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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 N 2 O 5 on the formation of nitrate (NO 3 − ) and ammonium (NH 4 + ) in Beijing. The results show that during heavy pollution days (e.g., during 14–17 September 2015, with PM 2.5 concentration over 100 μg/m 3 ), the concentrations of NO 2 and O 3 were high, with maxima of 90 and 240 μg/m 3 , respectively, providing high precursors for the formation of N 2 O 5 . In addition, the aerosol and sulfate concentrations were also high, with maxima of 201 μg/m 3 and 23 μg/m 3 respectively, providing reacting surface for the heterogeneous reaction. As a result, the hydrolysis of N 2 O 5 led to 21.0% enhancement of nitrate (NO 3 − ) and 7.5% enhancement of ammonium (NH 4 + ). It is worth to note that this important effect only occurred in high pollution days (PM 2.5 concentration over 100 μg/m 3 ). During low-pollution periods (PM 2.5 concentration < 100 μg/m 3 ), the effect of hydrolysis of N 2 O 5 on the formation of nitrate and ammonium was insignificant (variation rate < 5%). This study suggests that during heavy pollution periods, the hydrolysis of N 2 O 5 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. [ABSTRACT FROM AUTHOR]