Nitrogen oxides emissions from selected cities in North America, Europe, and East Asia observed by TROPOMI before and after the COVID-19 pandemic

Abstract

Abstract. Nitrogen oxides (NOx = NO + NO2) emissions are estimated in three regions in the Northern hemisphere, generally located in North America, Europe, and East Asia, by calculating the directional derivatives of NO2 column amounts observed by the TROPOMI instrument with respect to the horizontal wind vectors. We present monthly averaged emissions from 1 May 2018 to 31 January 2023 to capture variations before and after the COVID-19 pandemic. We focus on a diverse collection of 54 cities, 18 in each region. A spatial resolution of 0.04° resolves intracity emission variations and reveals NOx emission hot spots at city cores, industrial areas, and sea ports. For each selected city, COVID-19-induced changes in NOx emissions are estimated by comparing monthly and annually averaged values to the pre-COVID-19 year of 2019. While emission reductions are initially found during the first outbreak of COVID-19 in early 2020 in most cities, the cities' paths diverge afterwards. We group the selected cities into 4 clusters according to their normalized annual NOx emissions in 2019–2022 using an unsupervised learning algorithm. All but one selected North American cities fall into cluster 1 characterized by weak emission reduction in 2020 (−7 % relative to 2019) and increase in 2022 by +5 %. Cluster 2 contains mostly European cities and is characterized by the largest reduction in 2020 (−31 %), whereas the selected East Asian cities generally fall into clusters 3 and 4 with the largest impacts in 2022 (−25 % and −37 %). This directional derivative approach has been implemented in object-oriented, open-source Python and is available publicly for high-resolution and low-latency emission estimation for different regions, atmospheric species, and satellite instruments.

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“…Very different lockdown measures have been applied for COVID around the world resulting in various length and intensity of activity reductions and effect on air pollutant emissions (Sokhi et al, 2021). SLCF emissions have been seen to return to their pre-COVID levels by 2022 in some regions, sometimes with rebound effect, but not in all (Putaud et al, 2023, Lonsdale andSun, 2023) but quantification at the global scale is not yet available.…”