varying wind speeds and liquid diesel temperatures. Comprehensive composition measurements guided the development of a model to determine rates of evaporation and estimate pollutant production. Results show that reducing IVOC emissions can result in significant reductions in ozone formation, in addition to the expected reductions in SOA formation, and that IVOC emissions can continue over the course of a month. Ozone formation from IVOC emissions is equal to that from VOCs after 3 days of evaporation at 0.65 g-ozone/g-diesel released. SOA formation is dominated by IVOCs, reaching 0.2 g-SOA/g-diesel released after 30 days.

Photosensitized Reactions of a Phenolic Carbonyl from Wood Combustion in the Aqueous Phase-Chemical Evolution and Light Absorption Properties of AqSOA

Guaiacyl acetone (GA) is a phenolic carbonyl emitted in significant quantities by wood combustion that undergoes rapid aqueous-phase oxidation to produce aqueous secondary organic aerosol (aqSOA). We investigate the photosensitized oxidation of GA by an organic triplet excited state (³C*) and the formation and aging of the resulting aqSOA in wood smoke-influenced fog/cloud water. The chemical transformations of the aqSOA were characterized in situ using a high-resolution time-of-flight aerosol mass spectrometer. Additionally, aqSOA samples collected over different time periods were analyzed using high-performance liquid chromatography coupled with a photodiode array detector and a high-resolution Orbitrap mass spectrometer (HPLC-PDA-HRMS) to provide details on the molecular composition and optical properties of brown carbon (BrC) chromophores. Our results show efficient formation of aqSOA from GA, with an average mass yield around 80%. The composition and BrC properties of the aqSOA changed significantly over the course of reaction. Three generations of aqSOA products were identified via positive matrix factorization analysis of the aerosol mass spectrometry data. Oligomerization and functionalization dominated the production of the first-generation aqSOA, whereas fragmentation and ring-opening reactions controlled the formation of more oxidized second- and third-generation products. Significant formation of BrC was observed in the early stages of the photoreaction, while organic acids were produced throughout the experiment. High-molecular weight molecules (m/z > 180) with high aromaticity were identified via HPLC-PDA-HRMS and were found to account for a majority of the UV-vis absorption of the aqSOA.

Description of the NASA GEOS Composition Forecast Modeling System GEOS-CF v1.0

The Goddard Earth Observing System composition forecast (GEOS-CF) system is a high-resolution (0.25°) global constituent prediction system from NASA's Global Modeling and Assimilation Office (GMAO). GEOS-CF offers a new tool for atmospheric chemistry research, with the goal to supplement NASA's broad range of space-based and in-situ observations. GEOS-CF expands on the GEOS weather and aerosol modeling system by introducing the GEOS-Chem chemistry module to provide hindcasts and 5-days forecasts of atmospheric constituents including ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and fine particulate matter (PM2.5). The chemistry module integrated in GEOS-CF is identical to the offline GEOS-Chem model and readily benefits from the innovations provided by the GEOS-Chem community. Evaluation of GEOS-CF against satellite, ozonesonde and surface observations for years 2018-2019 show realistic simulated concentrations of O3, NO2, and CO, with normalized mean biases of -0.1 to 0.3, normalized root mean square errors between 0.1-0.4, and correlations between 0.3-0.8. Comparisons against surface observations highlight the successful representation of air pollutants in many regions of the world and during all seasons, yet also highlight current limitations, such as a global high bias in SO2 and an overprediction of summertime O3 over the Southeast United States. GEOS-CF v1.0 generally overestimates aerosols by 20%-50% due to known issues in GEOS-Chem v12.0.1 that have been addressed in later versions. The 5-days forecasts have skill scores comparable to the 1-day hindcast. Model skills can be improved significantly by applying a bias-correction to the surface model output using a machine-learning approach.

Molecular markers for fungal spores and biogenic SOA over the Antarctic Peninsula: Field measurements and modeling results

