Secondary Organic Aerosol Concentrations Research Articles

The synergistic effect of NOx and SO2 on the formation and light absorption of secondary organic aerosols from ο-xylene photooxidation

Both NOx and SO2 are important anthropogenic air pollutants that significantly affect secondary organic aerosols (SOA) formation. However, our understanding of the combined effects of NOx and SO2 on SOA formation is still insufficient. The concentrations, optical properties, and chemical compositions of SOA formed through ο-xylene photooxidation under various NOx concentrations were investigated with or without SO2 in the chamber. It was found that SOA mass concentration increased from 60 to 129 μg m−3 when the initial NOx concentration was increased from 0 to 900 ppb. The presence of 50 ppb SO2 not only enhanced ο-xylene SOA formation by 1.6–2.9 times, but also changed the evolution of SOA with NOx. The enhancement of ο-xylene SOA formation was primarily attributed to acid-catalyzed heterogeneous reactions initiated by nitric acid and sulfuric acid. However, SOA concentration decreased under 900 ppb NOx when SO2 was present, likely due to the decrease in sulfate production because of the competition between NOx and SO2 for ·OH. In addition, nitrogen-containing organic compounds (NOCs) concentrations and mass absorption coefficients (MACλ=365 nm) were promoted by both NOx and SO2. Our research indicates that the promotion of both NOCs formation by NOx, and oligomer formation by SO2 were the main reasons for the enhanced SOA light absorption. The present research provides a scientific basis for understanding the combined effects of both NOx and SO2 on ο-xylene SOA formation, and illustrates that the imbalanced reduction of anthropogenic air pollutants would lead to new characteristics in SOA pollution.

Secondary organic aerosol formed by Euro 5 gasoline vehicle emissions: chemical composition and gas-to-particle phase partitioning

Abstract. In this study we investigated the photo-oxidation of Euro 5 gasoline vehicle emissions during cold urban, hot urban and motorway Artemis cycles. The experiments were conducted in an environmental chamber with average OH concentrations ranging between 6.6 × 105–2.3 × 106 molec. cm−3, relative humidity (RH) between 40 %–55 % and temperatures between 22–26 °C. A proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) and the CHemical Analysis of aeRosol ON-line (CHARON) inlet coupled with a PTR-ToF-MS were used for the gas- and particle-phase measurements respectively. This is the first time that the CHARON inlet has been used for the identification of the secondary organic aerosol (SOA) produced from vehicle emissions. The secondary organic gas-phase products ranged between C1 and C9 with one to four atoms of oxygen and were mainly composed of small oxygenated C1–C3 species. The SOA formed contained compounds from C1 to C14, having one to six atoms of oxygen, and the products' distribution was centered at C5. Organonitrites and organonitrates contributed 6 %–7 % of the SOA concentration. Relatively high concentrations of ammonium nitrate (35–160 µg m−3) were formed. The nitrate fraction related to organic nitrate compounds was 0.12–0.20, while ammonium linked to organic ammonium compounds was estimated only during one experiment, reaching a fraction of 0.19. The SOA produced exhibited log C∗ values between 2 and 5. Comparing our results to theoretical estimations for saturation concentrations, we observed differences of 1–3 orders of magnitude, indicating that additional parameters such as RH, particulate water content, aerosol hygroscopicity, and possible reactions in the particulate phase may affect the gas-to-particle partitioning.

Contribution of intermediate-volatility organic compounds from on-road transport to secondary organic aerosol levels in Europe

Abstract. Atmospheric organic compounds with an effective saturation concentration (C∗) at 298 K between 103 and 106 µg m−3 are called intermediate-volatility organic compounds (IVOCs), and they have been identified as important secondary organic aerosol (SOA) precursors. In this work, we simulate IVOCs emitted from on-road diesel and gasoline vehicles over Europe with a chemical transport model (CTM), utilizing a new approach in which IVOCs are treated as lumped species that preserve their chemical characteristics. This approach allows us to assess both the overall contribution of IVOCs to SOA formation and the role of specific compounds. For the simulated early-summer period, the highest concentrations of SOA formed from the oxidation of on-road IVOCs (SOA-iv) are predicted for major European cities, like Paris, Athens, and Madrid. In these urban environments, on-road SOA-iv can account for up to a quarter of the predicted total SOA. Over Europe, unspeciated cyclic alkanes in the IVOC range are estimated to account for up to 72 % of the total on-road SOA-iv mass, with compounds with 15 to 20 carbons being the most prominent precursors. The sensitivity of the predicted SOA-iv concentrations to the selected parameters of the new lumping scheme is also investigated. Active multigenerational aging of the secondary aerosol products has the most significant effect as it increases the predicted SOA-iv concentrations by 67 %.

