Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary aerosols that affect Earth’s radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH₂SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate.

Nitryl chloride (ClNO₂) is a dominant source of chlorine radical in polluted environment, and can significantly affect the atmospheric oxidative chemistry. However, the abundance of ClNO₂ and its exact role are not fully understood under different environmental conditions. During the summer of 2014, we deployed a chemical ionization mass spectrometer to measure ClNO₂ and dinitrogen pentoxide (N₂O₅) at a rural site in the polluted North China Plain. Elevated mixing ratios of ClNO₂ (> 350 pptv) were observed at most of the nights with low levels of N₂O₅ (< 200 pptv). The highest ClNO₂ mixing ratio of 2070 pptv (1 min average) was observed in a plume from a megacity (Tianjin), and was characterized with a faster N₂O₅ heterogeneous loss rate and ClNO₂ production rate compared to average conditions. The abundant ClNO₂ concentration kept increasing even after sunrise, and reached a peak 4 h later. Such highly sustained ClNO₂ peaks after sunrise are discrepant from the previously observed typical diurnal pattern. Meteorological and chemical analysis shows that the sustained ClNO₂ morning peaks are caused by significant ClNO₂ production in the residual layer at night followed by downward mixing after breakup of the nocturnal inversion layer in the morning. We estimated that ∼ 1.7–4.0 ppbv of ClNO₂ would exist in the residual layer in order to maintain the observed morning ClNO₂ peaks at the surface site. Observation-based box model analysis show that photolysis of ClNO₂ produced chlorine radical with a rate up to 1.12 ppbv h⁻¹, accounting for 10–30 % of primary RO_x production in the morning hours. The perturbation in total radical production leads to an increase of integrated daytime net ozone production by 3 % (4.3 ppbv) on average, and with a larger increase of 13 % (11 ppbv) in megacity outflow that was characterized with higher ClNO₂ and a relatively lower oxygenated hydrocarbon (OVOC) to non-methane hydrocarbon (NMHC) ratio.

Nitrous acid (HONO) can strongly affect atmospheric photochemistry in polluted regions through the production of hydroxyl radicals (OHs). In January 2017, a severe pollution episode occurred in the Pearl River Delta (PRD) of China, with maximum hourly PM_2.5, ozone, and HONO levels reaching 400 µg m⁻³, 150 ppb, and 8 ppb, respectively, at a suburban site. The present study investigated the sources and processes generating such high HONO concentrations and the role of HONO chemistry in this severe winter episode. Four recently reported HONO sources were added to the Community Multiscale Air Quality (CMAQ) model, including RH-dependent (relative humidity) and light-enhancing effects on heterogeneous reactions, photolysis of particulate nitrate in the atmosphere, and photolysis of HNO₃ and nitrate on surfaces. The revised model reproduced the observed HONO and significantly improved its performance for O₃ and PM_2.5. The model simulations showed that the heterogeneous generation on surfaces (with RH and light effects) was the largest contributor (72 %) to the predicted HONO concentrations, with the RH-enhancing effects more significant at nighttime and the light-enhancing effects more important in the daytime. The photolysis of total nitrate in the atmosphere and deposited on surfaces was the dominant HONO source during noon and afternoon, contributing above 50 % of the simulated HONO. The HONO photolysis was the dominant contributor to HO_x production in this episode. With all HONO sources, the daytime average O₃ at the Heshan site was increased by 24 ppb (or 70 %), compared to the simulation results without any HONO sources. Moreover, the simulated mean concentrations of TNO₃ (HNO₃+ fine particle NO₃⁻) at the Heshan site, which was the key species for this haze formation, increased by about 17 µg m⁻³ (67 %) due to the HONO chemistry, and the peak enhancement reached 55 µg m⁻³. This study highlights the key role of HONO chemistry in the formation of winter haze in a subtropical environment.

Nitryl chloride (ClNO₂) plays potentially important roles in atmospheric chemistry, but its abundance and effect are not fully understood due to the small number of ambient observations of ClNO₂ to date. In late autumn 2013, ClNO₂ was measured with a chemical ionization mass spectrometer (CIMS) at a mountain top (957 m above sea level) in Hong Kong. During 12 nights with continuous CIMS data, elevated mixing ratios of ClNO₂ (>400 parts per trillion by volume) or its precursor N₂O₅ (>1000 pptv) were observed on six nights, with the highest ever reported ClNO₂ (4.7 ppbv, 1 min average) and N₂O₅ (7.7 ppbv, 1 min average) in one case. Backward particle dispersion calculations driven by winds simulated with a mesoscale meteorological model show that the ClNO₂/N₂O₅‐laden air at the high‐elevation site was due to transport of urban/industrial pollution north of the site. The highest ClNO₂/N₂O₅ case was observed in a later period of the night and was characterized with extensively processed air and with the presence of nonoceanic chloride. A chemical box model with detailed chlorine chemistry was used to assess the possible impact of the ClNO₂ in the well‐processed regional plume on next day ozone, as the air mass continued to downwind locations. The results show that the ClNO₂ could enhance ozone by 5–16% at the ozone peak or 11–41% daytime ozone production in the following day. This study highlights varying importance of the ClNO₂ chemistry in polluted environments and the need to consider this process in photochemical models for prediction of ground‐level ozone and haze.

