Disentangling the Impact of the COVID-19 Lockdowns on Urban NO From Natural Variability.

TROPOMI satellite data show substantial drops in nitrogen dioxide (NO) during COVID-19 physical distancing. To attribute NO changes to NO emissions changes over short timescales, one must account for meteorology. We find that meteorological patterns were especially favorable for low NO in much of the United States in spring 2020, complicating comparisons with spring 2019. Meteorological variations between years can cause column NO differences of ~15% over monthly timescales. After accounting for solar angle and meteorological considerations, we calculate that NO drops ranged between 9.2% and 43.4% among 20 cities in North America, with a median of 21.6%. Of the studied cities, largest NO drops (>30%) were in San Jose, Los Angeles, and Toronto, and smallest drops (<12%) were in Miami, Minneapolis, and Dallas. These normalized NO changes can be used to highlight locations with greater activity changes and better understand the sources contributing to adverse air quality in each city.