Biogenic organic aerosols are important components of atmospheric organic aerosols and play vital roles in atmospheric chemistry, global climate, and biogeochemical cycles of carbon. However, studies on biogenic organic aerosols in the vast regions of the Southern Ocean and over the coastal waters of the Antarctic, especially Antarctic Peninsula, are still extremely limited. To understand the concentrations, molecular composition and seasonality of biogenic organic aerosols in Antarctica, atmospheric aerosols were collected at the Palmer Station on the west Antarctic Peninsula experiencing dramatic climate warming. Molecular marker compounds of fungal spores and secondary organic aerosols formed from the photooxidation of isoprene and monoterpene were analyzed using gas chromatography/mass spectrometry. Concentrations of sugar alcohols and biogenic SOA tracers both presented seasonal patterns with higher average concentrations in summer (90.7 and 122 pg m^-3) than in winter (8.88 and 57.2 pg m^-3). Sugar alcohols and biogenic SOA tracers were predominated by mannitol and isoprene oxidation products. Relative contributions of fungal-spore organic carbon (OC), isoprene-derived secondary OC (SOC) and monoterpene-derived SOC estimated with tracer-based methods were 26.2%, 55.6% and 18.2%, respectively. The observed seasonality of total biogenic SOA and some molecular species at the Antarctic Peninsula was further supported by the results from the global model CESM/IMPACT. Model results also suggest higher biogenic SOA in East Antarctica than that in West Antarctica, which is attributed to the influence of vertical atmospheric circulation. Our results of air-mass trajectory indicate the potential influence of marine emissions on the biogenic organic aerosols over the Antarctic Peninsula.

Modern mass spectrometry in atmospheric sciences: Measurement of volatile organic compounds in the troposphere using proton-transfer-reaction mass spectrometry

Volatile organic compounds (VOCs) in the troposphere are emitted from a wide variety of natural and man-made sources and resulting in environmental issues such as air pollution and climate change. Proton-transfer-reaction mass spectrometry (PTR-MS), a chemical ionization mass spectrometry technique using H₃ O⁺ reagent ions, allows real-time measurements of various VOCs in air with high sensitivity (parts-per-trillion to parts-per-billion level) and fast time response (<1 s). Modern PTR-MS equipped with time-of-flight mass analyzers can measure hundreds of compounds simultaneously. The applications of PTR-MS have greatly promoted understanding VOC sources and their roles in environmental issues, and therefore, PTR-MS has become a well-established analytical technique for in situ VOC measurements in atmospheric sciences. Here, the principles and implementation of PTR-MS are described. We additionally highlight modern PTR-MS techniques for accurate identification and comprehensive quantification of various VOCs. Finally, recent applications for atmospheric science are presented, using examples from research in biomass burning emissions and chemistry.

PM2.5 characterization of primary and secondary organic aerosols in two urban-industrial areas in the East Mediterranean

Primary and secondary organic aerosols in PM_2.5 were investigated over a one-year campaign at Zouk Mikael and Fiaa, Lebanon. The n-alkanes concentrations were quite similar at both sites (26-29 ng/m³) and mainly explained by anthropogenic emissions rather than natural ones. The concentrations of total Polycyclic Aromatic Hydrocarbons (PAHs) were nearly three times higher at Zouk Mikael (2.56 ng/m³) compared to Fiaa (0.95 ng/m³), especially for indeno[1,2,3-c,d]pyrene linked to the presence of the power plant. A characteristic indeno[1,2,3-c,d]pyrene/(indeno[1,2,3-c,d]pyrene + benzo[g,h,i]perylene) ratio in the range 0.8-1.0 was determined for heavy fuel oil combustion from the power plant. Fatty acids and hopanes were also investigated and were assigned to cooking activities and vehicular emissions respectively. Phthalates were identified for the first time in Lebanon with high concentrations at Zouk and Fiaa (106.88 and 97.68 ng/m³ respectively). Moreover, the biogenic secondary aerosols revealed higher concentrations in summer. The total terpene concentration varied between 131 ng/m³ at Zouk Mikael in winter to 469 ng/m³ at Fiaa in summer. Additionnally, the concentrations of the dicarboxylic acids especially for adipic and phthalic acids were more influenced by anthropogenic sources.The analysis of molecular markers and diagnostic ratios indicated that the sites were strongly affected by anthropogenic sources such as waste open burning, diesel private generators, cooking activities, road transport, power plant, and industrial emissions. Moreover, results showed different pattern during winter and summer seasons. Whereas, higher concentrations of biogenic markers were clearly encountered during the summer period.

Primary and secondary organic aerosol in an urban/industrial site: Sources, health implications and the role of plastic enriched waste burning

