Effect of hydrolysis of N2O5 on nitrate and ammonium formation in Beijing China: WRF-Chem model simulation

Beijing, the capital of China, is a mega city with a population of >20 million. In recent years, the city has experienced heavy air pollution, with particulate matter (PM) being one of its top pollutants. In the last decade, extensive efforts have been made to characterize the sources, properties, and processes of PM in Beijing. Despite progress made by previous studies, there are still some important questions to be answered and addressed. The focus of this research is to study the impact of the heterogeneous hydrolysis of N₂O₅ on the formation of nitrate (NO₃^-) and ammonium (NH₄⁺) in Beijing. The results show that during heavy pollution days (e.g., during 14-17 September 2015, with PM_2.5 concentration over 100μg/m³), the concentrations of NO₂ and O₃ were high, with maxima of 90 and 240μg/m³, respectively, providing high precursors for the formation of N₂O₅. In addition, the aerosol and sulfate concentrations were also high, with maxima of 201μg/m³ and 23μg/m³ respectively, providing reacting surface for the heterogeneous reaction. As a result, the hydrolysis of N₂O₅ led to 21.0% enhancement of nitrate (NO₃^-) and 7.5% enhancement of ammonium (NH₄⁺). It is worth to note that this important effect only occurred in high pollution days (PM_2.5 concentration over 100μg/m³). During low-pollution periods (PM_2.5 concentration <100μg/m³), the effect of hydrolysis of N₂O₅ on the formation of nitrate and ammonium was insignificant (variation rate <5%). This study suggests that during heavy pollution periods, the hydrolysis of N₂O₅ enhances the level of aerosol pollution in Beijing, and needs to be further studied in order to perform efficient air pollution control and mitigation strategies.