A lockdown was implemented in Canada mid-March 2020 to limit the spread of COVID-19. In the wake of this, declines in nitrogen dioxide (NO2) were observed from the Tropospheric Monitoring Instrument (TROPOMI). A method is presented to quantify how much of this decrease is due to the lockdown itself as opposed to variability in meteorology and satellite sampling. The operational air quality forecast model, GEM-MACH, was used with TROPOMI to determine expected NO2 columns that represents what TROPOMI would have observed for a non-COVID scenario. Decreases in NO2 due to the lockdown were seen across southern Ontario, with an average 40% in Toronto and even larger declines in the city center. Natural and satellite sampling variability accounted for as much as 20-30%. A model run using a lockdown emissions scenario were found to be consistent with TROPOMI suggesting the prescribed declines in transportation and industry emissions are reasonable.

Satellite data show substantial drops in nitrogen dioxide (NO2) during COVID-19 physical distancing. To attribute NO2 changes to NOx emissions changes over short timescales, one must account for meteorological effects. We find that meteorological patterns were especially favorable for low NO2 in much of the U.S. in spring 2020, complicating comparisons with spring 2019. Meteorological variations between years can cause column NO2 differences of ~15% over monthly timescales. After accounting for sun angle and meteorological considerations, we calculate that NO2 drops ranged between 9.2 – 43.4% among twenty cities in North America, with a median of 21.6%. Of the studied cities, largest NO2 drops (>30%) were in San Jose, Los Angeles, and Toronto, and smallest drops (<12%) were in Miami, Minneapolis, and Dallas. These normalized NO2 changes can be used to highlight locations with greater activity changes and better understand the sources contributing to adverse air quality in each city.