Updating Biogenic Volatile Organic Compound (BVOC) Emissions With Locally Measured Emission Factors in South China and the Effect on Modeled Ozone and Secondary Organic Aerosol Production

AbstractBiogenic volatile organic compounds (BVOCs) emitted from terrestrial plants contribute substantially to ozone (O3) and secondary organic aerosol (SOA) formation in the troposphere. Accurate estimation of BVOC emissions is highly challengeable with a variety of uncertainties, one of which is the use of default emission factors (EFs) particularly for underrepresented regions without local data. In this study, locally measured BVOC‐EFs in south China, a subtropical region with abundant vegetation, were used to update regional BVOC emissions as estimated by the Model of Emissions of Gases and Aerosols from Nature (MEGAN). These EFs were recently determined in situ with characterized dynamic chambers for the emissions of isoprene, monoterpenes, and sesquiterpenes from tree species. The Community Multiscale Air Quality (CMAQ) model was then employed to see how much the regional O3 and SOA production is altered with the updated BVOC emissions. Results revealed lower BVOC emission estimates in south China when using the localized EFs than the MEGAN default ones, particularly for sesquiterpenes with a notable average reduction rate of approximately 40%. Using the updated BVOC emissions improved model O3 predictions in all seasons when compared to surface O3 monitoring, yet the lower BVOC emissions resulted in modeled O3 and SOA concentrations decreased by up to −6 ppb and −1.5 μg m−3, respectively, throughout south China. This study highlights the significance of localized EFs in refining emission estimates and air quality predictions in regions with a wealth of vegetation.

Evaluation of indoor secondary organic aerosol concentrations and contributions in Chinese residences: Insights from field testing

Indoor secondary organic aerosols (SOAs) generated from reactive organic gases (ROGs) reacting with O3 have a substantial impact on indoor PM2.5 levels. China's recent decrease in atmospheric PM2.5 and increase in atmospheric O3 have resulted in correspondingly lower levels of indoor PM2.5 and higher levels of indoor O3, which will further affect indoor SOA concentration. Thus, the aims of this study were to evaluate indoor SOA concentrations and their contribution to indoor PM2.5 and analyze the impact of the air exchange rate, window-opening status, proportion of indoor organic particles, and O3 concentrations on indoor SOA generation. To this end, measurements were conducted in Beijing, Shanghai, and Shenzhen, China by synchronously collecting samples of indoor ROGs and O3, along with indoor and outdoor PM2.5 concentrations. We found a maximum indoor SOA concentration of 35.23 μg/m3, contributing over 50% to indoor PM2.5, although most samples had an average concentration of 1.65 ± 4.89 μg/m3. Shanghai had the lowest average SOA concentration and contribution, whereas Shenzhen had the highest. Terpenoid compounds were identified as crucial precursors for indoor SOA generation, with the indoor O3 concentration being an important influencing factor. Compared to the measured benchmark scenario, when the O3 concentration reached 156 ppb, the average indoor SOA concentration increased approximately 20-fold while its contribution to indoor PM2.5 increased by approximately 7-fold. Our results provide a meticulous look at indoor SOA pollution and its determinants, the knowledge of which is pivotal for the implementation of effective strategies to mitigate indoor SOA pollution.

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.