Dinitrogen pentoxide (N₂O₅) and nitryl chloride (ClNO₂) are key species in nocturnal tropospheric chemistry and have significant effects on particulate nitrate formation and the following day's photochemistry through chlorine radical production and NO_x recycling upon photolysis of ClNO₂. To better understand the roles of N₂O₅ and ClNO₂ in the high-aerosol-loading environment of northern China, an intensive field study was carried out at a high-altitude site (Mt. Tai, 1465 m a.s.l.) in the North China Plain (NCP) during the summer of 2014. Elevated ClNO₂ plumes were frequently observed in the nocturnal residual layer with a maximum mixing ratio of 2.1 ppbv (1 min), whilst N₂O₅ was typically present at very low levels (< 30 pptv), indicating fast heterogeneous N₂O₅ hydrolysis. Combined analyses of chemical characteristics and backward trajectories indicated that the ClNO₂-laden air was caused by the transport of NO_x-rich plumes from the coal-fired industry and power plants in the NCP. The heterogeneous N₂O₅ uptake coefficient (γ) and ClNO₂ yield (ϕ) were estimated from steady-state analysis and observed growth rate of ClNO₂. The derived γ and ϕ exhibited high variability, with means of 0.061 ± 0.025 and 0.28 ± 0.24, respectively. These values are higher than those derived from previous laboratory and field studies in other regions and cannot be well characterized by model parameterizations. Fast heterogeneous N₂O₅ reactions dominated the nocturnal NO_x loss in the residual layer over this region and contributed to substantial nitrate formation of up to 17 µg m⁻³. The estimated nocturnal nitrate formation rates ranged from 0.2 to 4.8 µg m⁻³ h⁻¹ in various plumes, with a mean of 2.2 ± 1.4 µg m⁻³ h⁻¹. The results demonstrate the significance of heterogeneous N₂O₅ reactivity and chlorine activation in the NCP, and their unique and universal roles in fine aerosol formation and NO_x transformation, and thus their potential impacts on regional haze pollution in northern China.

Heterogeneous uptake of dinitrogen pentoxide (N₂O₅) and production of nitryl chloride (ClNO₂) are important nocturnal atmospheric processes that have significant implications for the production of secondary pollutants. However, the understanding of N₂O₅ uptake processes and ClNO₂ production remains limited, especially in China. This study presents a field investigation of the N₂O₅ heterogeneous uptake coefficient (γ(N₂O₅)) and ClNO₂ production yield (ϕ) in a polluted area of northern China during the summer of 2014. The N₂O₅ uptake coefficient and ClNO₂ yield were estimated by using the simultaneously measured ClNO₂ and total nitrate in 10 selected cases, which have concurrent increases in the ClNO₂ and nitrate concentrations and relatively stable environmental conditions. The determined γ(N₂O₅) and ϕ values varied greatly, with an average of 0.022 for γ(N₂O₅) (±0.012, standard deviation) and 0.34 for ϕ (±0.28, standard deviation). The variations in γ(N₂O₅) could not be fully explained by the previously derived parameterizations of N₂O₅ uptake that consider nitrate, chloride, and the organic coating. Heterogeneous uptake of N₂O₅ was found to have a strong positive dependence on the relative humidity and aerosol water content. This result suggests that the heterogeneous uptake of N₂O₅ in Wangdu is governed mainly by the amount of water in the aerosol, and is strongly water limited, which is different from most of the field observations in the US and Europe. The ClNO₂ yield estimated from the parameterization was also overestimated comparing to that derived from the observation. The observation-derived ϕ showed a decreasing trend with an increasing ratio of acetonitrile to carbon monoxide, an indicator of biomass burning emissions, which suggests a possible suppressive effect on the production yield of ClNO₂ in the plumes influenced by biomass burning in this region. The findings of this study illustrate the need to improve our understanding and to parameterize the key factors for γ(N₂O₅) and ϕ to accurately assess photochemical and haze pollution.