Short-term change in air pollution following the COVID-19 state of emergency: A national analysis for the United States

Lockdown measures taken in response to the COVID-19 pandemic produced sudden social and economic changes. We examined the extent of air pollution reduction that was attained under these extreme circumstances, whether these reductions occurred everywhere in the US, and the local factors that drove them. Employing counterfactual time series analysis based on seasonal autoregressive integrated moving average models, we found that these extreme lockdown measures led to a reduction in the weekly PM_2.5 average by up to 3.4 µg m^-3 and the weekly NO₂ average by up to 11 ppb. These values represent a substantial fraction of the annual mean NAAQS values of 12 µg m^-3 and 53 ppb, respectively. We found evidence of a statistically significant decline in NO₂ concentrations following the state-level emergency declaration in almost all states. However, statistically significant declines in PM_2.5 occurred mostly in the West Coast and the Northeast. Certain states experienced a decline in NO₂ but an increase in PM_2.5 concentrations, indicating that these two pollutants arise from dissimilar sources in these states. Finally, we found evidence that states with a higher percentage of mobile source emissions prior to the emergency measures experienced a greater decline in NO₂ levels during the pandemic. Although the current social and economic restrictions are not sustainable, our results provide a benchmark to estimate the extent to which air pollution reductions can be achieved. We also identify factors that contributed to the magnitude of pollutant reductions, which can help guide future state-level policies to sustainably reduce air pollution.