TROPOMI 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 meteorology. We find that meteorological patterns were especially favorable for low NO2 in much of the United States 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 solar angle and meteorological considerations, we calculate that NO2 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 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.
The
TROPOspheric Monitoring Instrument (TROPOMI) is used to derive
top-down NO
X
emissions for two large power
plants and three megacities in North America. We first re-process
the vertical column NO2 with an improved air mass factor
to correct for a known systematic low bias in the operational retrieval
near urban centers. For the two power plants, top-down NO
X
emissions agree to within 10% of the emissions reported
by the power plants. We then derive top-down NO
X
emissions rates for New York City, Chicago, and Toronto, and
compare them to projected bottom-up emissions inventories. In this
analysis of 2018 NO
X
emissions, we find
a +22% overestimate for New York City, a −21% underestimate
in Toronto, and good agreement in Chicago in the projected bottom-up
inventories when compared to the top-down emissions. Top-down NO
X
emissions also capture intraseasonal variability,
such as the weekday versus weekend effect (emissions are +45% larger
on weekdays versus weekends in Chicago). Finally, we demonstrate the
enhanced capabilities of TROPOMI, which allow us to derive a NO
X
emissions rate for Chicago using a single
overpass on July 7, 2018. The large signal-to-noise ratio of TROPOMI
is well-suited for estimating NO
X
emissions
from relatively small sources and for sub-seasonal timeframes.
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