Short-Term Exposure to Fine Particulate Matter and Ozone: Source Impacts and Attributable Mortalities.

Short-term exposure to particles with aerodynamic diameters less than 2.5 µm (PM2.5) and ozone (O3) are important risk factors for human health. Despite the awareness of reducing attributable health burden, region-specific and source-specific strategies remain less explored due to the gap between precursor emissions and health effects. In this study, we isolate the health burden of individual sector sources of PM2.5 and O3 precursors, nitrogen oxides (NOx) and volatile organic compounds (VOCs), across the globe. Specifically, we estimate mortalities attributable to short-term exposure using machine-learning-based daily exposure estimates and quantify sectoral impacts using chemical transport model simulations. Globally, short-term exposure to PM2.5 and O3 result in 713.5 (95% Confidence Interval: 598.8-843.3) thousand and 496.3 (371.3-646.1) thousand mortalities in 2019, respectively, of which 12.5% are contributed by fuel-related NOx emissions from transportation, energy, and industry. Sectoral impacts from anthropogenic NOx and VOC emissions on health burden vary significantly among seasons and regions, requiring a target shift from transportation in winter to industry in summer for East Asia, for instance. Emission control and health management are additionally complicated by unregulated natural influences during climatic events. Fire-sourced NOx and VOC emissions, respectively, contribute to 8.5 (95% CI: 6.2-11.7) thousand and 4.8 (3.6-5.9) thousand PM2.5 and O3 mortalities, particularly for tropics with high vulnerability to climate change. Additionally, biogenic VOC emissions during heatwaves contribute to 1.8 (95% CI: 1.5-2.2) thousand O3-introduced mortalities, posing challenges in urban planning for high-income regions, where biogenic contributions to health burden during heatwaves are 13% of anthropogenic contributions annually. Our study provides important implications for temporally dynamic and sector-targeted emission control and health management strategies, which are of urgency under the projection of continuously increasing energy consumption and changing climate.

Chemically Resolved Respiratory Deposition of Ultrafine Particles Characterized by Number Concentration in the Urban Atmosphere.

Ultrafine particles (UFPs) dominate the atmospheric particles in number concentration, impacting human health and climate change. However, existing studies primarily rely on mass-based approaches, leading to a restricted understanding of the number-based and chemically resolved health effects of atmospheric UFPs. In this study, we utilized a high-mass-resolution single-particle aerosol mass spectrometer to investigate the online chemical composition and number size distribution of ultrafine, fine, and coarse particles during the summertime in urban Shenzhen, China. Human respiratory deposition dose assessments of particles with varying chemical compositions were further conducted by a respiratory deposition model. The results showed that during our observation, particles containing elemental carbon (EC) were the dominant components in UFPs (0.05-0.1 µm). Compared to fine and coarse particles, UFPs can deposit more deeply into the respiratory tract with a daily dose of ∼2.08 ± 0.67 billion particles. Among the deposited UFPs, EC-cluster particles constituted ∼85.7% in number fraction, accounting for a daily number dose of ∼1.78 billion particles, which poses a greater impact on human health. Simultaneously, we found discrepancies in the chemically resolved particle depositions among number-, surface area-, and mass-based approaches, emphasizing the importance of an appropriate metric for particle health-risk evaluation.

Changing Responses of PM2.5 and Ozone to Source Emissions in the Yangtze River Delta Using the Adjoint Model.

China's industrial restructuring and pollution controls have altered the contributions of individual sources to varying air quality over the past decade. We used the GEOS-Chem adjoint model and investigated the changing sensitivities of PM2.5 and ozone (O3) to multiple species and sources from 2010 to 2020 in the central Yangtze River Delta (YRDC), the largest economic region in China. Controlling primary particles and SO2 from industrial and residential sectors dominated PM2.5 decline, and reducing CO from multiple sources and ≥C3 alkenes from vehicles restrained O3. The chemical regime of O3 formation became less VOC-limited, attributable to continuous NOX abatement for specific sources, including power plants, industrial combustion, cement production, and off-road traffic. Regional transport was found to be increasingly influential on PM2.5. To further improve air quality, management of agricultural activities to reduce NH3 is essential for alleviating PM2.5 pollution, while controlling aromatics, alkenes, and alkanes from industry and gasoline vehicles is effective for O3. Reducing the level of NOX from nearby industrial combustion and transportation is helpful for both species. Our findings reveal the complexity of coordinating control of PM2.5 and O3 pollution in a fast-developing region and support science-based policymaking for other regions with similar air pollution problems.