PM10 samples were collected from an urban/industrial site nearby Athens, where uncontrolled burning activities occur. PAHs, monocarboxylic, dicarboxylic, hydroxycarboxylic and aromatic acids, tracers from BVOC oxidation, biomass burning tracers and bisphenol A were determined. PAH, monocarboxylic acids, biomass burning tracers and bisphenol A were increased during autumn/winter, while BSOA tracers, dicarboxylic- and hydroxycarboxylic acids during summer. Regarding aromatic acids, different sources and formation mechanisms were indicated as benzoic, phthalic and trimellitic acids were peaked during summer whereas p-toluic, isophthalic and terephthalic were more abundant during autumn/winter. The Benzo[a]pyrene-equivalent carcinogenic power, carcinogenic and mutagenic activities were calculated showing significant (p < 0.05) increases during the colder months. Palmitic, succinic and malic acids were the most abundant monocarboxylic, dicarboxylic and hydrocarboxylic acids during the entire sampling period. Isoprene oxidation was the most significant contributor to BSOA as the isoprene-SOA compounds were two times more abundant than the pinene-SOA (13.4 ± 12.3 and 6.1 ± 2.9 ng/m³, respectively). Ozone has significant impact on the formation of many studied compounds showing significant correlations with: isoprene-SOA (r = 0.77), hydrocarboxylic acids (r = 0.69), pinene-SOA (r = 0.63),dicarboxylic acids (r = 0.58), and the sum of phthalic, benzoic and trimellitic acids (r = 0.44). PCA demonstrated five factors that could explain sources including plastic enriched waste burning (30.8%), oxidation of unsaturated fatty acids (23.0%), vehicle missions and cooking (9.2%), biomass burning (7.7%) and oxidation of VOCs (5.8%). The results highlight the significant contribution of plastic waste uncontrolled burning to the overall air quality degradation.

Comparison of physical and chemical characteristics and oxidative potential of fine particles emitted from rice straw and pine stem burning

Agricultural burning and forest fires are common in Northeast Asia and contribute to the elevation of fine particulate pollution, which greatly affects air quality. In this study, chemical and physical attributes, as well as the oxidative potential of fine particles produced from rice straw and pine stem burning in a laboratory-scale chamber were determined. The burning of rice straw generated notably lower emissions of fine particles and elemental carbon (EC) than did the burning of pine stems. The longer retention of ultrafine particles was observed for rice straw burning likely caused by this material's longer period of initial flaming combustion. Organic carbon (OC), OC/EC, K⁺/OC, K⁺/EC, Zn, and alkanoic acid were higher in the fine particles of rice straw burning, while EC, K⁺/Cl^-, Fe, Cr, Al, Cu, and levoglucosan were higher for pine stem burning particles. Chemical data were consistent with a higher hygroscopic growth factor and cloud formation potential and lower amount of agglomerated soot for rice straw burning particles. Rice straw burning particles displayed an oxidative potential seven times higher than that of pine stems.

Molecular characteristics of organic compositions in fresh and aged biomass burning aerosols

Biomass burning (BB) is the most important source of primary organic aerosols (OA) in the atmosphere that has significant impact on local/regional air quality and human health. However, few studies paid attention to the evolution of molecular characteristics of BB OA in the atmospheric aging processes. In this study, both fresh and aged PM_2.5 aerosols from burning of rice, maize, and wheat straws were collected from a combined system of combustion chamber and oxidation flow reactor, and were analyzed for >100 organic species. The emission factors (EFs) of anhydrosugars and some fatty acids showed slight variations between fresh and aged samples, indicating that these compounds are relatively stable. However, the EFs of n-alkanes, fatty alcohols, and parent-PAHs decreased 8-57% from fresh to aged samples, suggesting that they can undergo further oxidation to form other organic materials in the atmosphere. Phthalic acids, nitrophenols and isoprene-derived products were mainly secondarily formed by aging processes. Thus their EFs increased by 2-23 times from fresh to aged samples. Levoglucosan was the most abundant individual organic tracer, and its EF varied slightly between fresh and aged samples, proving its indicative role on BB emission. Moreover, the ratio of vanillic acid to levoglucosan and p-hydroxybenzoic acid to levoglucosan increased 2-13 times from fresh to aged samples. Therefore they can be used to investigate the impact of aging processes on BB aerosols in the atmosphere. RO₂ + HO₂ pathway derived 2-methyltetrols (2-MTs) predominated the EFs of isoprene-derived products (SOA_i) in the fresh samples. However, RO₂ + NO pathway derived 2-methylglyceric acid (2-MGA) increased by >30 times and became comparable with 2-MTs in aged particles. The ratio of 2-MGA/2-MTs increased from 0.06-0.27 in fresh samples to 0.94-1.18 in aged samples, because the high loading of NO_x in BB smoke enhanced the formation of SOA_i through RO₂ + NO reactions.