Simulating dust emissions and secondary organic aerosol formation over northern Africa during the mid-Holocene Green Sahara period

Abstract. Paleo-proxy data indicate that a “Green Sahara” thrived in northern Africa during the early- to mid-Holocene (MH; 11 000 to 5000 years before present), characterized by more vegetation cover and reduced dust emissions. Utilizing a state-of-the-art atmospheric chemical transport model, TM5-MP, we assessed the changes in biogenic volatile organic compound (BVOC) emissions, dust emissions and secondary organic aerosol (SOA) concentrations in northern Africa during this period relative to the pre-industrial (PI) period. Our simulations show that dust emissions reduced from 280.6 Tg a−1 in the PI to 26.8 Tg a−1 in the MH, agreeing with indications from eight marine sediment records in the Atlantic Ocean. The northward expansion in northern Africa resulted in an increase in annual emissions of isoprene and monoterpenes during the MH, around 4.3 and 3.5 times higher than that in the PI period, respectively, causing a 1.9-times increase in the SOA surface concentration. Concurrently, enhanced BVOC emissions consumed more hydroxyl radical (OH), resulting in less sulfate formation. This effect counteracted the enhanced SOA surface concentration, altogether leading to a 17 % increase in the cloud condensation nuclei at 0.2 % super saturation over northern Africa. Our simulations provide consistent emission datasets of BVOCs, dust and the SOA formation aligned with the northward shift of vegetation during the “Green Sahara” period, which could serve as a benchmark for MH aerosol input in future Earth system model simulation experiments.

Molecular compositions of marine organic aerosols over the Bohai and Yellow Seas: Influence of primary emission and secondary formation

To understand the influences of continental emissions on marine aerosols from East Asia, the total suspended particle (TSP) samples were collected from the Yellow Sea (YS) and Bohai Sea (BS) of China through a cruise campaign during the spring of 2018. >100 individual organic compounds were identified and determined in the samples, and they were classified into 6 major groups according to the functionality and sources. The mass concentrations of total quantified organic compounds in BS (307.6 ± 268.3 ng m−3) was approximately 3 times those of YS (111.0 ± 97.3 ng m−3). Organic compounds from biomass burning (23.2 ± 34.2 ng m−3), marine/microbial source (23.1 ± 6.2 ng m−3) and fossil fuel combustion (21.4 ± 11.3 ng m−3) were the most abundant in the YS samples, whereas primary biological origins (90.0 ± 107.1 ng m−3), biomass burning (52.1 ± 39.1 ng m−3) and fossil fuel combustion (41.9 ± 20.1 ng m−3) were the most important organic species in BS marine aerosols. The concentrations of total biogenic secondary organic aerosol (BSOA) tracers including six isoprene SOA (SOAI), three monoterpene SOA (SOAM), and one β-caryophyllene SOA (SOAC) were 2.4 ± 1.5 ng m−3 and 7.3 ± 4.4 ng m−3 in the YS and BS samples, respectively. A strong correlation between BSOA tracers and levoglucosan was shown in the marine aerosol samples, indicating that biomass burning emissions could also promote BSOA formations. The spatial distribution of chemical and source characteristics suggests a relatively large impacts of continental emissions to the BS aerosols. Based on the tracer-based method, the biomass burning (YS: 4.9 ± 6.8%; BS: 8.8 ± 8.0%) was the largest contributor to OC, followed by primary biogenic OC (YS: 0.59 ± 0.44%; BS: 2.9 ± 3.0%) and biogenic secondary OC (YS: 0.45 ± 0.23%; BS: 1.0 ± 0.59%). This study suggests important contributions of biomass burning emissions to organic aerosols in the YS and BS marine atmosphere, although marine source, fossil fuel combustion and secondary formation was also substantial.

Effects of simulated secondary organic aerosol water on PM1 levels and composition over the US

Abstract. Water is a key component of atmospheric aerosol, affecting many aerosol processes including gas-to-particle partitioning of semi-volatile compounds. Water related to secondary organic aerosol (SOAW) is often neglected in atmospheric chemical transport models and is not considered in gas-to-particle partitioning calculations for inorganic species. We use a new inorganic aerosol thermodynamics model, ISORROPIA-lite, which considers the effects of SOAW, to perform chemical transport model simulations for 1 year over the continental United States to quantify its effects on aerosol mass concentration and composition. SOAW can increase average fine aerosol water levels by up to a factor of 2 when secondary organic aerosol (SOA) is a major PM1 component. This is often the case in the south-eastern US, where SOA concentrations are higher. Although the annual average impact of this added water on total dry PM1 concentrations due to increased partitioning of nitrate and ammonium is small (up to 0.1 µg m−3), total dry PM1 increases of up to 2 µg m−3 (with nitrate levels increases of up to 200 %) can occur when RH levels and PM1 concentrations are high.