Formation of Nitrosamines from the Heterogeneous Reaction of Nitrous Acid and Organic Amines in Indoor Environments.

Carcinogenic nitrosamines have been widely studied due to their risk to human health. However, the universality and evolutionary processes of their generation, particularly concerning their secondary sources, remain unclear at present. We demonstrated through laboratory flow tube experiments that corresponding nitrosamines were generated from heterogeneous reactions of nitrous acid (HONO) with five structurally diverse amines commonly found indoors, including diphenylamine (DPhA), dibenzylamine (DBzA), dioctylamine (DOtA), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), and N-phenyl-1-naphthylamine (PANA). The heterogeneous reaction rate constants of DBzA and DOtA with HONO (∼70 ppb) were 1.21 × 10-3 and 2.13 × 10-3 min-1 at 30% relative humidity (RH), resulting in a lifetime of 13.8 and 7.8 h. As compared to higher RH (∼80%), more nitrosamines were produced from the reaction of HONO with surface-sorbed DBzA, DOtA, 6PPD, and PANA at lower RH (30%), with product yields ranging from <0.1% to 0.5%. Furthermore, we observed the formation of nitroso-6PPDs and nitro-6PPDs during room air exposure of 6PPD in a genuine indoor environment, in addition to various other transformation products indicative of reactions of 6PPD with HONO, NOx, and ozone indoors. This study confirmed the universality of the heterogeneous reaction of surface-sorbed amine with HONO as a source of nitrosamines indoors.

Satellite NO2 Retrieval Complicated by Aerosol Composition over Global Urban Agglomerations: Seasonal Variations and Long-Term Trends (2001-2018).

Tropospheric nitrogen dioxide (NO2) poses a serious threat to the environmental quality and public health. Satellite NO2 observations have been continuously used to monitor NO2 variations and improve model performances. However, the accuracy of satellite NO2 retrieval depends on the knowledge of aerosol optical properties, in particular for urban agglomerations accompanied by significant changes in aerosol characteristics. In this study, we investigate the impacts of aerosol composition on tropospheric NO2 retrieval for an 18 year global data set from Global Ozone Monitoring Experiment (GOME)-series satellite sensors. With a focus on cloud-free scenes dominated by the presence of aerosols, individual aerosol composition affects the uncertainties of tropospheric NO2 columns through impacts on the aerosol loading amount, relative vertical distribution of aerosol and NO2, aerosol absorption properties, and surface albedo determination. Among aerosol compositions, secondary inorganic aerosol mostly dominates the NO2 uncertainty by up to 43.5% in urban agglomerations, while organic aerosols contribute significantly to the NO2 uncertainty by -8.9 to 37.3% during biomass burning seasons. The possible contrary influences from different aerosol species highlight the importance and complexity of aerosol correction on tropospheric NO2 retrieval and indicate the need for a full picture of aerosol properties. This is of particular importance for interpreting seasonal variations or long-term trends of tropospheric NO2 columns as well as for mitigating ozone and fine particulate matter pollution.

Convolutional Neural Networks Facilitate Process Understanding of Megacity Ozone Temporal Variability.

Ozone pollution is profoundly modulated by meteorological features such as temperature, air pressure, wind, and humidity. While many studies have developed empirical models to elucidate the effects of meteorology on ozone variability, they predominantly focus on local weather conditions, overlooking the influences from high-altitude and broader regional meteorological patterns. Here, we employ convolutional neural networks (CNNs), a technique typically applied to image recognition, to investigate the influence of three-dimensional spatial variations in meteorological fields on the daily, seasonal, and interannual dynamics of ozone in Shenzhen, a major coastal urban center in China. Our optimized CNNs model, covering a 13° × 13° spatial domain, effectively explains over 70% of daily ozone variability, outperforming alternative empirical approaches by 7 to 62%. Model interpretations reveal the crucial roles of 2-m temperature and humidity as primary drivers, contributing 16% and 15% to daily ozone fluctuations, respectively. Regional wind fields account for up to 40% of ozone changes during the episodes. CNNs successfully replicate observed ozone temporal patterns, attributing -5-6 µg·m-3 of interannual ozone variability to weather anomalies. Our interpretable CNNs framework enables quantitative attribution of historical ozone fluctuations to nonlinear meteorological effects across spatiotemporal scales, offering vital process-based insights for managing megacity air quality amidst changing climate regimes.

