Influence of COVID-19 Lockdown on Atmospheric Nitrate Sources and Chemistry in the Northeastern US

Abstract The outbreak of COVID-19 and subsequent lockdowns caused widespread restrictions on human activities. This presented a unique opportunity to track atmospheric chemistry changes under rapid emission reductions. Using stable nitrogen (δ15N) and triple oxygen isotopes (δ18O and Δ17O) of particulate nitrate (pNO3) collected from 2019 to 2020 in the northeastern US, combined with 0-D box-model simulations, we investigate the response of atmospheric nitrate chemistry during the COVID-19 lockdown period (March 2020) in the northeastern US. Triple oxygen isotope measurements indicate that emission reductions altered the relative contributions of NOx oxidation pathways, with a shift toward the NO2 + OH pathway and a reduced contribution from N2O5 hydrolysis. This shift was consistent with the modeled increase in OH concentrations and the observed and modeled decrease in O3 during the COVID-19 lockdown, which limited N2O5 formation and subsequent heterogeneous nitrate formation. Moreover, nitrogen isotope-constrained sensitivity analysis suggests that a reduction of approximately 40(±10)% in transportation NOx emissions relative to the annual emission inventory estimates may be required to reproduce the observed decrease in δ15N(pNO3) during the COVID-19 period. These findings highlight the utility of isotopes to detail the complex interactions between emission reductions, atmospheric oxidation chemistry, and secondary aerosol formation. Understanding these interactions is critical for effectively determining air pollution mitigation strategies and testing air-quality models, particularly under rapidly evolving emission conditions associated with energy transitions and policy interventions.