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

Abstract. Nitrate ( NO 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 ( N 2 O 5 ) 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 µ g m −3 ) and O 3 ( ∼160 ppbv) were observed at a semi-rural site (Heshan) in the western PRD. Nitrate concentrations reached 108 µ g m −3 (1 h time resolution), and the contribution of nitrate to PM 2.5 was nearly 40 %. Concurrent increases in NO 3 - and ClNO 2 (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 N 2 O 5 heterogeneous uptake by aerosols. The formation potential of NO 3 - via N 2 O 5 heterogeneous reactions was estimated to be between 29.0 and 77.3 µ 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 NO 3 - concentration during the same time period. Daytime production of nitric acid from the gas-phase reaction of OH+NO 2 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 N 2 O 5 chemistry was comparable to or even higher than the nitric acid formed during the day. This study confirms that N 2 O 5 heterogeneous chemistry was a significant source of aerosol nitrate during hazy days in southern China.