Secondary organic aerosol (SOA) contributes a significant fraction to aerosol mass and toxicity.Low-volatility organic vapors are critical intermediates connecting the oxidation of volatile organic compounds (VOCs) to SOA formation. However, the direct measurement of intermediate vapors poses a great challenge, further compounded by the difficulty of linking them to specific precursors from a cocktail of complex emission sources in the vast urbanized areas. Here, we present coordinated measurements of low-volatility oxidation products, termed oxygenated organic molecules (OOMs) in three most urbanized regions in China. With a newly-developed analysis methodology, we are able to assign these OOMs to their likely precursors and ultimately connect SOA formation to various VOCs. At all measurement locations, we find similar OOM

Abstract. 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 PM2.5, 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 PM2.5. 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 HOx 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.

Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl2), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the daytime Cl2 concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl2 (2.7 pptv s−1 at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl2 production. The high Cl2 concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.

Abstract. Nitrate (NO3-) has become a major component of fine particulate matter (PM2.5) 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 PM2.5 (∼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 PM2.5 was nearly 40 %. Concurrent increases in NO3- 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 NO3- 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 NO3- 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.