Fingerprints of a New Normal Urban Air Quality in S5P TROPOMI Tropospheric NO2 Observations

Most countries around the world took actions to control COVID-19 spread that included social distancing, limiting air and ground travel, closing schools, suspending sports leagues, closing factories etc., leading to  economic shutdown. The reduced traffic and human movement compared to Business as Usual (BAU) scenario was tracked by Apple and Android cellphone use; the data showed substantial reductions in mobility in most metropolitan areas.  We analyzed reductions in on-road mobile NOx emissions from light and heavy duty vehicles in four major metropolitan and one rural areas in the United States that showed a reduction in NOx mobile emissions from 9% to 19% between February and March at the onset of lockdown in the middle of March; between March and April, the mobile NOx emissions dropped further by 8% to 31% when lockdown measures were the most stringiest.  These precipitous drops in NOx emissions correlated well with tropospheric NO₂ column amount observed by Sentinel 5 Precursor TROPospheric Ozone Monitoring Instrument (S5P TROPOMI).  Further, the changes in TROPOMI tropospheric NO₂ across the continental U.S. between 2020 and 2019 correlated well with changes in on-road NOx emissions (r=0.78) but correlated weakly with changes in emissions from the power plants (r=0.44). These findings confirm that power plants are no longer the major source of NO₂ in the United States. We also examined correlation between increase in unemployment rate between 2020 and 2019 to decrease in tropospheric NO₂ amount.  The negative correlation indicates that with increased unemployment rate combined with telework policies across the nation for non-essential workers, the NO₂ values decreased at the rate of 0.8 µmoles/m² decrease per unit percentage increase in unemployment rate.  There is a substantial amount of scatter in the data with some cities such as Atlanta, Dallas, and Houston showing no noticeable trend in tropospheric NO₂ changes during the time period when unemployment rate increased from 6% to 12%.   We examined the trends in on-road and power plant emissions for five different locations (four urban areas and one rural area) and show that the changes in NOx emissions during the lockdown are detectable in TROPOMI tropNO2 data, the economic indicators are consistent with emissions changes, and the trends reversing with the removal of lockdown measures in the major metro areas have not come back to pre-pandemic levels.  The COVID-19 pandemic experience has provided the scientific community an opportunity to identify emissions reductions scenarios that created a new normal for urban air quality and if the environmental protection agencies should look at this new normal as a guidance for instituting new policies.