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

S. Lama,S. Houweling,K. Boersma,I. Aben,Hugo Denier van der Gon,M. Krol

Published 2023 in Atmospheric Environment

ABSTRACT

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 NOx ) 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 NOx 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 NOx during Covid-19 lockdowns in all six megacities during calm days. The largest change in τ NOx 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 NOx are observed during Covid-19 lock-downs in New York and Chicago. However, over Riyadh τ NOx 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 NOx . The reduction in τ NOx 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.

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