Declines and peaks in NO.sub.2 pollution during the multiple waves of the COVID-19 pandemic in the New York metropolitan area

The COVID-19 pandemic created an extreme natural experiment in which sudden changes in human behavior and economic activity resulted in significant declines in nitrogen oxide (NO.sub.x) emissions, immediately after strict lockdowns were imposed. Here we examined the impact of multiple waves and response phases of the pandemic on nitrogen dioxide (NO.sub.2) dynamics and the role of meteorology in shaping relative contributions from different emission sectors to NO.sub.2 pollution in post-pandemic New York City. Long term ( 3.5 years), high frequency measurements from a network of ground-based Pandora spectrometers were combined with TROPOMI satellite retrievals, meteorological data, mobility trends, and atmospheric transport model simulations to quantify changes in NO.sub.2 across the New York metropolitan area. The stringent lockdown measures after the first pandemic wave resulted in a decline in top-down NO.sub.x emissions by approx. 30 % on top of long-term trends, in agreement with sector-specific changes in NO.sub.x emissions. Ground-based measurements showed a sudden drop in total column NO.sub.2 in spring 2020, by up to 36 % in Manhattan and 19 %-29 % in Queens, New Jersey (NJ), and Connecticut (CT), and a clear weakening (by 16 %) of the typical weekly NO.sub.2 cycle. Extending our analysis to more than a year after the initial lockdown captured a gradual recovery in NO.sub.2 across the NY/NJ/CT tri-state area in summer and fall 2020, as social restrictions eased, followed by a second decline in NO.sub.2 coincident with the second wave of the pandemic and resurgence of lockdown measures in winter 2021. Meteorology was not found to have a strong NO.sub.2 biassing effect in New York City after the first pandemic wave. Winds, however, were favorable for low NO.sub.2 conditions in Manhattan during the second wave of the pandemic, resulting in larger column NO.sub.2 declines than expected based on changes in transportation emissions alone. Meteorology played a key role in shaping the relative contributions from different emission sectors to NO.sub.2 pollution in the city, with low-speed ( 5 m s.sup.-1) SW-SE winds enhancing contributions from the high-emitting power-generation sector in NJ and Queens and driving particularly high NO.sub.2 pollution episodes in Manhattan, even during - and despite - the stringent early lockdowns. These results have important implications for air quality management in New York City, and highlight the value of high resoluti