Characteristics of Nocturnal Boundary Layer over a Subtropical Forest: Implications for the Dispersion and Fate of Atmospheric Species.

The nocturnal boundary layer (NBL) significantly influences the dispersion and fate of atmospheric species at night. Subtropical forests are crucial in carbon and water exchange between the biosphere and the atmosphere. However, the NBL characteristics and their impact on atmospheric species over these forests remain unknown. This study conducted vertical measurements of atmospheric species such as O3 and volatile organic compounds (VOCs), along with meteorological variables, over a national forest reserve in Southern China. Results reveal that the NBL height ranged from 180 to 300 m in the summer and from 80 to 160 m in the winter. The vertical distribution of chemical species varied by time and season, with greater concentration gradients observed in the summer. Over 90% of VOCs above the NBL were anthropogenic, while biogenic VOCs were mainly found within the NBL. Higher O3 concentration and VOC product-to-reactant ratios were observed in the residual layer, suggesting enhanced oxidation levels. This unique vertical distribution of atmospheric species at night is driven by factors, such as emission, deposition, turbulence, and atmospheric chemistry, potentially affecting ecosystem functions. Results from this study highlight the importance of incorporating NBL dynamics into atmospheric models to better understand the evolution of chemical species and their ecological effects over forests.

Temperature-Dependent Evaporative Anthropogenic VOC Emissions Significantly Exacerbate Regional Ozone Pollution.

The evaporative emissions of anthropogenic volatile organic compounds (AVOCs) are sensitive to ambient temperature. This sensitivity forms an air pollution-meteorology connection that has not been assessed on a regional scale. We parametrized the temperature dependence of evaporative AVOC fluxes in a regional air quality model and evaluated the impacts on surface ozone in the Beijing-Tianjin-Hebei (BTH) area of China during the summer of 2017. The temperature dependency of AVOC emissions drove an enhanced simulated ozone-temperature sensitivity of 1.0 to 1.8 µg m-3 K-1, comparable to the simulated ozone-temperature sensitivity driven by the temperature dependency of biogenic VOC emissions (1.7 to 2.4 µg m-3 K-1). Ozone enhancements driven by temperature-induced AVOC increases were localized to their point of emission and were relatively more important in urban areas than in rural regions. The inclusion of the temperature-dependent AVOC emissions in our model improved the simulated ozone-temperature sensitivities on days of ozone exceedance. Our results demonstrated the importance of temperature-dependent AVOC emissions on surface ozone pollution and its heretofore unrepresented role in air pollution-meteorology interactions.

Impacts of Ship Emissions on Air Quality in Southern China: Opportunistic Insights from the Abrupt Emission Changes in Early 2020.

In early 2020, two unique events perturbed ship emissions of pollutants around Southern China, proffering insights into the impacts of ship emissions on regional air quality: the decline of ship activities due to COVID-19 and the global enforcement of low-sulfur (<0.5%) fuel oil for ships. In January and February 2020, estimated ship emissions of NOx, SO2, and primary PM2.5 over Southern China dropped by 19, 71, and 58%, respectively, relative to the same period in 2019. The decline of ship NOx emissions was mostly over the coastal waters and inland waterways of Southern China due to reduced ship activities. The decline of ship SO2 and primary PM2.5 emissions was most pronounced outside the Chinese Domestic Emission Control Area due to the switch to low-sulfur fuel oil there. Ship emission reductions in early 2020 drove 16 to 18% decreases in surface NO2 levels but 3.8 to 4.9% increases in surface ozone over Southern China. We estimated that ship emissions contributed 40% of surface NO2 concentrations over Guangdong in winter. Our results indicated that future abatements of ship emissions should be implemented synergistically with reductions of land-borne anthropogenic emissions of nonmethane volatile organic compounds to effectively alleviate regional ozone pollution.

Investigation of Plant-Level Volatile Organic Compound Emissions from Chemical Industry Highlights the Importance of Differentiated Control in China.

The chemical industry is a significant source of nonmethane volatile organic compounds (NMVOCs), pivotal precursors to ambient ozone (O3), and secondary organic aerosol (SOA). Despite their importance, precise estimation of these emissions remains challenging, impeding the implementation of NMVOC controls. Here, we present the first comprehensive plant-level assessment of NMVOC emissions from the chemical industry in China, encompassing 3461 plants, 127 products, and 50 NMVOC compounds from 2010 to 2019. Our findings revealed that the chemical industry in China emitted a total of 3105 (interquartile range: 1179-8113) Gg of NMVOCs in 2019, with a few specific products accounting for the majority of the emissions. Generally, plants engaged in chemical fibers production or situated in eastern China pose a greater risk to public health due to their higher formation potentials of O3 and SOA or their proximity to residential areas or both. We demonstrated that targeting these high-risk plants for emission reduction could enhance health benefits by 7-37% per unit of emission reduction on average compared to the current situation. Consequently, this study provides essential insights for developing effective plant-specific NMVOC control strategies within China's chemical industry.

