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

Nitrate (NO₃^-) is an important contributor to PM_2.5 which can adversely affect the environment and human health. A noticeable decrease in NO_x 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_x and NO₃^-. Here, we collected PM_2.5 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 NO_x source remained largely unchanged throughout the lockdown periods. Nevertheless, the concentration of NO₃^- contributed by each NO_x 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 NO_x emissions from various sources.