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

Abstract

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.

Sentinel-5P TROPOMI NO<sub>2</sub> retrieval: impact of version v2.2 improvements and comparisons with OMI and ground-based data

“…4.2), in two cloud fraction regimes: small (f c ≤ 0.1; panel a) and medium (0.3 ≤ f c ≤ 0.6; panel b), with f c the cloud fraction in the NO 2 fit window. This example shows that the FRESCO-wide implementation in NO 2 v1.4 indeed leads to lower c p ; see also the recent study by Riess et al (2022). The use of level-1b v2.0 in NO 2 v2.2 has a smaller impact than the implemen- tation of the FRESCO-wide approach for the lowest cloud fractions (panel a) and appears to lead to somewhat higher cloud pressures in that range.…”

“…An improved FRESCO cloud pressure retrieval (Sect. 4.1), is used in the NO 2 processing as v1.4 since 29 November 2020, which reduces the bias in the tropospheric VCDs of polluted scenes considerably Riess et al, 2022).…”

Derivation of Emissions From Satellite‐Observed Column Amounts and Its Application to TROPOMI NO2 and CO Observations

“…Decrease of emissions relative to the corresponding month in the 2019 period is marked by blue shade, and increase is marked by red. For all the cities, the NO x emissions drop relative to the 2019 period after the onset of the COVID‐19 pandemic in March 2020, as observed by many studies globally (e.g., Ding et al., 2020; Goldberg et al., 2020; F. Liu et al., 2020; Miyazaki, Bowman, Sekiya, Jiang, et al., 2020; Sun et al., 2021; Riess et al., 2022). The relative emission decreases become less clear after 2021.…”

Comparing Sentinel-5P TROPOMI NO2 column observations with the CAMS regional air quality ensemble

“…The operational TROPOMI NO 2 product is described in van Geffen et al ( 2020), which focuses on the DOAS slant column retrieval, , which describes the tropospheric vertical column retrieval and air mass factor aspects up to processor version 1.4.0, van Geffen et al (2022), which discusses the upgrade to version 2.2.0 on July 2021, and Riess et al (2022), which discusses the improvement in the cloud pressure product and how that improves NO 2 air mass factors and retrievals. Users of the data are advised to first consult the product README file (PRF; which provides a short introduction to the processor versions, data quality remarks, algorithm changes, and links to the relevant documentation and routine validation.…”

“…The upgrade to version 1.4.0 on 29 November 2020 (for the offline product) brought only one change, namely an update of the FRESCO cloud retrieval, which led to an overall decrease in the effective cloud pressure of about 5 %. Because of the large sensitivity of the NO 2 retrieval to this quantity, this resulted in a considerable change and general increase in NO 2 in the more polluted regions Riess et al, 2022). The datasets were filtered with the "qa_value" quality parameter provided in the data product, and only observations with a qa_value > 0.75 were kept to produce the results described below.…”