Preprints
https://doi.org/10.5194/amt-2021-321
https://doi.org/10.5194/amt-2021-321

  03 Nov 2021

03 Nov 2021

Review status: this preprint is currently under review for the journal AMT.

Improved monitoring of shipping NO2 with TROPOMI: decreasing NOx emissions in European seas during the COVID-19 pandemic

T. Christoph V. W. Riess1, K. Folkert Boersma1,2, Jasper van Vliet3, Wouter Peters1,4, Maarten Sneep2, Henk Eskes2, and Jos van Geffen2 T. Christoph V. W. Riess et al.
  • 1Department of Meteorology and Air Quality, Wageningen University, Wageningen, the Netherland
  • 2Climate Observations Department, Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
  • 3Human Environment and Transport Inspectorate, the Netherlands
  • 4University of Groningen, Centre for Isotope Research, Groningen, the Netherlands

Abstract. TROPOMI measurements of tropospheric NO2 columns provide powerful information on emissions of air pollution by ships on open sea. This information is potentially useful for authorities to help determine the (non-)compliance of ships with increasingly stringent NOx emission regulations. We find that the information quality is improved further by recent upgrades in the TROPOMI cloud retrieval and an optimal data selection. We show that the superior spatial resolution of TROPOMI allows the detection of several lanes of NO2 pollution ranging from the Aegean Sea near Greece to the Skagerrak in Scandinavia, which have not been detected with other satellite instruments before. Additionally, we demonstrate that under conditions of sun glint TROPOMI's vertical sensitivity to NO2 in the marine boundary layer increases by up to 60 %. The benefits of sun glint are most prominent under clear-sky situations when sea surface winds are low, but slightly above zero (±2 m/s). Beyond spatial resolution and sun glint, we examine for the first time the impact of the recently improved cloud algorithm on the TROPOMI NO2 retrieval quality, both over sea and over land. We find that the new FRESCO+wide algorithm leads to 50 hPa lower cloud pressures, correcting a known high bias, and produces 1–4·1015 molec/cm2 higher retrieved NO2 columns, thereby at least partially correcting for the previously reported low bias in the TROPOMI NO2 product. By training an artificial neural network on the 4 available periods with standard and FRESCO+wide test-retrievals, we develop a historic, consistent TROPOMI NO2 data set spanning the years 2019 and 2020. This improved data set shows stronger (35–75 %) and sharper (10–35 %) shipping NO2 signals compared to co-sampled measurements from OMI. We apply our improved data set to investigate the impact of the COVID-19 pandemic on ship NO2 pollution over European seas and find indications that NOx emissions from ships reduced by 20–25 % during the pandemic. The reductions in ship NO2 pollution start in March–April 2020, in line with changes in shipping activity inferred from AIS data.

T. Christoph V. W. Riess et al.

Status: open (until 08 Dec 2021)

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T. Christoph V. W. Riess et al.

T. Christoph V. W. Riess et al.

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Short summary
This paper reports on improved monitoring of ship nitrogen oxide emissions by TROPOMI. With its fantastic resolution we can identify lanes of ship nitrogen dioxide (NO2) pollution not detected from space before. The quality of TROPOMI NO2 data over sea improves further by recent upgrades in cloud retrievals and the use of sun glint scenes. Lastly, we study the impact of COVID-19 on ship NO2 in European Seas and compare the found reductions to emission estimates gained from ship location data.