Chemical characteristics and sources of water-soluble organic aerosol in southwest suburb of Beijing

PM_2.5 filter sampling and components measurement were conducted in autumn and winter from 2014 to 2015 at a suburban site (referred herein as "LLH site") located in the southwest of Beijing. The offline aerosol mass spectrometry (offline-AMS) analysis and positive matrix factorization (PMF) were applied for measurement and source apportionment of water-soluble organic aerosol (WSOA). Organic aerosol (OA) always dominated PM_2.5 during the sampling period, especially in winter. WSOA pollution was serious during the polluted period both in autumn (31.1 µg/m³) and winter (31.9 µg/m³), while WSOA accounted for 54.4% of OA during the polluted period in autumn, much more than that (21.3%) in winter. The oxidation degree of WSOA at LLH site was at a high level (oxygen-to-carbon ratio, O/C=0.91) and secondary organic aerosol (SOA) contributed more mass ratio of WSOA than primary organic aerosol (POA) during the whole observation period. In winter, coal combustion OA (CCOA) was a stable source of OA and on average accounted for 25.1% of WSOA. In autumn, biomass burning OA (BBOA) from household combustion contributed 38.3% of WSOA during polluted period. In addition to oxygenated OA (OOA), aqueous-oxygenated OA (aq-OOA) was identified as an important factor of SOA. During heavy pollution period, the mass proportion of aq-OOA to WSOA increased significantly, implying the significant SOA formation through aqueous-phase process. The result of this study highlights the concentration on controlling the residential coal and biomass burning, as well as the research needs on aqueous chemistry in OA formation.

The formation and evolution of secondary organic aerosol during haze events in Beijing in wintertime

A regional air quality model system (RAQMS) with a volatility basis set approach for secondary organic aerosol (SOA) formation and an emission inventory of semi-volatile (SVOC) and intermediate volatile organic compounds (IVOC) are applied to investigate the distribution and evolution of organic aerosols over the Beijing-Tianjin-Hebei (BTH) region in winter 2014, with focus on Beijing. Model validation demonstrates the model is capable of reproducing meteorological variables and major aerosol components, and the model significantly improves SOA and organic aerosol (OA) simulations by taking S/IVOCs (SVOC + IVOC) and relevant aging processes into account. SVOC and IVOC emissions in the BTH region are estimated to be 0.47 Tg and in a range of 0.09-0.36 Tg, respectively, which are about 18% and 4-14% of the emission amounts of volatile organic compounds (VOCs). The distribution of mean organic aerosols is characterized by a high concentration belt oriented southwest-northeast from southern Hebei to Beijing, with the maximum concentration up to 50 μg m^-3 in Beijing and Shijiazhuang. The simulated SOA concentration is comparable in magnitude to primary organic aerosol (POA) concentration, and the SOA/OA ratio is around 50% in most areas of the BTH region. In terms of domain average, the percentage contributions to SOA mass concentration from anthropogenic volatile organic compounds (AVOCs), SVOCs, IVOCs and biogenic VOCs are estimated to be 46.1%, 40.1%, 9.4% and 4.4%, respectively, in the BTH region during the study period, which indicates an important role of S/IVOCs in SOA formation. From clean to haze periods, both POA and SOA concentrations apparently increase, with an increasing (decreasing) trend of the SOA/OA (POA/OA) ratio. SOA dominates over POA in fine organic aerosols during the haze periods. The increase of POA in hazy days is mainly due to the weakened vertical diffusion and accumulation near the surface, whereas the increase of SOA is likely attributed to both the reduced diffusivity and a series of competing chemical processes, in which the decreased photolysis rate by aerosol attenuation tends to decrease SOA concentration by about 6% during the most severe haze day, whereas the lower surface air temperature and higher POA and S/IVOC concentrations in haze days both enhance gas to particle partition, and consequently lead to higher SOA concentration.

A perspective on the development of gas-phase chemical mechanisms for Eulerian air quality models

An essential component of a three-dimensional air quality model is its gas-phase mechanism. We present an overview of the necessary atmospheric chemistry and a discussion of the types of mechanisms with some specific examples such as the Master Chemical Mechanism, the Carbon Bond, SAPRC and the Regional Atmospheric Chemistry Mechanism (RACM). The first versions of the Carbon Bond and SAPRC mechanisms were developed through a hierarchy of chemical species approach that relied heavily on chemical environmental chamber data. Now a new approach has been proposed where the first step is to develop a highly detailed explicit mechanism such as the Master Chemical Mechanism and the second step is to test the detailed explicit mechanism against laboratory and field data. Finally, the detailed mechanism is condensed for use in a three-dimensional air quality model. Here it is argued that the development of highly detailed explicit mechanisms is very valuable for research, but we suggest that combining the hierarchy of chemical species and the detailed explicit mechanism approaches would be better than either alone.Implication: Many gas-phase mechanisms are available for urban, regional and global air quality modeling. A "hierarchy of chemical species approach," relying heavily on smog-chamber data was used for the development of the early series of mechanisms. Now the development of large, explicit master mechanisms that may be condensed is a significant, trend. However, a continuing problem with air quality mechanism development is due to the high complexity of atmospheric chemistry and the current availability of laboratory measurements. This problem requires a balance between completeness and speculation so that models maintain their utility for policymakers.

