Comment on egusphere-2023-346

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Nitrogen oxides (NO_<em>x</em> = 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&deg; resolves intracity emission variations and reveals NO<em>_x</em> emission hot spots at city cores, industrial areas, and sea ports. For each selected city, COVID-19-induced changes in NO_<em>x</em> 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 NO_<em>x</em> emissions in 2019&ndash;2022 using an unsupervised learning algorithm. All but one selected North American cities fall into cluster 1 characterized by weak emission reduction in 2020 (&minus;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 (&minus;31 %), whereas the selected East Asian cities generally fall into clusters 3 and 4 with the largest impacts in 2022 (&minus;25 % and &minus;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.