Source Of Secondary Organic Aerosol Research Articles

Combined-phase source apportionment of ambient PM2.5, PAHs and VOCs from an industrialized environment: Consequences of photochemical initial concentrations

Air pollutants in the particulate (PM2.5 species, PAHs) and gaseous phases (VOCs) collected between 2013 and 2019 once every three or six days for a period of 24 hours in an industrialized city in Ontario were analyzed to apportion their common sources. The consequences of using these species jointly for receptor modelling were assessed via combined-phase source apportionment that used the data as is, and in a protocol that factored in the potential for photochemical losses of gas-phase species. Thus, photochemically corrected initial concentrations (PIC) were calculated. Analyses of the inputs followed either with positive matrix factorization or its dispersion-normalized variant (DN-PMF). Comparisons of applying PMF to the originally observed input data (BASE) and DN-PMF on data with PIC corrections were made. When the inputs consisted only of VOCs, three factors were resolved with BASE PMF: natural gas, vehicular emissions, and industrial emissions co-emitted with summertime gasoline evaporation. A fourth factor was obtained, representing reactive VOCs when DN-PIC PMF was used. When the combined phase input data were analyzed, nine factors were resolved for both BASE and DN-PIC PMF. These factors in order of diminishing average PM mass contributions were: particulate sulphate, secondary organic aerosol (SOA), particulate nitrate (pNO3), biomass burning with natural gas, crustal matter, winter blend of gasoline, coking/coal combustion, steelmaking, and summer blend/light duty vehicular emissions. When BASE and DN-PIC PMF results are compared, the average PM mass contribution of the summer gasoline fuel factor increased from 2% in BASE case to 5%, suggesting severe underestimation of this source's contributions without DN-PIC. Also, substantial increases of reactive VOCs in the SOA factor, and PAHs with ≥four rings in the pNO3 and steelmaking factors were observed with DN-PIC PMF compared to the BASE PMF case, indicating that for SOA, reactive VOCs at this location contributed to SOA sources.

Interaction between marine and terrestrial biogenic volatile organic compounds: Non-linear effect on secondary organic aerosol formation

Biogenic volatile organic compounds (BVOCs) are the largest source of secondary organic aerosols (SOA) globally. However, the complex interactions between marine and terrestrial BVOCs remain unclear, inhibiting our in-depth understanding of the SOA formation in the coastal areas and its environmental impacts. Here, we performed smog chamber experiments with mixed α-pinene (a typical monoterpene) and dimethyl sulfide (DMS, a typical marine emission BVOC) to investigate their possible interactions and subsequent SOA formation. It is found that DMS has a non-linear effect on SOA generation: The mass concentration and yield of SOA show increasing and then decreasing trends with the increase of the initial concentration of DMS. The increasing trend can be attributed to OH regeneration from isomerization of the CH3SCH2OO radical together with acid-catalyzed heterogeneous reactions by the oxidation of DMS, while the decreasing trend is explained by the less contribution of isomerization reaction and the high OH reactivity that inhibits the formation of low volatility products. The results from infrared spectra and mass spectra together reveal the contribution of sulfur-containing molecules in the mixed system. Moreover, the mass spectra results indicate that acidic products generated by DMS photooxidation enhance the O:C ratio, while organosulfates are produced to contribute to the formation of mixed SOA. In addition, the trends in relative abundance of highly oxygenated organic molecules (HOMs) with C8 - C10 multiple functional groups in different mixed systems agree well with the turning point of the SOA yield. The findings of this study have significant implications for understanding binary or more complex systems in the atmosphere in the coastal areas.

Substantial contribution of transported emissions to organic aerosol in Beijing

Haze in Beijing is linked to atmospherically formed secondary organic aerosol, which has been shown to be particularly harmful to human health. However, the sources and formation pathways of these secondary aerosols remain largely unknown, hindering effective pollution mitigation. Here we have quantified the sources of organic aerosol via direct near-molecular observations in central Beijing. In winter, organic aerosol pollution arises mainly from fresh solid-fuel emissions and secondary organic aerosols originating from both solid-fuel combustion and aqueous processes, probably involving multiphase chemistry with aromatic compounds. The most severe haze is linked to secondary organic aerosols originating from solid-fuel combustion, transported from the Beijing–Tianjing–Hebei Plain and rural mountainous areas west of Beijing. In summer, the increased fraction of secondary organic aerosol is dominated by aromatic emissions from the Xi’an–Shanghai–Beijing region, while the contribution of biogenic emissions remains relatively small. Overall, we identify the main sources of secondary organic aerosol affecting Beijing, which clearly extend beyond the local emissions in Beijing. Our results suggest that targeting key organic precursor emission sectors regionally may be needed to effectively mitigate organic aerosol pollution.

