Nitrate formation from heterogeneous uptake of dinitrogen pentoxide during a severe winter haze in southern China

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 (similar to 400 mu g m(-3)) and O-3 (similar to 160 ppbv) were observed at a semi-rural site (Heshan) in the western PRD. Nitrate concentrations reached 108 mu g m(-3) 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 mu g 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.