Improved detection of global NO₂ signals from shipping using TROPOMI observations: advanced filtering and comparison with CAMS

Shipping is an important source of atmospheric NOx worldwide, negatively affecting marine environments and human health. For decades, some of the busiest shipping lanes have been tracked by satellites from space. With TROPOMI aboard the Sentinel 5-Precursor (S5P), the potential for detecting ship emissions has increased due to its low noise and high spatial resolution of 5.5 x 3.5 km². Previous studies have demonstrated that even individual ship plumes can be identified from TROPOMI data.

In this study, we use TROPOMI tropospheric NO₂ slant columns (tSCDs) to qualitatively identify global shipping routes. Advanced preprocessing techniques, including iterative high-pass and Fourier filtering, markedly improve the detection of shipping lanes, revealing many previously undetectable routes. The impact of high-pass filter box sizes is analyzed, demonstrating that smaller sizes enhance the visibility of narrow shipping features, whereas larger box sizes increase overall NO₂ signals. In addition, various flagging criteria are investigated that affect the distribution of the NO₂ signal, highlighting the critical importance of careful selection for accurate emission monitoring. The filtered TROPOMI NO₂ tSCDs over oceans show a strong correlation with shipping activities, as confirmed by comparison with the CAMS-GLOB-SHIP inventory, and reveal unknown shipping routes. TROPOMI also effectively captures NO₂ signals from offshore oil and gas platforms. In the next step, filtered TROPOMI tropospheric NO₂ vertical columns are compared with those from the CAMS global model. While both datasets show consistent NO₂ enhancements along major shipping lanes, the CAMS NO₂ values are systematically higher than the TROPOMI measurements.

This study demonstrates the potential of advanced filtering techniques applied to TROPOMI observations to detect as many global NO₂ signals from shipping as possible. It contributes to the ongoing progress of satellite remote sensing of ship emissions.