Large contribution of in-cloud production of secondary organic aerosol from biomass burning emissions

Organic compounds released from wildfires and residential biomass burning play a crucial role in shaping the composition of the atmosphere. The solubility and subsequent reactions of these compounds in the aqueous phase of clouds and fog remain poorly understood. Nevertheless, these compounds have the potential to become an important source of secondary organic aerosol (SOA). In this study, we simulated the aqueous SOA (aqSOA) from residential wood burning emissions under atmospherically relevant conditions of gas-liquid phase partitioning, using a wetted-wall flow reactor (WFR). We analyzed and quantified the specific compounds present in these emissions at a molecular level and determined their solubility in clouds. Our findings reveal that while 1% of organic compounds are fully water-soluble, 19% exhibit moderate solubility and can partition into the aqueous phase in a thick cloud. Furthermore, it is found that the aqSOA generated in our laboratory experiments has a substantial fraction being attributed to the formation of oligomers in the aqueous phase. We also determined an aqSOA yield of 20% from residential wood combustion, which surpasses current estimates based on gas-phase oxidation. These results indicate that in-cloud chemistry of organic gases emitted from wood burning can serve as an efficient pathway to produce organic aerosols, thus potentially influencing climate and air quality.

On the potential use of highly oxygenated organic molecules (HOMs) as indicators for ozone formation sensitivity

Abstract. Ozone (O3), an important and ubiquitous trace gas, protects lives from harmful solar ultraviolet (UV) radiation in the stratosphere but is toxic to living organisms in the troposphere. Additionally, tropospheric O3 is a key oxidant and a source of other oxidants (e.g., OH and NO3 radicals) for various volatile organic compounds (VOCs). Recently, highly oxygenated organic molecules (HOMs) were identified as a new compound group formed from the oxidation of many VOCs, making up a significant source of secondary organic aerosol (SOA). The pathways forming HOMs from VOCs involve autoxidation of peroxy radicals (RO2), formed ubiquitously in many VOC oxidation reactions. The main sink for RO2 is bimolecular reactions with other radicals, such as HO2, NO, or other RO2, and this largely determines the structure of the end products. Organic nitrates form solely from RO2 + NO reactions, while accretion products (“dimers”) form solely from RO2 + RO2 reactions. The RO2 + NO reaction also converts NO into NO2, making it a net source for O3 through NO2 photolysis. There is a highly nonlinear relationship between O3, NOx, and VOCs. Understanding the O3 formation sensitivity to changes in VOCs and NOx is crucial for making optimal mitigation policies to control O3 concentrations. However, determining the specific O3 formation regimes (either VOC-limited or NOx-limited) remains challenging in diverse environmental conditions. In this work we assessed whether HOM measurements can function as a real-time indicator for the O3 formation sensitivity based on the hypothesis that HOM compositions can describe the relative importance of NO as a terminator for RO2. Given the fast formation and short lifetimes of low-volatility HOMs (timescale of minutes), they describe the instantaneous chemical regime of the atmosphere. In this work, we conducted a series of monoterpene oxidation experiments in our chamber while varying the concentrations of NOx and VOCs under different NO2 photolysis rates. We also measured the relative concentrations of HOMs of different types (dimers, nitrate-containing monomers, and non-nitrate monomers) and used ratios between these to estimate the O3 formation sensitivity. We find that for this simple system, the O3 sensitivity could be described very well based on the HOM measurements. Future work will focus on determining to what extent this approach can be applied in more complex atmospheric environments. Ambient measurements of HOMs have become increasingly common during the last decade, and therefore we expect that there are already a large number of groups with available data for testing this approach.

