Improved monitoring of shipping NO<sub>2</sub> with TROPOMI: decreasing NO<sub><i>x</i></sub> emissions in European seas during the COVID-19 pandemic

Abstract. TROPOMI (TROPOspheric Monitoring Instrument) measurements of tropospheric NO2 columns provide powerful information on emissions air pollution by ships open sea. This is potentially useful for authorities to help determine the (non-)compliance with increasingly stringent NOx emission regulations. We find that quality improved further recent upgrades in cloud retrieval and an optimal data selection. show superior spatial resolution allows detection several lanes ranging from Aegean Sea near Greece Skagerrak Scandinavia, which have not been detected other satellite instruments before. Additionally, we demonstrate under conditions sun glint TROPOMI's vertical sensitivity marine boundary layer increases up 60 %. The benefits are most prominent clear-sky situations when sea surface winds low but slightly above zero (±2 m s?1). Beyond glint, examine first time impact recently algorithm quality, both over land. new FRESCO+ (Fast Retrieval Scheme Clouds Oxygen A band) wide leads 50 hPa lower pressures, correcting a known high bias, produces 1–4×1015 molec. cm?2 higher retrieved columns, thereby at least partially previously reported bias product. By training artificial neural network four available periods standard test retrievals, develop historic, consistent set spanning years 2019 2020. shows stronger (35 %–75 %) sharper (10 %–35 shipping signals compared co-sampled OMI. apply our investigate COVID-19 pandemic ship European seas indications reduced 10 %–20 % during beginning reductions start March–April 2020, line changes activity inferred automatic identification system (AIS) location, speed, engine.