Iron and Steel Industry Emissions: A Global Analysis of Trends and Drivers.

The iron and steel industry (ISI) is important for socio-economic progress but emits greenhouse gases and air pollutants detrimental to climate and human health. Understanding its historical emission trends and drivers is crucial for future warming and pollution interventions. Here, we offer an exhaustive analysis of global ISI emissions over the past 60 years, forecasting up to 2050. We evaluate emissions of carbon dioxide and conventional and unconventional air pollutants, including heavy metals and polychlorinated dibenzodioxins and dibenzofurans. Based on this newly established inventory, we dissect the determinants of past emission trends and future trajectories. Results show varied trends for different pollutants. Specifically, PM2.5 emissions decreased consistently during the period 1970 to 2000, attributed to adoption of advanced production technologies. Conversely, NOx and SO2 began declining recently due to stringent controls in major contributors such as China, a trend expected to persist. Currently, end-of-pipe abatement technologies are key to PM2.5 reduction, whereas process modifications are central to CO2 mitigation. Projections suggest that by 2050, developing nations (excluding China) will contribute 52-54% of global ISI PM2.5 emissions, a rise from 29% in 2019. Long-term emission curtailment will necessitate the innovation and widespread adoption of new production and abatement technologies in emerging economies worldwide.

Impact of Ship Emissions on Air Quality in the Greater Bay Area in China under the Latest Global Marine Fuel Regulation.

As the main anthropogenic source in open seas and coastal areas, ship emissions impact the climate, air quality, and human health. The latest marine fuel regulation with a sulfur content limit of 0.5% went into effect globally on January 1, 2020. Investigations of ship emissions after fuel switching are necessary. In this study, online field measurements at an urban coastal site and modeling simulations were conducted to detect the impact of ship emissions on air quality in the Greater Bay Area (GBA) in China under new fuel regulation. By utilizing a high mass-resolution single particle mass spectrometer, the vanadium(V) signal was critically identified and was taken as a robust indicator for ship-emitted particles (with relative peak area > 0.1). The considerable number fractions of high-V particles (up to 30-40% during ship plumes) indicated that heavy fuel oils via simple desulfurization or blending processes with low-sulfur fuels were extensively used in the GBA to meet the global 0.5% sulfur cap. Our results showed that ship-emitted particulate matter and NOx contributed up to 21.4% and 39.5% to the ambient, respectively, in the summertime, significantly affecting the air quality in the GBA. The sea-land breeze circulation also played an important role in the transport pattern of ship-emitted pollutants in the GBA.

Efficient Atmospheric Transport of Microplastics over Asia and Adjacent Oceans.

We developed a regional atmospheric transport model for microplastics (MPs, 10 µm to 5 mm in size) over Asia and the adjacent Pacific and Indian oceans, accounting for MPs' size- and shape-dependent aerodynamics. The model was driven by tuned atmospheric emissions of MPs from the land and the ocean, and the simulations were evaluated against coastal (n = 19) and marine (n = 56) observations. Our tuned atmospheric emissions of MPs from Asia and the adjacent oceans were 310 Gg y-1 (1 Gg = 1 kton) and 60 Gg y-1, respectively. MP lines and fragments may be transported in the atmosphere >1000 km; MP pellets in our model mostly deposited near-source. We estimated that 1.4% of the MP mass emitted into the Asian atmosphere deposited into the oceans via atmospheric transport; the rest deposited over land. The resulting net atmospheric transported MP flux from Asia into the oceans was 3.9 Gg y-1, twice as large as a previous estimate for the riverine-transported MP flux from Asia into the oceans. The uncertainty of our simulated atmospheric MP budget was between factors of 3 and 7. Our work highlighted the impacts of the size and morphology on the aerodynamics of MPs and the importance of atmospheric transport in the source-to-sink relationship of global MP pollution.

Absorption Enhancement of Black Carbon Aerosols Constrained by Mixing-State Heterogeneity.