Insights into the origin and evolution of carbonaceous aerosols in a mediterranean urban environment

Organic carbon (OC) and elemental carbon (EC) concentrations were measured in PM₁ and PM₁₀ daily samples collected at an urban station in Elche (southeastern Spain) from February 2015 to February 2018. The effect of seasonal weather conditions, traffic, and specific pollution events (Saharan dust outbreaks and local pollution episodes) on the variability of carbonaceous aerosol levels was studied in this work. The joint contribution of carbonaceous species to PM₁ and PM₁₀ mass concentrations was, respectively, 48% and 26%. Both OC and EC concentrations were higher in winter than in summer because of the poor dispersion conditions and lower temperatures leading to the condensation of semivolatile species. Secondary organic carbon (SOC), estimated using the EC tracer method, also exhibited higher concentrations during winter, indicating that the prevailing meteorological conditions during the cold season are more favorable for the formation of secondary organic aerosols. Our results suggest different formation pathways of secondary organic components during summer and winter. At the sampling site, EC was primarily derived from traffic emissions, independently of the season and the type of event, with a modest contribution from biomass burning (<20%). The estimated contribution from this source to OC levels was similar. Local pollution episodes lead to a significant increase in the concentrations of carbonaceous species, in particular of SOC, influencing its temporal variation. On average, African dust outbreaks showed a moderate impact on the levels of carbonaceous aerosols; however, the effect was significantly stronger during winter Saharan events.

Impact of Aerosol-Cloud Cycling on Aqueous Secondary Organic Aerosol Formation

Chemical processing of organic material in aqueous atmospheric aerosols and cloudwater is known to form secondary organic aerosols (SOA), although the extent to which each of these processes contributes to total aerosol mass is unclear. In this study, we use GAMMA 5.0, a photochemical box model with coupled gas and aqueous-phase chemistry, to consider the impact of aqueous organic reactions in both aqueous aerosols and clouds on isoprene epoxydiol (IEPOX) SOA over a range of pH for both aqueous phases, including cycling between cloud and aerosol within a single simulation. Low pH aqueous aerosol, in the absence of organic coatings or other morphology which may limit uptake of IEPOX, is found to be an efficient source of IEPOX SOA, consistent with previous work. Cloudwater at pH 4 or lower is also found to be a potentially significant source of IEPOX SOA. This phenomenon is primarily attributed to the relatively high uptake of IEPOX to clouds as a result of higher water content in clouds as compared with aerosol. For more acidic cloudwater, the aqueous organic material is comprised primarily of IEPOX SOA and lower-volatility organic acids. Both cloudwater pH and the time of day or sequence of aerosol-to-cloud or cloud-to-aerosol transitions impacted final aqueous SOA mass and composition in the simulations. The potential significance of cloud processing as a contributor to IEPOX SOA production could account for discrepancies between predicted IEPOX SOA mass from atmospheric models and measured ambient IEPOX SOA mass, or observations of IEPOX SOA in locations where mass transfer limitations are expected in aerosol particles.

Spherical tarball particles form through rapid chemical and physical changes of organic matter in biomass-burning smoke

Biomass burning (BB) emits enormous amounts of aerosol particles and gases into the atmosphere and thereby significantly influences regional air quality and global climate. A dominant particle type from BB is spherical organic aerosol particles commonly referred to as tarballs. Currently, tarballs can only be identified, using microscopy, from their uniquely spherical shapes following impaction onto a grid. Despite their abundance and potential significance for climate, many unanswered questions related to their formation, emission inventory, removal processes, and optical properties still remain. Here, we report analysis that supports tarball formation in which primary organic particles undergo chemical and physical processing within ∼3 h of emission. Transmission electron microscopy analysis reveals that the number fractions of tarballs and the ratios of N and O relative to K, the latter a conserved tracer, increase with particle age and that the more-spherical particles on the substrates had higher ratios of N and O relative to K. Scanning transmission X-ray spectrometry and electron energy loss spectrometry analyses show that these chemical changes are accompanied by the formation of organic compounds that contain nitrogen and carboxylic acid. The results imply that the chemical changes increase the particle sphericity on the substrates, which correlates with particle surface tension and viscosity, and contribute to tarball formation during aging in BB smoke. These findings will enable models to better partition tarball contributions to BB radiative forcing and, in so doing, better help constrain radiative forcing models of BB events.