Effects of VOC emissions from chemical industrial parks on regional O3-PM2.5 compound pollution in the Yangtze River Delta

Ozone (O3) and fine particulate matter (PM2.5) compound pollution has emerged as a primary form of air pollution in Chinese urban. Volatile organic compounds (VOCs), as common precursors of O3 and PM2.5, play a significant role in air pollution control. Chemical industrial parks (CIPs) are crucial emission sources of VOCs and have garnered significant attention. This study focused on 142 CIPs located in the Yangtze River Delta (YRD) to investigate the characteristics of VOC emissions from CIPs and their impact on O3-PM2.5 compound pollution, considering the enhanced atmospheric oxidation capacity (AOC). The Comprehensive Air Quality Model with Extensions (CAMx) model was employed for this analysis. The results show that VOC emissions from CIPs contributed significantly to regional O3 and secondary organic aerosol (SOA), accounting for 17.1 % and 18.18 % of the anthropogenic sources, respectively. Regions exhibiting the highest contributions were located along the Hangzhou Bay. Compared with 2014, an elevation in the contribution of VOC emissions from CIPs to the annual average concentrations of MDA8 O3 and SOA in the YRD in 2017 by 0.069 μg/m3 and 0.007 μg/m3, respectively. During episodes of compound pollution, the concentration of atmospheric oxidant (HOx + NO3) was 28.65 % higher than during clean days, and significant positive correlations were observed between hydrogen oxygen radicals (HOx) and maximum daily 8-h average (MDA8 O3) as well as between HOx and SOA, exhibiting correlation coefficients of 0.86 and 0.48, respectively. Effective control measures for VOC emissions, particularly from the pharmaceutical and petrochemical industry parks located along Hangzhou Bay, are essential in curtailing the production rate of HOx and in regulating AOC levels in the YRD. Maintaining the daily average HOx concentration below 10 ppt would be a valuable strategy in achieving coordinated control of O3 and SOA, thus aiding in the alleviation of O3-PM2.5 compound pollution in the YRD.

Deposition of secondary organic aerosol in human lung model: Effect of photochemically aged aerosol on human respiratory system

Ultrafine particles (UFP) of Secondary Organic Aerosol (SOA) penetrate deep into the human respiratory system and exert fatal effects on human health. However, there is little data on the potential deposited doses of UFP-generated SOA in the human respiratory tract. This study is to estimate the fraction of aerosol deposition using a multiple-path-particle-dosimetry (MPPD) model. For relevancy of real life, the model employed measured concentrations of toluene-derived fresh and aged SOA produced within serially connected smog chamber and PAM-OFR (Potential Aerosol Mass-Oxidation Flow Reactor) under atmospheric environmental conditions (NOx and relative humidity). The number concentrations and chemical composition of fresh and aged aerosols produced within the chambers were measured using Scanning Mobility Particle Sizer (SMPS) and High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS), while the morphology of individual particles was analyzed using Scanning Electron Microscopy (SEM). The number concentration of aged SOA-w/s was more than double compared to that of fresh SOA-w/s (maximum reached after 10 h) with its size less than 100 nm. The O:C ratio for aged SOA-w/s were 0.96 and 1.15 depending on RH (0.96 at 3% RH and 1.15 at 50% RH), and individual spherical particles containing water were present in agglomerates with its size of less than 1 µm. In all inhalable fresh and aged SOA produced in the two chambers, 5–22% of aerosol is deposited in the Head airways, 4–8% in the tracheobronchial, and 8–34% in the alveolar regions. The predominant deposition of the aged aerosol occurred in the alveoli (in the generation 20th lobe), and the deposition faction in the alveoli was 2–3 times higher in the children group than the adults group. This study presented a quantitative exposure assessment of SOA generated under a realistic simulation and suggested the possibility of evaluating long-term exposure to SOA and potential health effects by determining the potential inhalable aerosol doses and the fraction of deposition in the human respiratory system.

