The impact of COVID-19 lockdowns on urban photochemistry as inferred from TROPOMI.

Movement restrictions were imposed in 2020 to mitigate the spread of Covid-19. These lock-down episodes provide a unique opportunity to study the sensitivity of urban photochemistry to temporary emission reductions and test air quality models. This study uses Tropospheric Monitoring Instrument (TROPOMI) nitrogen dioxide/carbon monoxide (NO 2 /CO) ratios in urban plumes in combination with an exponential fitting procedure to infer changes in the NOx lifetime (τ N O x) during Covid-19 lock-downs in the cities of Denver, Chicago, New York, Riyadh, Wuhan and Sao Paulo compared with the year before. The strict lockdown policy in Wuhan led to a 65–80% reduction in NO 2 , compared to 30–50% in the other cities that were studied. In New York and Wuhan, CO concentration was reduced by 10–15%, whereas over Riyadh, Denver, Chicago, and Sao Paulo the CO background concentration increased by 2–5 ppb. τ N O x has been derived for calm (0.0 < U (m/s) < 3.5) and windy (5.0 < U (m/s) < 8.5) days to study the influence of wind speed. We find reductions in τ N O x during Covid-19 lockdowns in all six megacities during calm days. The largest change in τ N O x during calm days is found for Sao-Paulo (31.8 ± 9.0%), whereas the smallest reduction is observed over Riyadh (22 ± 6.6%). During windy days, reductions in τ N O x are observed during Covid-19 lockdowns in New York and Chicago. However, over Riyadh τ N O x is almost similar for windy days during the Covid-19 lockdown and the year before. Ground-based measurements and the Chemistry Land-surface Atmosphere Soil Slab (CLASS) model have been used to validate the TROPOMI-derived results over Denver. CLASS simulates an enhancement of ozone (O 3) by 4 ppb along with reductions in NO (38.7%), NO 2 (25.7%) and CO (17.2%) during the Covid-19 lockdown in agreement with the ground-based measurements. In CLASS, decreased NO x emissions reduce the removal of OH in the NO 2 + OH reaction, leading to higher OH concentrations and decreased τ N O x . The reduction in τ N O x inferred from TROPOMI (28 ± 9.0%) is in agreement with CLASS. These results indicate that TROPOMI derived NO 2 /CO ratios provide useful information about urban photochemistry and that changes in photochemical lifetimes can successfully be detected. • The impact of Covid-19 lockdowns on the NOx lifetime is derived using TROPOMI retrieved NO 2 /CO ratios for six megacities. • Covid-19 lockdowns reduced the NOx lifetime compared to the year before across the six megacities. • TROPOMI derived reduction in NOx lifetime over Denver agrees with CLASS model intialized using ground-based observations. [ABSTRACT FROM AUTHOR]