Aging Effects on Biomass Burning Aerosol Mass and Composition: A Critical Review of Field and Laboratory Studies

Biomass burning is a major source of atmospheric particulate matter (PM) with impacts on health, climate, and air quality. The particles and vapors within biomass burning plumes undergo chemical and physical aging as they are transported downwind. Field measurements of the evolution of PM with plume age range from net decreases to net increases, with most showing little to no change. In contrast, laboratory studies tend to show significant mass increases on average. On the other hand, similar effects of aging on the average PM composition (e.g., oxygen-to-carbon ratio) are reported for lab and field studies. Currently, there is no consensus on the mechanisms that lead to these observed similarities and differences. This review summarizes available observations of aging-related biomass burning aerosol mass concentrations and composition markers, and discusses four broad hypotheses to explain variability within and between field and laboratory campaigns: (1) variability in emissions and chemistry, (2) differences in dilution/entrainment, (3) losses in chambers and lines, and (4) differences in the timing of the initial measurement, the baseline from which changes are estimated. We conclude with a concise set of research needs for advancing our understanding of the aging of biomass burning aerosol.

Kinetic and Theoretical Study of the Atmospheric Aqueous-Phase Reactions of OH Radicals with Methoxyphenolic Compounds

Methoxyphenols, which are emitted through biomass burning, are an important species in atmospheric chemistry. In the present study, temperature-dependent aqueous-phase OH radical reactions of six methoxyphenols and two related phenols have been investigated through laser flash photolysis and the density functional theory. The rate constants obtained were in a range of (1.1-1.9) × 10¹⁰ L mol^-1 s^-1 with k(3-MC) > k(Cre) ≈ k(Syr) ≈ k(MEP) > k(Res) > k(3-MP) > k(2-EP) ≈ k(2-MP). We derived the parameters of these reactions from the obtained T-dependent rate constants and found a mean Arrhenius activation energy of 16.9 kJ mol^-1. The diffusion rate constants were calculated for each case and compared to the measured ones. Generally, the rate constants are found to be close to fully diffusion-controlled (k_diff = (1.4-1.5) × 10¹⁰ L mol^-1 s^-1 for all reactions). A structure-function relationship was established through the measurement result, which could be used for predicting unknown rate constants of other phenolic compounds. All of these findings are expected to enhance the predictive capabilities of models, such as the chemical aqueous-phase radical mechanism.

Application of multivariate statistical analysis using organic compounds: Source identification at a local scale (Napajedla, Czechia)

The study aimed to apply novel source classification tool for local scale air pollution assessment reducing the total number of organic compounds in the model. Samples of particulate matter (PM) were collected in the town of Napajedla (South-eastern Czech Republic) in 2016. The industrial sector of the town is represented by plastics processing and manufacturing, as well as by mechanical engineering. Analytical technique of pyrolysis chromatography with mass spectroscopy detection was employed to identify organic species in the PM₁₀ fraction. Two datasets (465 determined organic compounds and 50 selected organic markers) were used and compared by multivariate analysis - principal component analysis followed with hierarchical clustering on principal components incorporating compositional data approach. Three resulting clusters were observed in both cases. The cluster representing measurements near plastic processing and manufacturing plants was identical in both the analysed datasets with the same organic compounds that characterized resulting cluster Consequently, leading markers for plastic processing and manufacturing sources were suggested (bumetrizole, bis(tridecyl)phthalate, mono(2-ethylhexyl)phthalate). Other two clusters varied among the analysed datasets, however, dataset with selected markers showed more reliable outcomes. The results imply that concept of using only selected organic marker species with the compositional approach in multivariate statistical methods is sufficient and allows properly distinguishing the main air pollution sources between sampling locations even at a small urban scale.

Secondary Organic Aerosol Formation from Urban Roadside Air in Hong Kong

Motor vehicle emissions are an important but poorly constrained source of secondary organic aerosol (SOA). Here, we investigated in situ SOA formation from urban roadside air in Hong Kong during winter time using an oxidation flow reactor (OFR), with equivalent atmospheric oxidation ranging from several hours to several days. The campaign-average mass enhancement of OA, nitrate, sulfate, and ammonium upon OFR aging was 7.0, 7.2, 0.8, and 2.6 μg m^-3, respectively. To investigate the sources of SOA formation potential, we performed multilinear regression analysis between measured peak SOA concentrations from OFR and the concentrations of toluene that represent motor vehicle emissions and cooking OA from positive matrix factorization (PMF) analysis of ambient OA. Traffic-related SOA precursors contributed 92.3%, 92.4%, and 83.1% to the total SOA formation potential during morning rush hours, noon and early afternoon, and evening meal time, respectively. The SOA production factor (PF) was approximately 5.2 times of primary OA (POA) emission factor (EF) and the secondary particulate matter (PM) PF was approximately 2.6 times of primary particles EF. This study highlights the potential benefit of reducing secondary PM production from motor vehicle emissions in mitigating PM pollutions.

