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Secondary inorganic aerosols, including sulfate, nitrate, and ammonium contribute to a large extent to the severe haze pollution events in China. Understanding their formation mechanisms is critical for designing effective mitigation strategies to control haze pollution, especially as the role of nitrate seemed to become more important recently, especially in some megacities. In the present study, simultaneous observations were conducted in two megacities (Chengdu and Chongqing) in Sichuan Basin of southwest China, one of the regions suffering from severe aerosol pollution. One typical long-lasting pollution event in Chengdu and Chongqing was captured during wintertime from December 25, 2016 to January 5, 2017. The campaign-average of hourly concentrations of PM 2.5 , sulfate, and nitrate, measured by an Aerosol Analyzer (ZSF) were 101 ± 73.8 μg/m 3 , 15.9 ± 11.8 μg/m 3 , and 24.9 ± 20.6 μg/m 3 , respectively, in Chengdu, and were 87.7 ± 53.8 μg/m 3 , 19.7 ± 13.5 μg/m 3 , and 15.1 ± 10.1 μg/m 3 , respectively, in Chongqing. Nitrate contributed substantially to PM 2.5 pollution when PM 2.5 was lower than 150 μg/m 3 , largely due to the strong secondary transformation of NO X to nitrate during the occurrence of the pollution episode. Heterogeneous hydrolysis of N 2 O 5 dominated nitrate formation during nighttime, while photochemical reactions and high-RH enhanced gas- to aqueous-phase dissolution of NH 3 and HNO 3 or cloud processes likely played important roles for nitrate formation during daytime. RH-dependent heterogeneous reactions contributed greatly to the formation of sulfate. NO X is confirmed to play an important role as an oxidant in accelerating the secondary transformation of SO 2 to sulfate at high RH and low O 3 levels under neutralization condition during heavy PM 2.5 pollution episode. Results from this study identified the formation mechanism of nitrate, especially during the daytime, and addressed the importance of heterogeneous inorganic reactions in the formation of heavy aerosol pollution events., (Crown Copyright © 2019. Published by Elsevier Ltd. All rights reserved.)