Improved detection of global NO₂ signals from shipping in Sentinel-5P TROPOMI data

<p>Shipping is an important source of nitrogen oxide (NO<span class="inline-formula">_<i>x</i></span>) emissions worldwide, contributing to air pollution and negatively affecting marine environments, ecosystems, and biodiversity. TROPOMI (TROPOspheric Monitoring Instrument) on board the Sentinel-5 Precursor (S5P) has significantly enhanced the ability to detect ship emissions from space due to its low measurement noise levels and high spatial resolution of <span class="inline-formula">5.5×3.5</span> <span class="inline-formula">km²</span> at nadir. This study uses the TROPOMI tropospheric NO<span class="inline-formula">₂</span> slant column density (tSCD) to identify global shipping routes qualitatively. Preprocessing techniques, including iterative high-pass and Fourier filtering, markedly improve the detection of shipping lanes, revealing previously undetectable routes. Our analysis examines the impact of high-pass-filter box sizes, demonstrating that smaller sizes enhance the visibility of narrow shipping features, whereas larger box sizes increase overall NO<span class="inline-formula">₂</span> signals. Additionally, we investigate various flagging criteria that affect NO<span class="inline-formula">₂</span> signal distribution, highlighting the critical importance of careful selection for accurate emission monitoring. Filtered TROPOMI NO<span class="inline-formula">₂</span> tSCDs over oceans show a strong correlation with shipping activities, as confirmed by comparison with the CAMS-GLOB-SHIP (Copernicus Atmospheric Monitoring Service for Global Shipping) inventory, and also reveal unknown shipping routes in regions such as the Bering Sea. Furthermore, TROPOMI effectively captures NO<span class="inline-formula">₂</span> emissions from offshore oil and gas platforms, with NO<span class="inline-formula">₂</span> hotspots in the TROPOMI data aligning well with locations of offshore installations listed in the OSPAR (Oslo and Paris Commission) and BOEM (Bureau of Ocean Energy Management) inventories. Lastly, the filtered TROPOMI NO<span class="inline-formula">₂</span> tropospheric vertical column densities (tVCDs) are compared with the high-pass-filtered NO<span class="inline-formula">₂</span> tVCDs from the CAMS (Copernicus Atmospheric Monitoring Service) model, which has a coarse spatial resolution of 0.4°. While both datasets effectively identify global shipping lanes, the high-pass-filtered CAMS NO<span class="inline-formula">₂</span> tVCDs are significantly higher than the filtered TROPOMI NO<span class="inline-formula">₂</span> tVCDs in the North Atlantic and strongly depend on the masking threshold in the high-pass filtering method in the South Atlantic Ocean.</p>