Formation and evolution of aqueous organic aerosols via concurrent condensation and chemical aging

We review recent results on the formation and evolution of aqueous organic aerosols via concurrent nucleation/condensation and chemical aging processes obtained mostly using the formalism of classical nucleation theory In this framework, an aqueous organic aerosol was modeled as a spherical particle of liquid solution of water and hydrophilic and hydrophobic condensable organic compounds; besides these compounds, the surrounding air contained some chemically reactive, non-condensable species. Hydrophobic organic molecules on the aerosol surface can be processed by chemical reactions with some atmospheric species; this affects the hygroscopicity of the aerosol and hence its ability to become a cloud droplet. Such processing is most probably triggered by atmospheric hydroxyl radicals that abstract hydrogen atoms from surfactant molecules located on the aerosol surface (first step), resulting radicals being quickly oxidized by ubiquitous atmospheric oxygen molecules to produce surface-bound peroxyl radicals (second step). These two reactions play a crucial role in the enhancement of the Köhler activation of the aerosol. Taking them and a third reaction (next in the multistep chain of relevant heterogeneous reactions) into account, one can derive an explicit expression for the free energy of formation of a four-component aqueous droplet on a ternary aqueous organic aerosol as a function of four independent variables of state of a droplet. This approach was also applied to study a large subset of primary marine aerosols which can be initially treated using an "inverted micelle" model whereof the core consists of aqueous "salt" solution. Numerical evaluations suggest that the formation of cloud droplets on such (both aqueous hydrophilic/hydrophobic organic and marine) aerosols is most likely to occur via Köhler activation rather than via nucleation. The models allow one to determine the threshold parameters necessary for the Köhler activation of such aerosols. Furthermore, heterogeneous chemical reactions involved in the chemical aging of aerosols are most likely exothermic. Due to the release of the enthalpy of these reactions, the temperature of an aerosol particle during its chemical aging may become greater than the ambient (air) temperature. The analysis of the characteristic timescales of four most important processes involved suggests that this effect may play a significant impeding role in the formation of an ensemble of aqueous secondary organic aerosols via nucleation and, hence, must be taken into account in atmospheric aerosol and global climate models.

Influence of biomass burning on atmospheric aerosols over the western South China Sea: Insights from ions, carbonaceous fractions and stable carbon isotope ratios

Total suspended particle (TSP) samples were collected during a cruise campaign over the western South China Sea (SCS) from August to September 2014. Ten water-soluble ions (WSI), organic carbon (OC), elemental carbon (EC) and stable carbon isotope ratios of total carbon (δ¹³C_TC) were measured. The average concentrations of total WSI, OC and EC were 7.91 ± 3.44 μg/m³, 2.04 ± 1.25 μg/m³ and 0.30 ± 0.22 μg/m³, respectively. Among the investigated WSI, sulfate (SO₄^2-), sodium (Na⁺) and chloride (Cl^-) were the most abundant species, accounting for 39.2%, 24.5% and 14.3% of the total mass of the WSI, respectively. Significantly positive correlations of OC and EC with non-sea-salt potassium (nss-K⁺), a tracer for biomass burning, suggest that biomass burning is the major source of carbonaceous aerosols. The values of δ¹³C_TC ranged from -26.6‰ to -24.4‰ with an average of -25.3 ± 0.7‰. Based on the literature data of δ¹³C_TC, back-trajectory analysis and satellite fire spots, we propose that C₃ plant burning in Southeast Asia significantly contributes to carbonaceous aerosols over the western SCS. This is also supported by a good correlation between δ¹³C_TC and the mass ratios of nss-K⁺/TC. Furthermore, high Cl^- depletion (73 ± 23%) was observed in the aerosols over the western SCS. Given the neutralization of SO₄^2- by ammonium (NH₄⁺), excess nss-SO₄^2- and oxalate (C₂O₄^2-) made major contributions to Cl^- depletion in the samples strongly influenced by biomass burning. This study provides useful information to better understand the influence of biomass burning on atmospheric aerosols over the SCS.

Photochemical Cloud Processing of Primary Wildfire Emissions as a Potential Source of Secondary Organic Aerosol

We investigated the gas-phase chemical composition of biomass burning (BB) emissions and their role in aqueous secondary organic aerosol (aqSOA) formation through photochemical cloud processing. A high-resolution time-of-flight chemical ionization mass spectrometer using iodide reagent ion chemistry detected more than 100 gas-phase compounds from the emissions of 30 different controlled burns during the 2016 Fire Influence on Regional and Global Environments Experiment (FIREX) at the Fire Science Laboratory. Compounds likely to partition to cloudwater were selected based on high atomic oxygen-to-carbon ratio and abundance. Water solubility was confirmed by detection of these compounds in water after mist chamber collection during controlled burns and analysis using ion chromatography and electrospray ionization interfaced to high-resolution time-of-flight mass spectrometry. Known precursors of aqSOA were found in the primary gaseous BB emissions (e.g., phenols, acetate, and pyruvate). Aqueous OH oxidation of the complex biomass burning mixtures led to rapid depletion of many compounds (e.g., catechol, levoglucosan, methoxyphenol) and formation of others (e.g., oxalate, malonate, mesoxalate). After 150 min of oxidation (approximatively 1 day of cloud processing), oxalate accounted for 13-16% of total dissolved organic carbon. Formation of known SOA components suggests that cloud processing of primary BB emissions forms SOA.

