Nitrate (NO3-) is an important contributor to PM2.5 which can adversely affect the environment and human health. A noticeable decrease in NOx 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 NOx and NO3-. Here, we collected PM2.5 samples in two periods, before and during the lockdown of COVID-19 in Shanghai. Dual isotopes (δ18O-NO3- and δ15N-NO3-) of NO3- were measured to investigate the formation pathways and potential sources of NO3-. The results showed that the concentration of NO3- decreased significantly during the lockdown period compared to the period before the lockdown. Additionally, the hydroxyl pathway was the dominant contributor to NO3- production during the lockdown period, while N2O5 hydrolyses dominated the formation of NO3- 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 NOx source remained largely unchanged throughout the lockdown periods. Nevertheless, the concentration of NO3- contributed by each NOx source exhibited a notable decline, particularly the mobile sources and coal combustion. Furthermore, the reduction extent of NO3- 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 NO3- concentration due to changes in the formation pathway and reductions in NOx emissions from various sources.
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