Simultaneous decreases in NO2 levels and disparities in California during the COVID-19 pandemic

This study investigated the changes in nitrogen dioxide (NO2) air pollution levels and their disparities in California, U.S. during the pandemic of coronavirus disease 2019 (COVID-19). From pre-COVID (February 11-March 10, 2020) to peri-COVID (March 22-April 21, 2020) periods, ground-level measured NO2 concentrations decreased by 6.9 ppb (54.2%) on average across California. Our novel statistical approach showed that the meteorology-adjusted NO2 concentrations declined to a smaller extent (4.3 ppb, 34.4%), implying that the meteorological parameters helped reduce NO2 concentrations along with lockdown-related NOx emission reductions. In addition, satellite TROPOspheric Monitoring Instrument (TROPOMI) tropospheric NO2 data demonstrated substantial decreases in NO2 levels in urban (49.7% versus 16.5% in rural areas) and high-traffic (50.1%) areas. On average, 32.8%, 62.8%, and 14.9% higher TROPOMI NO2 levels were identified for socially vulnerable populations, as defined by the CalEnviroScreen 3.0, race/ethnicity, and educational attainment, respectively. The NO2 exposure differences generally decreased as average TROPOMI NO2 levels decreased during the pandemic (Pearson correlations = 0.61-0.95). From pre- to peri-COVID periods, the decreasing NO2 disparities were pronounced in relation to race/ethnicity [from 39.0 (78.6%) to 10.4 (36.7%) mu mol/m2]. Despite the remaining NO2 disparities during the pandemic, our findings indicated that the mitigation of NO2 disparities (particularly racial inequity) would be achievable by the continued regulatory actions emphasizing the reduction of traffic-related NOx emissions such as electrification of passenger cars and diesel trucks.