Measurement and Modeling of Ship-Related Ultrafine Particles and Secondary Organic Aerosols in a Mediterranean Port City.

Maritime transport emerges as a major source of ultrafine particle (UFP) pollution in coastal regions with consequences for the health of people living in port cities. Inhalation of UFPs can cause inflammation and oxidative stress, which are starting points for further diseases. In addition to primary particles, secondary organic aerosol (SOA) may form through the photo-oxidation of volatile organic compounds emitted in ship exhaust. The characterization of size-segregated and chemical properties of particles is essential for assessing the health implications related to shipping. We applied a coupled regional-local chemistry transport modeling system to study the effects of ship emissions on atmospheric concentrations of UFP and SOA in the Mediterranean port city Marseille (France), which is characterized by the combination of high port activity, industrialized emissions, and active photochemistry in summer. Our results show that the average potential impact from local shipping in the port area was 6-9% for SOA and 27-51% for total particle number concentration in July 2020. The estimated oxidative potential of daily mean particulate organic matter related to shipping was lower than the oxidative potential reported for heavy fuel oil (HFO). The lower oxidative potential in this study is very likely due to the low share of ships using HFO during stopover.

Relative humidity-dependent evolution of molecular composition of α-pinene secondary organic aerosol upon heterogeneous oxidation by hydroxyl radicals

Heterogeneous oxidation by gas-phase oxidants is an important chemical transformation pathway of secondary organic aerosol (SOA) and plays an important role in controlling the abundance, properties, as well as climate and health impacts of aerosols. However, our knowledge on this heterogeneous chemistry remains inadequate. In this study, the heterogeneous oxidation of α-pinene ozonolysis SOA by hydroxyl (OH) radicals was investigated under both low and high relative humidity (RH) conditions, with an emphasis on the evolution of molecular composition of SOA and its RH dependence. It is found that the heterogeneous oxidation of SOA at an OH exposure level equivalent to 12 hr of atmospheric aging leads to particle mass loss of 60% at 25% RH and 95% at 90% RH. The heterogeneous oxidation strongly changes the molecular composition of SOA. The dimer-to-monomer signal ratios increase dramatically with rising OH exposure, in particular under high RH conditions, suggesting that aerosol water stimulates the reaction of monomers with OH radicals more than that of dimers. In addition, the typical SOA tracer compounds such as pinic acid, pinonic acid, hydroxy pinonic acid and dimer esters (e.g., C17H26O8 and C19H28O7) have lifetimes of several hours against heterogeneous OH oxidation under typical atmospheric conditions, which highlights the need for the consideration of their heterogeneous loss in the estimation of monoterpene SOA concentrations using tracer-based methods. Our study sheds lights on the heterogeneous oxidation chemistry of monoterpene SOA and would help to understand their evolution and impacts in the atmosphere.

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

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

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.

Synergistic generation mechanisms of SOA and ozone from the photochemical oxidation of 1,3,5-trimethylbenzene: Influence of precursors ratio, temperature and radiation intensity

Aromatic hydrocarbons (AHs) are important anthropogenic precursors for both secondary organic aerosol (SOA) and ozone (O3) in urban areas. Knowledge about the synergistic processes and influence mechanisms of SOA and O3 during the photochemical oxidation of AHs is still relatively limited. In this study, the synergistic formation processes of SOA and O3 through the photochemical reactions of a representative AHs (1,3,5-trimethylbenzene, 135TMB) under different initial volatile organic compound/nitrogen oxides ([VOC]/[NOX]) ratios, temperatures and radiation intensities were investigated. The results showed that temperature and radiation significantly affected the reaction products and yields: the O3 production rates under high temperatures and radiation were greater than those under low temperatures and radiation, whereas the mass concentrations and yields of SOA under low temperatures and radiation were much higher than those under high temperatures and radiation. It was also found that the SOA yield increased with increasing NOX concentration within a specific initial [135TMB]/[NOX] ratio, but it decreased with increasing NOX concentration beyond a specific initial [135TMB]/[NOX] ratio. In addition, the concentrations of SOA and O3 in all the experiments increased simultaneously regardless of low temperatures or high temperatures. From these phenomena, it can be inferred that appropriate strategies for O3 precursor abatement can simultaneously reduce the concentration of SOA, which means that the coordinated control of fine particles and O3 could be achieved by taking coordinated emission reduction measures for VOCs and NOX precursors. The analysis of the organic components revealed that intramolecular hydrogen transfer occurred in some reactions in 135TMB photooxidation, and low-volatility and high-oxidative state organics were formed. The master chemical mechanism (MCM) results showed that the existing MCM mechanism does not yet fully meet the need to simulate the atmospheric photochemical reaction processes of 135TMB in the real atmosphere in China, and a localized MCM mechanism needs to be developed in the future.

