The volatilities of ambient organic aerosol (OA) components are important to forecasting OA formation with models. However, providing the OA volatility distribution inputs for models is challenging, and models often rely on measurements from chamber experiments. We measured the volatility of submicron ambient OA in Seoul during May/June of 2019 by connecting a thermodenuder to an Aerodyne Time-of-Flight Aerosol Mass Spectrometer (AMS). We calculated a volatility basis set (VBS) of the organic aerosol with a thermodenuder mass transfer model and data from the thermodenuder set to various temperatures (30–200 °C). We found a large discrepancy between the measured ambient VBS and a reference VBS used in air quality models, with the ambient organics being less volatile. The results suggest that a modeling study that tries to account for this discrepancy may be needed to identify the impact it has on modeling outcomes. Chamber experiments aiming to determine VBSs for specific chemical systems should address limitations caused by wall losses and incomplete modeling parameters.

Nitrous acid (HONO), an essential precursor of hydroxyl radicals (OH) in the troposphere, plays an integral role in atmospheric photochemistry. However, potential HONO sources remain unclear, particularly in rural areas, where long-term (including seasonal) measurements are scarce. HONO and related parameters were measured at a rural site in the North China Plain (NCP) during the winter of 2017 and summer and autumn of 2020. The mean HONO level was higher in winter (1.79 ± 1.44 ppbv) than in summer (0.67 ± 0.50 ppbv) and autumn (0.83 ± 0.62 ppbv). Source analysis revealed that the heterogeneous conversion (including photo-enhanced conversion) of NO₂ on the ground surface dominated the daytime HONO production in the three seasons (43.1% in winter, 54.3% in summer, and 62.0% in autumn), and the homogeneous reaction of NO and OH contributed 37.8, 12.2, and 28.4% of the daytime HONO production during winter, summer, and autumn, respectively. In addition, the total contributions of other sources (direct vehicle emissions, particulate nitrate photolysis, NO₂ uptake and its photo-enhanced reaction on the aerosol surface) to daytime HONO production were less than 5% in summer and autumn and 12.0% in winter. Unlike winter and autumn, an additional HONO source was found in summer (0.45 ± 0.21 ppbv h⁻¹, 31.4% to the daytime HONO formation), which might be attributed to the HONO emission from the fertilized field. Among the primary radical sources (photolysis of HONO, O₃, and formaldehyde), HONO photolysis was dominant, with contributions of 82.6, 49.3, and 63.2% in winter, summer, and autumn, respectively. Our findings may aid in understanding HONO formation in different seasons in rural areas and may highlight the impact of HONO on atmospheric oxidation capacity.

Intermediate-volatility and semi-volatile organic compounds (I/SVOCs) are key precursors of secondary organic aerosol (SOA). However, the comprehensive characterization of I/SVOCs has long been an analytical challenge. Here, we develop a novel method of speciating and quantifying I/SVOCs using two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-ToF-MS) by constructing class-screening programs based on their characteristic fragments and mass spectrum patterns. Using this new approach, we then present a comprehensive analysis of gaseous I/SVOC emissions from heavy-duty diesel vehicles (HDDVs). Over three-thousand compounds are identified and classified into twenty-one categories. The dominant compound groups of I/SVCOs emitted by HDDVs are alkanes (including normal and branched alkanes, 37–66%), benzylic alcohols (7–20%), alkenes (3–11%), cycloalkanes (3–9%), and benzylic ketones (1–4%). Oxygenated I/SVOCs (O–I/SVOCs, e.g., benzylic alcohols and ketones) are first quantified and account for >20% of the total I/SVOC mass. Advanced aftertreatment devices largely reduce the total I/SVOC emissions but increase the proportion of O–I/SVOCs. With the speciation data, we successfully map the I/SVOCs into the two-dimensional volatility basis set space, which facilitates a better estimation of SOA. As aging time goes by, approximate 45% difference between the two scenarios after seven-day aging is observed, which confirms the significant impact of speciated I/SVOC emission data on SOA prediction.