Secondary Organic Aerosol Formation Potential from Vehicular Non-tailpipe Emissions under Real-World Driving Conditions.

Traffic emissions are a dominant source of secondary organic aerosol (SOA) in urban environments. Though tailpipe exhaust has drawn extensive attention, the impact of non-tailpipe emissions on atmospheric SOA has not been well studied. Here, a closure study was performed combining urban tunnel experiments and dynamometer tests using an oxidation flow reactor in situ photo-oxidation. Results show a significant gap between field and laboratory research; the average SOA formation potential from real-world fleet is 639 ± 156 mg kg fuel-1, higher than the reconstructed result (188 mg kg fuel-1) based on dynamometer tests coupled with fleet composition inside the tunnel. Considering the minimal variation of SOA/CO in emission standards, we also reconstruct CO and find the critical role of high-emitting events in the real-world SOA burden. Different profiles of organic gases are detected inside the tunnel than tailpipe exhaust, such as more abundant C6-C9 aromatics, C11-C16 species, and benzothiazoles, denoting contributions from non-tailpipe emissions to SOA formation. Using these surrogate chemical compounds, we roughly estimate that high-emitting, evaporative emission, and asphalt-related and tire sublimation share 14, 20, and 10% of the SOA budget, respectively, partially explaining the gap between field and laboratory research. These experimental results highlight the importance of non-tailpipe emissions to atmospheric SOA.

Uncovering the dominant contribution of intermediate volatility compounds in secondary organic aerosol formation from biomass-burning emissions.

Organic vapors from biomass burning are a major source of secondary organic aerosols (SOAs). Previous smog chamber studies found that the SOA contributors in biomass-burning emissions are mainly volatile organic compounds (VOCs). While intermediate volatility organic compounds (IVOCs) are efficient SOA precursors and contribute a considerable fraction of biomass-burning emissions, their contribution to SOA formation has not been directly observed. Here, by deploying a newly-developed oxidation flow reactor to study SOA formation from wood burning, we find that IVOCs can contribute ∼70% of the formed SOA, i.e. >2 times more than VOCs. This previously missing SOA fraction is interpreted to be due to the high wall losses of semi-volatile oxidation products of IVOCs in smog chambers. The finding in this study reveals that SOA production from biomass burning is much higher than previously thought, and highlights the urgent need for more research on the IVOCs from biomass burning and potentially other emission sources.

Impacts of α-pinene ozonolysis products on the morphology and optical properties of black carbon

Black carbon (BC) is known to absorb solar radiation and thus have a warming impact on climate. The atmospheric aging of fresh BC has been suggested to enhance BC's light absorption, but the extent of variation observed varies largely in field and laboratory measurements. Biogenic VOCs with much higher abundance than anthropogenic emissions are reactive towards atmospheric oxidants, and act as major sources of secondary organic aerosol (SOA) in the atmosphere. In this study, we performed experiments of BC aging in the presence of condensable organic vapors generated from the ozonolysis of α-pinene. Significant changes in the particle size, mass, effective density, morphology as well as optical properties are observed upon coating of BC by α-pinene ozonolysis products. While the BC aging rate depends on the initial concentrations of precursors forming condensable materials and the particle size, the extent of transformation of BC at a given size is mainly controlled by the volume equivalent coating thickness. The highly fractal BC core is converted to a more compact morphology for a coating thickness of ∼25–30 nm depending on the initial particle size. The absorption and scattering of light by BC also increase by more than 1.29- and 5.30- fold during the coating process. The enhancement of optical properties can be attributed to both formation of coatings and restructuring of the BC core, with the latter accounting for a large fraction of optical enhancement. Our results indicate that the ozonolysis of α-pinene has profound effects on the morphology and optical properties of BC, with a magnitude comparable to sulfuric acid and ammonium nitrate, but much higher than other reported anthropogenic hydrocarbons. Although present in relatively low concentrations, α-pinene oxidation reactions can considerably enhance the impacts of BC on air quality and climate forcing.

