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.

Nitrate (${\mathrm{NO}}_{\mathrm{3}}^{-}$ ) has become a major component of fine particulate matter (PM_2.5) during hazy days in China. However, the role of the heterogeneous reactions of dinitrogen pentoxide (N₂O₅) in nitrate formation is not well constrained. In January 2017, a severe haze event occurred in the Pearl River Delta (PRD) of southern China during which high levels of PM_2.5 (∼400 µg m⁻³) and O₃ (∼160 ppbv) were observed at a semi-rural site (Heshan) in the western PRD. Nitrate concentrations reached 108 µg m⁻³ (1 h time resolution), and the contribution of nitrate to PM_2.5 was nearly 40 %. Concurrent increases in ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and ClNO₂ (with a maximum value of 8.3 ppbv at a 1 min time resolution) were observed in the first several hours after sunset, indicating an intense N₂O₅ heterogeneous uptake by aerosols. The formation potential of ${\mathrm{NO}}_{\mathrm{3}}^{-}$ via N₂O₅ heterogeneous reactions was estimated to be between 29.0 and 77.3 µg m⁻³ in the early hours (2 to 6 h) after sunset based on the measurement data, which could completely explain the measured increase in the ${\mathrm{NO}}_{\mathrm{3}}^{-}$ concentration during the same time period. Daytime production of nitric acid from the gas-phase reaction of OH+NO₂ was calculated with a chemical box model built using the Master Chemical Mechanism (MCM v3.3.1) and constrained by the measurement data. The integrated nocturnal nitrate formed via N₂O₅ chemistry was comparable to or even higher than the nitric acid formed during the day. This study confirms that N₂O₅ heterogeneous chemistry was a significant source of aerosol nitrate during hazy days in southern China.

Heterogeneous reactivity of N₂O₅ on aerosols is a critical parameter in assessing NO_x fate, nitrate production, and particulate chloride activation. Accurate measurement of its uptake coefficient (${\mathit{\gamma }}_{{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ ) and representation in air quality models are challenging, especially in the polluted environment. With an in situ aerosol flow-tube system, the ${\mathit{\gamma }}_{{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ was directly measured on ambient aerosols at two rural sites in northern and southern China. The results were analyzed together with the ${\mathit{\gamma }}_{{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ derived from previous field studies in China to obtain a holistic picture of N₂O₅ uptake and the influencing factors under various climatic and chemical conditions. The field-derived or measured ${\mathit{\gamma }}_{{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ was generally promoted by the aerosol water content and suppressed by particle nitrate. Significant discrepancies were found between the measured ${\mathit{\gamma }}_{{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ and that estimated from laboratory-determined parameterizations. An observation-based empirical parameterization was derived in the present work, which better reproduced the mean value and variability of the observed ${\mathit{\gamma }}_{{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ . Incorporating this new parameterization into a regional air quality model (WRF-CMAQ) has improved the simulation of N₂O₅, nitrogen oxides, and secondary nitrate in the polluted regions of China.

ClNO₂ and Cl₂ can affect atmospheric oxidation and thereby the formation of ozone and secondary aerosols, yet their sources and production mechanisms are not well understood or quantified. In this study we present field observations of ClNO₂ and Cl₂ at a suburban site in eastern China during April 2018. Persistent high levels of ClNO₂ (maximum: ∼3.7 ppbv; 1 min average) were frequently observed at night, due to the high ClNO₂ yield (φ (ClNO₂), 0.56±0.20) inferred from the measurements. The φ (ClNO₂) value showed a positive correlation with the [Cl⁻] / [H₂O] ratio, and its parameterization was improved at low to median yields (0–0.75) by the incorporation of [Cl⁻] / [H₂O] and the suppression effect of aerosol organics. ClNO₂ and Cl₂ showed a significant correlation on most nights. We show that the Cl₂ at our site was more likely a co-product with ClNO₂ from N₂O₅ uptake on acidic aerosols that contain chloride than being produced by ClNO₂ uptake as previously suggested. We propose a mechanism in which ${\mathrm{NO}}_{\mathrm{2}}^{+}$ can react with Cl⁻ to produce Cl₂ and ClNO₂ simultaneously. Under a new framework which regards Cl₂, ClNO₂, and nitrate as products of N₂O₅ uptake, the Cl₂ yield (φ (Cl₂)) was derived using ambient data. φ (Cl₂) exhibited significant correlations with [Cl⁻] and [H⁺], based on which a parameterization of φ (Cl₂) was developed. The derived parameterizations of φ (ClNO₂) and φ (Cl₂) can be used in models to evaluate the nighttime production of ClNO₂ and Cl₂ and their impact on the next day's photochemistry.