Aircraft validation reveals a 20% low bias in TROPOMI NO2 over sea caused by TM5 a priori profiles

Scattering of light in the atmosphere and low sea surface albedo decrease the sensitivity of satellites to air pollution close to the sea surface. To reliably retrieve tropospheric nitrogen dioxide (NO₂) columns using the TROPOspheric Monitoring Instrument (TROPOMI), it is therefore necessary to have good a priori knowledge of the vertical distribution of NO₂. In this study, we used an aircraft of the Royal Belgian Institute of Natural Sciences, part of the Belgian coastguard structure, which was already equipped with a sniffer sensor system, measuring CO₂, NO_x and SO₂. This instrumentation enables us (1) to capture pollution plumes originating from ships sailing within an Emission Control Area, and (2) to validate TROPOMI tropospheric NO₂ columns over the polluted North Sea in summer 2021 and (3) to evaluate vertical profile shapes from several chemical models. We observe multiple clear signatures of ship plumes from seconds after emission to multiple kilometers downwind. Besides that, our results show that the chemical transport model TM5, which is used in the retrieval of the operational TROPOMI data, tends to underestimate surface level pollution while overestimating NO₂ at higher levels over the polluted North Sea. The higher horizontal resolutions in the regional CAMS ensemble mean and LOTOS EUROS improve the surface level pollution estimates, but the models still systematically overestimate NO₂ levels at higher altitudes, indicating exaggerated vertical mixing in the models. When replacing the TM5 a priori NO₂ profiles with the aircraft-measured NO₂ profiles in the air mass factor (AMFs) calculation, we find that recalculated AMFs reduce, and the retrieved NO₂ columns increase by 20%. This indicates a significant low bias in TROPOMI tropospheric NO₂ measurements over the North Sea. This low bias has important implications for estimating emissions over the sea. While TROPOMI NO₂ low biases caused by the TM5 a priori profiles have previously also been reported over land, the reduced vertical mixing and smaller surface albedo over sea makes this issue especially relevant over sea and coastal regions.