Floating reactive barriers to mitigate secondary organic aerosol formation from oil sands tailings ponds

Oil sands operations to recover bitumen in Alberta, Canada generate large volumes of contaminated water which are stored in tailings ponds. These ponds emit large quantities of intermediate / semi- volatile organic compounds (IVOC/SVOCs) which upon emission can react or partition to form secondary organic aerosols (SOAs), a significant issue concerning human and environmental health. These ponds are one of the largest sources of SOAs in North America, and the responsible VOCs (polycyclic aromatic hydrocarbons (PAHs), alkanes, single ring aromatics) are difficult to treat. Passive technologies that can reduce aqueous emissions and degrade volatiles in-situ are desired. A promising technology is buoyant photocatalysts (BPCs), which float on the surface of water bodies and absorb sunlight to oxidize contaminants. In this work, a glass microsphere TiO2-coated BPC is studied, which forms a semi-porous floating reactive barrier on the surface of water, intercepting VOC emissions and photocatalytically oxidizing them in water before they can be released to air. Based on a group of F2 – F3 hydrocarbons, 16 PAHs (C10 – C22) and 8 alkanes (C11-C18), this BPC technology reduced 89% of these SOA precursor emissions while degrading 84% of the compounds in the system under 2 days of simulated sunlight. In a synthetic tailings pond environment, BPCs were demonstrated to block and treat VOCs of particular concern well (phenanthrene, pyrene, chrysene), while further preventing emissions from free phase hydrocarbons. In context of tailings pond SOA precursor emissions, application of this floating reactive barrier technology to all ponds could reduce the environmental impact of oil sands operations by preventing release of between 2400 to 11000 tonnes of SOAs per year. The proven ability to intercept and treat VOCs in water extends the potential of this technology past the oil sands industry, providing one of the first options for passive fugitive emission management for various industrial water bodies. SynopsisBuoyant Photocatalysts permanently prevent aqueous gas emissions and likely subsequent secondary organic aerosol formation by preventing release while oxidizing aqueous volatiles.

Direct formation of HONO through aqueous-phase photolysis of organic nitrates

Abstract. Organic nitrates (RONO2) are secondary compounds whose fate is closely related to the transport and removal of NOx in the atmosphere. Despite their ubiquitous presence in submicron aerosols, the photochemistry of RONO2 has only been investigated in the gas phase, leaving their reactivity in condensed phases poorly explored. This work aims to address this gap by investigating, for the first time, the reaction products and the mechanisms of aqueous-phase photolysis of four RONO2 (i.e., isopropyl nitrate, isobutyl nitrate, α-nitrooxy acetone, and 1-nitrooxy-2-propanol). The results show that the reactivity of RONO2 in the aqueous phase differs significantly from that in the gas phase. In contrast to the gas phase, where RONO2 release NOx upon photolysis, the aqueous-phase photolysis of RONO2 leads primarily to the direct formation of nitrous acid (HONO or HNO2), which was confirmed by quantum chemistry calculations. Hence, the aqueous-phase photolysis of RONO2 represents both a NOx sink and a source of atmospheric nitrous acid, a significant precursor of ⋅ OH and ⋅ NO. These secondary radicals (⋅ OH and ⋅ NO) are efficiently trapped in the aqueous phase, leading to the formation of HNO3 and functionalized RONO2. This reactivity can thus potentially contribute to the aging of secondary organic aerosol (SOA) and serves as an additional source of aqueous-phase SOA.

On the sources of ambient SOA in PM2.5: An integrated analysis over Jinan city of China

Secondary organic aerosol (SOA) accounts for a significant fraction of fine particulate matter (PM2.5) and is a crucial component that affects climate and public health. However, the formation of SOA in urban areas remains unclear and sources contributed to SOA are poorly characterized. In this study, we combined ambient measurements and the Weather Research and Forecasting (WRF)-Community Multi-scale Air Quality (CMAQ) modeling system to investigate the evolution of SOA during winter over Jinan city in China. Relative contributions of different anthropogenic sources to ambient SOA were subsequently probed by the Integrated Source Apportionment Method (ISAM) to explore the impact of anthropogenic emissions. The mean concentration of SOA was 11.2 μg m−3, accounting for 29.4% in PM2.5, and SOA concentrations on clean days (4.9 μg m−3) increased by 4.8 times on polluted days (23.3 μg m−3), and strong correlation between the elevated PM2.5 episodes and increased SOA concentrations was found (R2 > 0.5). In addition, SOA concentrations and the proportion of SOA in PM2.5 were higher during the daytime (24.9 μg m−3, 32.4%) compared to nighttime (21.9 μg m−3, 25.1%) on polluted days, which is primarily attributed to the intense atmospheric photochemical reactions and anthropogenic activities. Mobile sources act as the dominant contributor to SOA (57.6%) and road diesel emissions (30.9%) made the largest contribution to SOA, followed by non-road mobile sources (17.4%) among subsectors in mobile sources. This study sheds new insights into ambient SOA formation and SOA source apportionment for typical megacities in China, guiding the design of source-oriented regulation efforts on SOA mitigation.

