Dual isotope analysis reveals the COVID-19 lockdown impact on nitrate aerosol sources and formation pathways in Shanghai.

Nitrate (NO) is an important contributor to PM which can adversely affect the environment and human health. A noticeable decrease in NO concentrations has been reported due to the lockdown measures implemented to curb the spread of Corona Virus Disease 2019 (COVID-19). However, questions remain, regarding the nonlinear relationship between NO and NO. Here, we collected PM samples in two periods, before and during the lockdown of COVID-19 in Shanghai. Dual isotopes (δO-NO and δN-NO) of NO were measured to investigate the formation pathways and potential sources of NO. The results showed that the concentration of NO decreased significantly during the lockdown period compared to the period before the lockdown. Additionally, the hydroxyl pathway was the dominant contributor to NO production during the lockdown period, while NO hydrolyses dominated the formation of NO before the lockdown. This change is largely attributable to alterations in the oxidative potential of the environment. In comparison to the period preceding the lockdown, the relative contributions of each NO source remained largely unchanged throughout the lockdown periods. Nevertheless, the concentration of NO contributed by each NO source exhibited a notable decline, particularly the mobile sources and coal combustion. Furthermore, the reduction extent of NO due to the lockdown period was also greater than the reduction during the Clean Air Actions (2013-2017). Our findings provide evidence that the COVID-19 lockdown led to a decrease in NO concentration due to changes in the formation pathway and reductions in NO emissions from various sources.