Insights into characteristics and formation mechanisms of secondary organic aerosols in the Guangzhou urban area

Abstract. Emission controls have substantially brought down aerosol pollution in China; however, aerosol mass reductions have slowed down in recent years in the Pearl River Delta (PRD) region, where secondary organic aerosol (SOA) formation poses a major challenge for air quality improvement. In this study, we characterized the roles of SOA in haze formation in urban Guangzhou in the PRD using year-long aerosol mass spectrometer measurements for the first time and discussed possible pathways of SOA formations. On average, organic aerosols (OA) contribute dominantly (50 %) to nonrefractory submicron aerosol mass (NR-PM1). The average mass concentration of SOA (including by less- and more-oxidized OA, LOOA and MOOA) contributed most to NR-PM1, reaching about 1.7 times that of primary organic aerosols (POA, including hydrocarbon-like and cooking-related OA) and accounting for 32 % of NR-PM1, even more than sulfate (22 %) and nitrate (16 %). Seasonal variations of NR-PM1 revealed that haze-formation mechanisms differed much among distinct seasons. Sulfate mattered more than nitrate in fall, while nitrate was more important than sulfate in spring and winter, with SOA contributing significantly to haze formations in all the seasons. Daytime SOA formation was weak in winter under low oxidant levels and air relative humidity, whereas prominent daytime SOA formation was observed in fall, spring and summer almost on a daily basis, suggesting important roles of photochemistry in SOA formations. Further analysis showed that the coordination of gas-phase photochemistry and subsequent aqueous-phase reactions likely played significant roles in quick daytime SOA formations. Obvious nighttime SOA formations were also frequently observed in spring, fall and winter, and it was found that daytime and nighttime SOA formations together had resulted in the highest SOA concentrations in these seasons and contributed substantially to severe haze formations. Simultaneous increases in nitrate with SOA after sunset suggested the important roles of NO3 radical chemistry in nighttime SOA formations, and this was further confirmed by a continuous increase in the NO+ / NO2+ fragment ratio that related to measured particulate nitrate after sunset. The findings of this study have promoted our understanding of haze pollution characteristics of the PRD and laid down future directions for investigations of SOA-formation mechanisms in urban areas of southern China that share similar emission sources and meteorological conditions.

Yields and molecular composition of gas phase and secondary organic aerosol from the photooxidation of the volatile consumer product benzyl alcohol: formation of highly oxygenated and hydroxy nitroaromatic compounds.

Recently, volatile chemical products (VCPs) have been increasingly recognized as important precursors for secondary organic aerosol (SOA) and ozone in urban areas. However, their atmospheric chemistry, physical transformation, and their impact on climate, environment and human health remain poorly understood. Here, the yields and chemical composition at the molecular level of gas and particle phase products originating from the photooxidation of one of these VCPs, benzyl alcohol (BnOH), is reported. The SOA was generated in the presence of seed aerosol from nebulized ammonium sulfate solution in a 14.5 m3 smog chamber operated in flow mode. More than 50 organic compounds containing nitrogen and/or up to seven oxygen atoms were identified by mass spectrometry. While a detailed non-targeted analysis has been made, our primary focus has been to examine highly oxygenated and nitro-aromatic compounds. The major components include ring-opening products with high oxygen to carbon ratio (e.g., malic acid, tartaric acids, arabic acid, trihydroxy-oxo-pentanoic acids, and pentaric acid), and ring-retaining products (e.g., benzaldehyde, benzoic acid, catechol, 3-nitrobenzyl alcohol, 4-nitrocatechol, 2-hydroxy-5-nitrobenzyl alcohol, 2-nitrophloroglucinol, 3,4-dihydroxy-5-nitrobenzyl alcohol). The presence of some of these products in the gas and particle phases simultaneously provides evidence of their gas/particle partitioning. These oxygenated oxidation products made dominant contributions to the SOA particle composition in both low and high NOx systems. Yields, organic mass to organic carbon ratio, and proposed reaction schemes for selected compounds are provided. The aerosol yield was 5.2% for BnOH/H2O2 at SOA concentration of 52.9 µg m-3 and ranged between 1.7-8.1 % for BnOH/NOx at SOA concentration of 40.0-119.5 µg m-3.