Primary and Secondary Organic Aerosol Formation from Asphalt Pavements.

Asphalt is ubiquitous across cities and a source of organic compounds spanning a wide range of volatility and may be an overlooked source of urban organic aerosols. The emission rate and composition depend strongly on temperature, but emissions have been observed at both application temperatures and surface temperatures during warm sunny days. Here we report primary organic aerosol (POA) emissions and secondary organic aerosol (SOA) production from asphalt. We reheated real-world asphalt samples to application-relevant temperatures (∼130 °C) and typical summertime road-surface temperatures (∼55 °C) and then flushed the emitted vapors into an environmental oxidation chamber containing ammonium sulfate seed particles. SOA was then formed following the photo-oxidation of emissions under high-NOx conditions typical of urban atmospheres. We find that POA only forms at application temperature as it does not require further oxidation, whereas SOA forms under both conditions; with the resulting POA and SOA both being semi-volatile. While total OA formation rates were substantially greater under the limited time spent under application conditions, SOA formation from passive asphalt heating presents a potential long-term source, as heating continues for the lifetime of the road surface. This suggests that persistent asphalt solar heating is likely a considerable and continued source of summertime SOA in urban environments.

Effects of driving conditions on aerosol formation from photooxidation of gasoline vehicles exhaust in Hong Kong

The emissions before and after atmospheric oxidation from three gasoline vehicles were characterized under different driving conditions (idling, cruising and New European Driving Cycle (NEDC)). The total quantified volatile organic compounds (VOCs) emission factors for the vehicles ranged from 18.4 to 2728.7 mg/kg depending on driving conditions. The organic aerosol mass concentrations showed substantial increase (∼7.6–10.8 times) after ageing process, implying that vehicular emissions were an important source of secondary organic aerosols (SOAs). The total quantified organic aerosol mass concentrations in aged samples ranged from 77.7 to 480.5 ng m−3 under different test conditions. Aromatic and alkenoic acids were identified as major organic aerosols after ageing. The emission factors of VOCs were higher under idling condition compared to the other driving modes for all vehicles. High VOCs emissions can potentially lead to high secondary organic aerosol (SOA) formation inside the reactor and more robust SOA formation under idling condition in all analysis. The gasoline direct injection (GDI) vehicle with latest year of manufacture in EURO 4 standard can pose lower primary emissions of organic substances, but nevertheless the composition of emitted VOCs demonstrates higher SOA yield and SOA formation. Further analysis proposes engine type is the key factor on primary VOC composition under idling. The findings suggest a need to revise existing regulation of composition of organic substances.

Sequential Interaction of Biogenic Volatile Organic Compounds and SOAs in Urban Forests Revealed Using Toeplitz Inverse Covariance-Based Clustering and Causal Inference