The Community Multi-Scale Air Quality (CMAQ) model was applied to evaluate the air quality in the coastal city of Kannur, India, during the 2020 COVID-19 lockdown. From the Pre1 (March 1–24, 2020) period to the Lock (March 25–April 19, 2020) and Tri (April 20–May 9, 2020) periods, the Kerala state government gradually imposed a strict lockdown policy. Both the simulations and observations showed a decline in the PM₂.₅ concentrations and an enhancement in the O₃ concentrations during the Lock and Tri periods compared with that in the Pre1 period. Integrated process rate (IPR) analysis was employed to isolate the contributions of the individual atmospheric processes. The results revealed that the vertical transport from the upper layers dominated the surface O₃ formation, comprising 89.4%, 83.1%, and 88.9% of the O₃ sources during the Pre1, Lock, and Tri periods, respectively. Photochemistry contributed negatively to the O₃ concentrations at the surface layer. Compared with the Pre1 period, the O₃ enhancement during the Lock period was primarily attributable to the lower negative contribution of photochemistry and the lower O₃ removal rate by horizontal transport. During the Tri period, a slower consumption of O₃ by gas-phase chemistry and a stronger vertical import from the upper layers to the surface accounted for the increase in O₃. Emission and aerosol processes constituted the major positive contributions to the net surface PM₂.₅, accounting for a total of 48.7%, 38.4%, and 42.5% of PM₂.₅ sources during the Pre1, Lock, and Tri periods, respectively. The decreases in the PM₂.₅ concentrations during the Lock and Tri periods were primarily explained by the weaker PM₂.₅ production from emission and aerosol processes. The increased vertical transport rate of PM₂.₅ from the surface layer to the upper layers was also a reason for the decrease in the PM₂.₅ during the Lock periods.

We quantify for the first time marine aerosol properties and their differences in the offshore and remote ocean in the mid-latitude South Asian waters, low-latitude South Asian waters, and equatorial waters of the Western Pacific Ocean, based on shipboard cruise observations conducted by the Western Pacific Ocean Scientific Observation Network in winter 2018, and further investigate the effects of long-range transport of continental aerosols on the marine environment. During the overall observation period, the average number concentration of particle matter which aerodynamic diameters<2.5 μm (PM₂.₅N) was 35.1 ± 87.4 cm⁻³ and the mass concentration (PM₂.₅M) was 12.3 ± 9.1 μg/m³. The PM₂.₅N and PM₂.₅M during the continental air mass transport period were 7.2 and 1.3 times higher than those during the non-transport period (109.2 ± 169.3 cm⁻³, 15.9 ± 14.9 μg/m³), respectively. Excluding transport period, the average PM₂.₅N and PM₂.₅M are reduced by 120% and 7%. Coarse mode particle number concentration (PM₂.₅–₁₀N) and mass concentration (PM₂.₅–₁₀M) are not significantly influenced by continental air masses (only a reduction of 7% and 2%). The variation of marine aerosol concentrations in different latitudes zones is greatly influenced by continental aerosol transport. The offshore PM₂.₅M/PM₁₀M was 30%, 21%, and 22% in the mid-latitude sea of South Asia, a low-latitude sea of South Asia, and the equatorial sea, respectively. In comparison, in the remote ocean, the distribution ratio of PM₂.₅M/PM₁₀M tended to be steady (22%–23%), and the background characteristics of marine aerosols were clearly represented. The aerosol concentration decreases with the increase of wind speed during the transport period, and the wind speed reflects the scavenging effect on aerosol. In the non-transport period, the wind speed at the sea surface promotes the generation of marine aerosols, and the impact in wind speed is strongest in the PM₂.₅–PM₅ particle size range.

Various precursor emissions and chemical mechanisms for secondary organic aerosol (SOA) formation were incorporated into a regional air quality model system (RAQMS) and applied to investigate the distribution, composition, and source contribution of SOA over east China in summer 2018. Model comparison against a variety of observations at a national scale demonstrated that the model was able to reasonably reproduce meteorological variables, O₃ and PM₂.₅ concentrations, and the model simulated SOA concentration generally agreed with observations, with the overall NMB of 7.0% and R of 0.4 in 10 cities over east China. The simulated period-mean SOA concentrations of 4–15 μg m⁻³ were mainly distributed over the North China Plain (NCP), the middle and lower reaches of the Yangtze River and Chongqing district. SOA dominated organic aerosol (OA) over China in summertime (90%). The percentage contributions to SOA from ASOA (SOA produced from anthropogenic volatile organic compounds (AVOC)), BSOA (SOA produced from biogenic volatile organic compounds (BVOC)), DSOA (SOA produced from aqueous uptake of glyoxal and methylglyoxal) and S/I-SOA (SOA produced from semi-volatile and intermediate volatile organic compounds) were estimated to be 48.3%, 28.6%, 14.3%, and 8.8% respectively, over east China in summertime. In terms of domain and period average, ASOA contributed most to SOA (59%) in north China, while BSOA contributed most to SOA (37.3%) in northeast China. The percentage contribution of DSOA to SOA reached 21.5% in southwest China. S/I-SOA accounted for approximately 10% of SOA in most areas of east China. This study reveals that while AVOC dominates SOA formation on average over east China, the SOA source contributions differ considerably in different regions of China. BVOC makes the same contribution to SOA formation as AVOC in northeast China and southwest China, where forest coverage and BVOC emission are higher and anthropogenic emissions are relatively low, highlighting the significant role of BVOC in summer SOA formation in China.