Double-Edged Role of VOCs Reduction in Nitrate Formation: Insights from Observations during the China International Import Expo 2018.

Aerosol nitrate (NO 3 - ) constitutes a significant component of fine particles in China. Prioritizing the control of volatile organic compounds (VOCs) is a crucial step toward achieving clean air, yet its impact on NO 3 - pollution remains inadequately understood. Here, we examined the role of VOCs in NO 3 - formation by combining comprehensive field measurements conducted during the China International Import Expo (CIIE) in Shanghai (from 10 October to 22 November 2018) and multiphase chemical modeling. Despite a decline in primary pollutants during the CIIE, NO 3 - levels increased compared to pre-CIIE and post-CIIE─NO 3 - concentrations decreased in the daytime (by -10 and -26%) while increasing in the nighttime (by 8 and 30%). Analysis of the observations and backward trajectory indicates that the diurnal variation in NO 3 - was mainly attributed to local chemistry rather than meteorological conditions. Decreasing VOCs lowered the daytime NO 3 - production by reducing the hydroxyl radical level, whereas the greater VOCs reduction at night than that in the daytime increased the nitrate radical level, thereby promoting the nocturnal NO 3 - production. These results reveal the double-edged role of VOCs in NO 3 - formation, underscoring the need for transferring large VOC-emitting enterprises from the daytime to the nighttime, which should be considered in formulating corresponding policies.