Atmospheric black carbon (BC) has a large yet highly uncertain contribution to global warming. When mixed with non-BC/coating material during atmospheric aging, the BC light absorption can be enhanced through the lensing effect. Laboratory and modeling studies have consistently found strong BC absorption enhancement, while the results in ambient measurements are conflicting, with some reporting weak absorption enhancement even for particles with large bulk coating amounts. Here, from our direct field observations, we report both large and minor absorption enhancement factors for different BC-containing particle populations with large bulk non-BC-to-BC mass ratios. By gaining insights into the measured coating material distribution across each particle population, we find that the level of absorption enhancement is strongly dependent on the particle-resolved mixing state. Our study shows that the greater mixing-state heterogeneity results in the larger difference between observed and predicted absorption enhancement. We demonstrate that by considering the variability in coating material thickness in the optical model, the previously observed model measurement discrepancy of absorption enhancement can be reconciled. The observations and improved optical models reported here highlight the importance of mixing-state heterogeneity on BC's radiative forcing, which should be better resolved in large-scale models to increase confidence when estimating the aerosol radiation effect.

Production of Formate via Oxidation of Glyoxal Promoted by Particulate Nitrate Photolysis.

Particulate nitrate photolysis can produce oxidants (i.e., OH, NO2, and NO2-/HNO2) in aqueous droplets and may play a potential role in increased atmospheric oxidative capacity. Our earlier works have reported on the SO2 oxidation promoted by nitrate photolysis to produce sulfate. Here, we used glyoxal as a model precursor to examine the role of particulate nitrate photolysis in the formation of secondary organic aerosol (SOA) from particle-phase oxidation of glyoxal by OH radicals. Particles containing sodium nitrate and glyoxal were irradiated at 300 nm. Interestingly, typical oxidation products of oxalic acid, glyoxylic acid, and higher-molecular-weight products reported in the literature were not found in the photooxidation process of glyoxal during nitrate photolysis in the particle phase. Instead, formic acid/formate production was found as the main oxidation product. At glyoxal concentration higher than 3 M, we found that the formic acid/formate production rate increases significantly with increasing glyoxal concentration. Such results suggest that oxidation of glyoxal at high concentrations by OH radicals produced from nitrate photolysis in aqueous particles may not contribute significantly to SOA formation since formic acid is a volatile species. Furthermore, recent predictions of formic acid/formate concentration from the most advanced chemical models are lower than ambient observations at both the ground level and high altitude. The present study reveals a new insight into the production of formic acid/formate as well as a sink of glyoxal in the atmosphere, which may partially narrow the gap between model predictions and field measurements in both species.

Impacts of Chemical Degradation on the Global Budget of Atmospheric Levoglucosan and Its Use As a Biomass Burning Tracer.

Levoglucosan has been widely used to quantitatively assess biomass burning's contribution to ambient aerosols, but previous such assessments have not accounted for levoglucosan's degradation in the atmosphere. We develop the first global simulation of atmospheric levoglucosan, explicitly accounting for its chemical degradation, to evaluate the impacts on levoglucosan's use in quantitative aerosol source apportionment. Levoglucosan is emitted into the atmosphere from the burning of plant matter in open fires (1.7 Tg yr-1) and as biofuels (2.1 Tg yr-1). Sinks of atmospheric levoglucosan include aqueous-phase oxidation (2.9 Tg yr-1), heterogeneous oxidation (0.16 Tg yr-1), gas-phase oxidation (1.4 × 10-4 Tg yr-1), and dry and wet deposition (0.27 and 0.43 Tg yr -1). The global atmospheric burden of levoglucosan is 19 Gg with a lifetime of 1.8 days. Observations show a sharp decline in levoglucosan's concentrations and its relative abundance to organic carbon aerosol (OC) and particulate K+ from near-source to remote sites. We show that such features can only be reproduced when levoglucosan's chemical degradation is included in the model. Using model results, we develop statistical parametrizations to account for the atmospheric degradation in levoglucosan measurements, improving their use for quantitative aerosol source apportionment.

Sensitivities of Ozone Air Pollution in the Beijing-Tianjin-Hebei Area to Local and Upwind Precursor Emissions Using Adjoint Modeling.