Impacts of land cover changes on biogenic emission and its contribution to ozone and secondary organic aerosol in China

Abstract. The greening impacts on China from 2000 to 2017 led to an increase in vegetated areas and thus enhanced biogenic volatile organic compound (BVOC) emissions. BVOCs are regarded as important precursors for ozone (O3) and secondary organic aerosol (SOA). As a result, accurate estimation of BVOC emissions is critical to understand their impacts on air quality. In this study, the Model of Emissions of Gases and Aerosols from Nature (MEGAN) v2.1 was used to investigate the impact of different leaf area index (LAI) and land cover (LC) datasets on BVOC emissions in China in 2016, and the effects on O3 and SOA were evaluated based on the Community Multiscale Air Quality (CMAQ) modeling system. Three LAI satellite datasets of the Global LAnd Surface Satellite (GLASS), the Moderate Resolution Imaging Spectroradiometer (MODIS) MOD15A2H version 6 (MOD15), and the Copernicus Global Land Service (CGLS), as well as three LC satellite datasets of the MODIS MCD12Q1 LC products, the Copernicus Climate Change Service (C3S) LC products, and the CGLS LC products, were used in five parallel experiments (cases: C1–C5). Results show that changing LAI and LC datasets of the model input has an impact on BVOC estimations. BVOC emissions in China ranged from 25.42 to 37.39 Tg in 2016 and were mainly concentrated in central and southeastern China. Changing the LC inputs for the MEGAN model has a more significant difference in BVOC estimates than using different LAI datasets. The combination of C3S LC and GLASS LAI performs better in the CMAQ model, indicating that it is the better choice for BVOC estimations in China. The highest contribution of BVOCs to O3 and SOA can reach 12 ppb and 9.8 µg m−3, respectively. Changing the MEGAN inputs further impacts the concentrations of O3 and SOA, especially changing LC datasets. The relative difference between MCD12Q1 LC and C3S LC is over 52 % and 140 % in O3 and biogenic SOA (BSOA) in central and eastern China. The BSOA difference is mainly attributed to the isoprene SOA (ISOA), a major contributor to BSOA. The relative differences in ISOA between different cases are up to 160 % in eastern China. Therefore, our results suggest that the uncertainties in MEGAN inputs should be fully considered in future O3 and SOA simulations.

Drivers of High Concentrations of Secondary Organic Aerosols in Northern China during the COVID-19 Lockdowns.

During the COVID-19 lockdown in early 2020, observations in Beijing indicate that secondary organic aerosol (SOA) concentrations increased despite substantial emission reduction, but the reasons are not fully explained. Here, we integrate the two-dimensional volatility basis set into a state-of-the-art chemical transport model, which unprecedentedly reproduces organic aerosol (OA) components resolved by the positive matrix factorization based on aerosol mass spectrometer observations. The model shows that, for Beijing, the emission reduction during the lockdown lowered primary organic aerosol (POA)/SOA concentrations by 50%/18%, while deteriorated meteorological conditions increased them by 30%/119%, resulting in a net decrease in the POA concentration and a net increase in the SOA concentration. Emission reduction and meteorological changes both led to an increased OH concentration, which accounts for their distinct effects on POA and SOA. SOA from anthropogenic volatile organic compounds and organics with lower volatility contributed 28 and 62%, respectively, to the net SOA increase. Different from Beijing, the SOA concentration decreased in southern Hebei during the lockdown because of more favorable meteorology. Our findings confirm the effectiveness of organic emission reductions and meanwhile reveal the challenge in controlling SOA pollution that calls for large organic precursor emission reductions to rival the adverse impact of OH increase.