Figure 1. TROPOMI NO2 signals from shipping activity in the European Seas with high-pass filters at different box sizes: approximately (a) 0.25° and (b) 1° in both longitude and latitude. The 0.25° box size reveals details of more minor shipping routes, such as the two shipping lanes in the North Atlantic Ocean, while weaker signals, such as in the Baltic Sea, and NOx emissions from oil and gas platforms in the North Sea (see also Fig. 5), are more visible with the 1° box size.

Figure 2. TROPOMI NO2 signals from a great number of shipping lanes in (a) the Red and Arabian Seas, (b) the Seas around Middle America, and (c) the South China Sea and one towards Australia. NOx emissions from offshore platforms are visible as elevated dot-shaped NO2 values, e.g., in (a) the Persian Gulf, (b) the Gulf of Mexico (see also Fig. 6), and (c) the Gulf of Thailand.

Figure 3. Filtered TROPOMI NO2 signals from shipping activity in the Red and Arabian Seas with the non-iterative high-pass filtering at a box size of approximately 0.25° in both longitude and latitude. Two separated shipping lanes are visible in the Gulf of Aden. In the Persian Gulf, a distinct shipping route is visible near the coast of Iran, and the dot-shaped elevated NO2 values indicate NOx emissions from offshore platforms. However, the shipping NO2 signals in the Arabian Sea are not as pronounced as in the filtered TROPOMI data using the iterative approach with a box size of 1° (see Fig. 2a).

Figure 4. (a) TROPOMI and (b) CAMS-GLOB-SHIP NO2 signals from shipping activity in the North Pacific correlate well for the two shipping routes between Asia and North America. The northern NO2 signal in the Bering Sea is only identified in the filtered TROPOMI data and not tracked through the AIS-based CAMS-GLOB-SHIP data, possibly due to disabled trackers onboard military or unauthorized fishing vessels. The CAMS-GLOB-SHIP data cover the years from 2018 to 2021 (ECCAD, 2018).

Figure 5. (a) TROPOMI NO2 signals in the North Sea and (b) locations of oil and gas platforms (dots). The hotspot regions where the NO2 signals exceed 3e13 molec cm-2 (a) correlate well with locations of offshore installations, and those are indicated by the red dots (b). In other regions where oil and gas platforms are located, the NO2 values show no significant enhancement, possibly due to variations in the emission rates and the high-pass filter effect. The offshore installations are classified as "operational" in the OSPAR emission inventories for 2019 and 2021 (ODIMS, 2021).