Wintertime nitrate formation pathways in the north China plain: Importance of N₂O₅ heterogeneous hydrolysis

Strict emission control measures have been implemented in the North China Plain (NCP) to improve air quality since 2013. However, heavy particulate matter (PM) pollution still frequently occurs in the region especially during wintertime, and the nitrate contribution to fine PM (PM_2.5) has substantially increased in recent several years. Nitrate aerosols, which are formed via nitric acid (HNO₃) to balance inorganic cations in the particle phase, have become a major fraction of PM_2.5 during wintertime haze events in the NCP. HNO₃ is mainly produced through homogeneous (NO₂+OH, NO₃+VOCs) and heterogeneous pathways (N₂O₅ heterogeneous hydrolysis) in the atmosphere, but the contribution of the two pathways to the nitrate formation remains elusive. In this study, the Weather Research and Forecasting model with Chemistry (WRF-Chem) was applied to simulate a heavy haze episode from 16 to December 31, 2016 in the North China Plain, and the source-oriented method (SOM) and brute force method (BFM) were both used to evaluate contributions of the heterogeneous pathway to the nitrate formation. The results demonstrated that the near-surface nitrate contributions of the heterogeneous pathway were about 30.8% based on the BFM, and 51.6% based on the SOM, indicating that the BFM might be subject to underestimating importance of the heterogeneous pathway to the nitrate formation. The SOM simulations further showed that the heterogeneous pathway dominated the nighttime HNO₃ production in the planetary boundary layer, with an average contribution of 83.0%. Although N₂O₅ was photolytically liable during daytime, the heterogeneous N₂O₅ hydrolysis still contributed 10.1% of HNO₃, which was caused by substantial attenuation of incident solar radiation by clouds and high PM_2.5 mass loading. Our study highlighted the significantly important role of N₂O₅ heterogeneous hydrolysis in the nitrate formation during wintertime haze days.