COVID‐19 Induced Fingerprints of a New Normal Urban Air Quality in the United States

Most countries around the world, including the United States, took actions to control COVID‐19 spread that led to an abrupt shift in human activity. On‐road NOx emissions from light and heavy‐duty vehicles decreased by 9%–19% between February and March at the onset of the lockdown period in the middle of March in most of the US; between March and April, the on‐road NOx emissions dropped further by 8%–31% when lockdown measures were most stringent. These precipitous drops in NOx emissions correlated well (r = 0.75) with tropospheric NO2 column amount observed by the Sentinel 5 Precursor TROPOspheric Monitoring Instrument (S5P TROPOMI). Furthermore, the changes in TROPOMI tropospheric NO2 column amount across continental US between 2020 and 2019 correlated well with the changes in on‐road NOx emissions (r = 0.68) but correlated weakly with changes in emissions from the power plants (r = 0.35). At the height of lockdown‐related unemployment in the second quarter of 2020, the tropospheric NO2 column values decreased at the rate of 0.8 µmoles/m2 per unit percentage increase in the unemployment rate. Despite the lifting of lockdown measures, parts of the US continued to have ∼20% below normal on‐road NOx emissions. To achieve this new normal urban air quality in the US, continuing remote work policies that do not impede economic growth may become one of the many options. Plain Language Summary This study documents the different phases of COVID‐19 lockdown in 2020 and how traffic emissions changed accordingly across the US, particularly in five different cities, namely Los Angeles, San Francisco, San Joaquin Valley, New York City, and Atlanta. Analysis of data for these cities from measurements on the ground and satellites indicate that a downturn in the economy and telework policies reduced the number of cars and trucks on the road in March and April due to which air quality got better. The recovery of traffic emissions after the lockdowns was lifted was slow and below normal emissions were observed even at the end of 2020. While the cities in the east reached near normal levels, the west coast showed below normal traffic emissions. The air quality in 2020 provided a window into the future as to how improvements can be achieved. Key Points COVID‐19 lockdown‐related reductions in on‐road NOx emissions tracked by TROPOspheric Monitoring Instrument (TROPOMI) tropospheric nitrogen dioxide column observations On‐road NOx emissions continued to be ∼20% below normal th