Urban forests play a crucial role in both emitting and absorbing atmospheric pollutants. Understanding the ecological processes of biogenic volatile organic compounds (BVOCs) and secondary organic aerosols (SOAs) and their interactions in urban forests can help to assess how they influence air quality. Additionally, exploring the adaptation and feedback mechanisms between urban forests and their surrounding environments can identify new pollutants and potential risks in urban forests. However, the relationship between BVOC emissions and SOA formation is complex due to the influence of meteorological conditions, photochemical reactions, and other factors. This complexity makes it challenging to accurately describe this relationship. In this study, we used time-of-flight mass spectrometry and aerosol particle size spectrometry to monitor concentrations of BVOCs and particulate matter with a diameter less than 1 µm (PM1; representing SOAs) at a frequency of 10–12 times per min in an urban forest near Shanghai. We then analyzed the temporal changes in concentrations of BVOCs, SOAs, and other chemical pollutants in different periods of the day by using subsequence clustering and causal inference methods. The results showed that after using this method for diurnal segmentation, PM1 prediction accuracy was improved by 26.77%–47.51%, and the interaction rules of BVOCs and SOAs had sequential interaction characteristics. During the day, BVOCs are an important source of SOAs and have a negative feedback relationship with O3. From night to early morning, BVOCs have a positive, balanced relationship with O3, SOAs are affected by wind speed or deposition, BVOCs have no obvious relationship with O3, and SOAs are affected by temperature or humidity. This study is the first to apply Toeplitz inverse covariance-based clustering and causal inference methods for the high-frequency monitoring of BVOCs and SOAs, revealing the temporal effects and characteristics of BVOCs and SOAs and providing a scientific basis and new methods for understanding the dynamic effects of urban forest communities on the environment.

Real-world emission characteristics and inventory of volatile organic compounds originating from construction and agricultural machinery

Volatile organic compound (VOC) emissions originating from nonroad mobile sources constitute an important but uncertain source of secondary organic aerosols (SOAs) and ozone (O3). In this study, we investigated the emission factors (EFs) of 120 individual VOC species for 40 machines via gas chromatography–mass spectrometry/flame ionization detection and high-performance liquid chromatography. The results showed that the diesel-based VOC EF for the tested machines was 4.18 ± 2.55 (average ± standard deviation) g/kg fuel, dominated by alkanes (38.20 % ± 18.08 %) and oxygenated VOCs (OVOCs; 30.94 % ± 15.71 %). The machine type, rated power, emission standards, and operating conditions affected the emissions of VOCs and their components, and this effect maybe mostly depends on the fuel combustion efficiency. The VOC species were primarily distributed in the C1–C2 and C4–C6 (based on the carbon number) and B4–B6 (based on the saturated vapor concentration) intervals. Furthermore, the estimated formation potential (FP) values of SOAs and O3 from VOCs were 21.02 ± 15.57 mg/kg fuel and 15.96 ± 11.87 g/kg fuel, respectively. VOC control based on the SOA formation potential (SOAFP) and ozone formation potential (OFP) could be more effective in the mitigation of fine particulate matter (PM2.5) and O3 pollution because the top 5 species ranked by percentage contribution accounted for 83.09 % ± 9.59 % and 51.78 % ± 14.38 % of the estimated SOAFP and OFP, respectively. Finally, the emission estimates showed that the VOC emissions originating from construction and agricultural machinery in China (2020) reached 64.05 and 95.24 Gg, respectively. We provide species-specific VOC EFs and detailed emission characteristics to facilitate a comprehensive understanding of gas emissions originating from nonroad mobile sources and an update of emission inventories and atmospheric chemistry models.

Distribution characteristics of secondary organic aerosol tracers in PM2.5 in Jinzhong

In this study, 13 secondary organic aerosol (SOA) tracers in PM2.5 were measured in Jinzhong, Shanxi, to investigate the distribution characteristics of SOAs. According to the results, isoprene SOA tracers had the highest annual average concentration at 7.35 ± 14.23 ng/m3, accounting for 57.8% of all tracers by concentration, followed by β-caryophyllene SOA tracer (3.53 ± 2.67 ng/m3, 22.6%), α/β-pinene SOA tracers (2.68 ± 2.23 ng/m3, 18.9%) and toluene SOA tracer (0.11 ± 0.08 ng/m3, 0.7%). The concentrations of isoprene and α/β-pinene SOA tracers in summer and autumn were higher than those in winter and spring, whereas the concentration of β-caryophyllene SOA tracer was higher in autumn and winter than in spring and summer. However, no statistically significant seasonal variation was observed in the concentration of the toluene SOA tracer. According to results for pinolic acid and 3-methyl-1,2,3-butanetricarboxylic acid, α/β-pinene SOA tracers in Jinzhong were fresh in winter and the contribution ratio of β-pinene to SOA formation was higher than that of α-pinene. A correlation analysis also indicated that the high concentrations of isoprene and α/β-pinene SOA tracers in summer were correlated with temperature, whereas the high concentration of the β-caryophyllene SOA tracer in autumn and winter was primarily correlated with biomass burning. According to tracer yield estimation results, plant-derived volatile organic compounds were the main source of SOAs in PM2.5 in Jinzhong, with a contribution ratio of 94%. Isoprene contributed the most to secondary organic carbon concentrations in spring and summer, whereas β-caryophyllene contributed the most in autumn and winter.