Effective mitigation of surface ozone pollution entails detailed knowledge of the contributing precursors' sources. We use the GEOS-Chem adjoint model to analyze the precursors contributing to surface ozone in the Beijing-Tianjin-Hebei area (BTH) of China on days of different ozone pollution severities in June 2019. We find that BTH ozone on heavily polluted days is sensitive to local emissions, as well as to precursors emitted from the provinces south of BTH (Shandong, Henan, and Jiangsu, collectively the SHJ area). Heavy ozone pollution in BTH can be mitigated effectively by reducing NOx (from industrial processes and transportation), ≥C3 alkenes (from on-road gasoline vehicles and industrial processes), and xylenes (from paint use) emitted from both BTH and SHJ, as well as by reducing CO (from industrial processes, transportation, and power generation) and ≥C4 alkanes (from industrial processes, paint and solvent use, and on-road gasoline vehicles) emissions from SHJ. In addition, reduction of NOx, xylene, and ≥C3 alkene emissions within BTH would effectively decrease the number of BTH ozone-exceedance days. Our analysis pinpoint the key areas and activities for locally and regionally coordinated emission control efforts to improve surface ozone air quality in BTH.

Global Significant Changes in Formaldehyde (HCHO) Columns Observed From Space at the Early Stage of the COVID-19 Pandemic.

Satellite HCHO data are widely used as a reliable proxy of non-methane volatile organic compounds (NMVOCs) to constrain underlying emissions and chemistry. Here, we examine global significant changes in HCHO columns at the early stage of the COVID-19 pandemic (January-April 2020) compared with the same period in 2019 with observations from the TROPOspheric Monitoring Instrument (TROPOMI). HCHO columns decline (11.0%) in the Northern China Plain (NCP) because of a combination of meteorological impacts, lower HCHO yields as NO x emission plunges (by 36.0%), and reduced NMVOC emissions (by 15.0%) resulting from the lockdown. HCHO columns change near Beijing (+8.4%) due mainly to elevated hydroxyl radical as NO x emission decreases in a NO x -saturated regime. HCHO columns change in Australia (+17.5%), Northeastern Myanmar of Southeast Asia (+14.9%), Central Africa (+7.8%), and Central America (+18.9%), consistent with fire activities. Our work also points to other changes related to temperature and meteorological variations.

Significant Changes in Chemistry of Fine Particles in Wintertime Beijing from 2007 to 2017: Impact of Clean Air Actions.

The Beijing government implemented a number of clean air action plans to improve air quality in the last 10 years, which contributed to changes in the concentration of fine particles and their compositions. However, quantifying the impacts of these interventions is challenging as meteorology masks the real changes in observed concentrations. Here, we applied a machine learning technique to decouple the effect of meteorology and evaluate the changes in the chemistry of nonrefractory PM1 (particulate matter less than 1 µm) in winter 2007, 2016, and 2017 as a result of the clean air actions. The observed mass concentrations of PM1 were 74.6, 90.2, and 36.1 µg m-3 in the three winters, while the deweathered concentrations were 74.2, 78.7, and 46.3 µg m-3, respectively. The deweathered concentrations of PM1, organics, sulfate, ammonium, chloride, SO2, NO2, and CO decreased by -38, -46, -59, -24, -51, -89, -16, and -52% in 2017 in comparison to 2007. On the contrary, the deweathered concentration of nitrates increased by 4%. Our results indicate that the clean air actions implemented in 2017 were highly effective in reducing ambient concentrations of SO2, CO, and PM1 organics, sulfate, ammonium, and chloride, but the control of nitrate and PM1 organics remains a major challenge.

Parameterized Yields of Semivolatile Products from Isoprene Oxidation under Different NO x Levels: Impacts of Chemical Aging and Wall-Loss of Reactive Gases.

We developed a parametrizable box model to empirically derive the yields of semivolatile products from VOC oxidation using chamber measurements, while explicitly accounting for the multigenerational chemical aging processes (such as the gas-phase fragmentation and functionalization and aerosol-phase oligomerization and photolysis) under different NO x levels and the loss of particles and gases to chamber walls. Using the oxidation of isoprene as an example, we showed that the assumptions regarding the NO x-sensitive, multigenerational aging processes of VOC oxidation products have large impacts on the parametrized product yields and SOA formation. We derived sets of semivolatile product yields from isoprene oxidation under different NO x levels. However, we stress that these product yields must be used in conjunction with the corresponding multigenerational aging schemes in chemical transport models. As more mechanistic insights regarding SOA formation from VOC oxidation emerge, our box model can be expanded to include more explicit chemical aging processes and help ultimately bridge the gap between the process-based understanding of SOA formation from VOC oxidation and the bulk-yield parametrizations used in chemical transport models.