Reconciling Ground-Based Remote Sensing and In-Situ Observations of COVID-Related Air Quality Changes in the Boston Area

Abstract:

At the peak of COVID-related restrictions, the Massachusetts Department of Transportation recorded declines in traffic of up to 70% compared to the same time in the previous year. With mobile sources making up 60-70% of all nitrogen oxide emissions in Boston, this disruption of traffic was captured by our ground-based monitoring of nitrogen dioxide (NO2). Here we report long-term observations by Pandora spectrometer systems in the Boston area since April 2019, and compare the measurements with ground-level NO2 measured in-situ by the Massachusetts Department of Environmental Protection. Total column NO2 recorded by the direct-sun Pandora measurements across April-June 2020 was, on average, 40% lower than the same time in 2019; declines in the highest (95th percentile) NO2 were even more dramatic (~60%). These ground-based column observations are consistent with evidence from traffic data and in-situ NO2 measurements. We explore the role of meteorological variability and long-term background trends in the difference between 2019 and 2020. We further leverage the disparate time-of-day, day-of-week, and meteorological controls on the COVID-related impacts in the remotely sensed column and ground-level in-situ observations to gain novel insight into column-to-surface relationships of NO2 in general.