Water Plays Multifunctional Roles in the Intervening Formation of Secondary Organic Aerosols in Ozonolysis of Limonene: A Valence Photoelectron Spectroscopy and Density Functional Theory Study

Although water may affect aqueous aerosol chemistry, how it intervenes in the formation of secondary organic aerosols (SOAs) at the molecular level remains elusive. Ozonolysis of limonene is one of the most important sources of indoor SOAs. Here, we report the valence electronic properties of limonene aerosols and SOAs derived from limonene ozonolysis (Lim-SOAs) via aerosol vacuum ultraviolet photoelectron spectroscopy, with a focus on the effects of water on Lim-SOAs. The first vertical ionization energy of limonene aerosols is measured to be 8.79 ± 0.07 eV. While water significantly increases the total photoelectron yield of Lim-SOAs, three photoelectron features attributable to Lim-SOAs each exhibit distinct dependence on the fraction of water in aerosols, implying that different formation pathways and molecular origins are involved in the formation of Lim-SOAs. Combined with density functional theory calculation and mass spectrometry measurements, this study reveals that water, particularly the water dimer, enhances the formation of Lim-SOAs by altering the ozonolysis energetics and pathways by intervening in its Criegee chemistry, acting as both a catalyst and a reactant. The atmospheric implication is discussed.

Sources of Organic Aerosol in China from 2005 to 2019: A Modeling Analysis.

Organic aerosol (OA) is a key component of fine particulate matter (PM2.5) and affects the human health and leads to climate change. With strict control measures for air pollutants during the last decade, the OA concentration in China declined slowly, while its sources remain unclear. In this study, we simulate the primary OA (POA) and secondary OA (SOA) concentrations from 2005 to 2019 with a state-of-the-art air quality model, Community Multiscale Air Quality (CMAQ, version 5.3.2) coupled with a Two-Dimensional Volatility Basis Set (2D-VBS) module, and a long-term emission inventory of full-volatility organic compounds in China and conduct source apportionment and sensitivity analysis. The simulation results show that, from 2005 to 2019, the OA concentration in China decreased from 24.0 to 12.8 μg/m3 with most of the reduction from POA. The OA pollution from residential biomass burning declined 75% from 2005 to 2019, while it is still the major OA source in China. OA pollution from VCP increased by more than 2-fold and became the largest SOA source in China. From 2014 to 2019, the NOx control in China slightly offset the decrease of SOA concentration due to elevated oxidation capacity.

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.

Modeling of wintertime regional formation of secondary organic aerosols around Beijing: sensitivity analysis and anthropogenic contributions

Modeling of secondary organic aerosol (SOA) has remained a big challenge due to the various precursors and complex processes involved. In this study, the WRF-CAMx model was used to predict the ambient SOA concentrations in urban Beijing as well as the North China Plain (NCP) during a polluted period in winter. To identify the major uncertainties and improve the model performance, a series of model tests were performed to assess the sensitivity of model prediction to the key factors. Then the sources of SOA in Beijing were identified using the optimized model. Both the volatility basis set (VBS) approach and the two-product approach were used for SOA simulation. Although the modeled SOA was underpredicted compared with the SOA estimated through filter-based measurements, the VBS scheme produced higher SOA than the traditional two-product scheme. According to the sensitivity tests with the VBS scheme, the emissions of volatile organic compounds (VOC) and intermediate volatility organic compounds (IVOC) as well as the oxidant levels were the key factors that affected SOA prediction. Based on the optimized simulation scenario, the potential contributions from different anthropogenic sources and source areas were identified, with over 80% of SOA in urban Beijing from regional transport of SOA or its precursors from the surrounding areas during the polluted period. Residential emission in the North China Plain appeared as the dominant source of SOA in urban Beijing from the perspective of regional contribution.Graphical