Additional Benefits of Federal Air-Quality Rules: Model Estimates of Controllable Biogenic Secondary Organic Aerosol

Atmospheric models that accurately describe the fate and transport of trace species for the right reasons aid in the development of effective air-quality management strategies that safeguard human health. Controllable emissions facilitate the formation of biogenic secondary organic aerosol (BSOA) to enhance the atmospheric fine particulate matter (PM2.5) burden. Previous modeling with the EPA's Community Multiscale Air Quality (CMAQ) model predicted that anthropogenic primary organic aerosol (POA) emissions had the greatest impact on BSOA. That experiment included formation processes involving semivolatile partitioning but not aerosol liquid water (ALW), a ubiquitous PM constituent. We conduct 17 summertime CMAQ simulations with updated chemistry and evaluate changes in BSOA due to the removal of individual pollutants and source sectors for the contiguous U.S. CMAQ predicts SO2 from electricity generating units, and mobile source NOX emissions have the largest impacts on BSOA. The removal of anthropogenic NOX, SO2, and POA emissions during the simulation reduces the nationally averaged BSOA by 23, 14, and 8% and PM2.5 by 9.2, 14, and 5.3%, respectively. ALW mass concentrations decrease by 10 and 35% in response to the removal of NOX and SO2 emissions. This work contributes chemical insight into ancillary benefits of Federal NOX and SO2 rules that concurrently reduce organic PM2.5 mass.

Spatial Distribution of Secondary Organic Aerosol Formation Potential in China Derived from Speciated Anthropogenic Volatile Organic Compound Emissions

Fine particulate matter (PM_2.5), largely composed of secondary organic aerosol (SOA), is currently one of the most intractable environmental problems in China. As crucial precursors for SOA, understanding the formation propensity of various volatile organic compound (VOC) species and sources is useful for pollution control. In this work, we estimated the SOA formation potential (SOAP) of anthropogenic VOC emissions based on an improved speciated VOC emission inventory and investigated its distribution in China. According to our estimates, toluene had the largest SOAP, followed by n-dodecane, m-/p-xylene, styrene, n-decane, and n-undecane, while passenger cars, chemical fiber manufacturing, asphalt paving, and building coating were the top five SOAP-contributing sources nationwide. The spatial distribution of SOAP in China shows a distinct pattern of high values in the southeast and low values in the northwest. Beijing-Tianjin-Hebei and surroundings, the Yangtze River Delta, Pearl River Delta, and Sichuan-Chongqing District were found to have the highest SOAP, particularly in urban areas. The major SOAP-contributing species and sources differed among these regions, which was attributed to local industrial and energy structures. Our results suggest that to mitigate PM_2.5 pollution in China, more efficient SOAP-based control measures should be implemented instead of current emissions-based policies, and VOC control strategies should be adapted to local conditions.

Seasonally Varying Secondary Organic Aerosol Formation From In-Situ Oxidation of Near-Highway Air

The extent to which motor vehicles contribute to ambient secondary organic aerosol (SOA) remains uncertain. Here, we present in situ measurements of SOA formation at a near-highway site with substantial tree-cover 10 m from Interstate 40 near Durham, North Carolina. In July 2015 (summer) and February 2016 (winter), we exposed ambient air to a range of oxidant (O₃ and OH) concentrations in an oxidation flow reactor (OFR), resulting in hours to weeks of equivalent atmospheric aging. We observed substantial seasonal variation in SOA formation upon OFR aging; diurnally varying OA enhancements of ∼3-8 μg m^-3 were observed in summer and significantly lower enhancements (∼0.5-1 μg m^-3) in winter. Measurements in both seasons showed consistent changes in bulk OA properties (chemical composition; volatility) with OFR aging. Mild increases in traffic-related SOA precursors during summer partly explains the seasonal variation. However, biogenic emissions, with sharp temperature dependence, appear to dominate summer OFR-SOA. Our analysis indicates that SOA observed in the OFR is similar (within a factor of 2) to that predicted to form from traffic and biogenic precursors using literature yields, especially in winter. This study highlights the utility of the OFR for studying the prevalence of SOA precursors in complex real-world settings.