Extreme NO2 at Global Maritime Chokepoints: Regional Atmospheric Background, Maritime Activity, and Temporal Validation

Satellite nitrogen dioxide (NO2) over busy straits mixes ship exhaust with regional pollution. This study tests how much daily shipping activity helps predict extreme-NO2 days at 24 maritime chokepoints once regional NO2 is known, using TROPOMI NO2, ERA5 meteorology, and PortWatch transit and trade-volume estimates. A day was extreme when corridor NO2 exceeded the site’s own 90th percentile. Logistic regression and gradient-boosted trees were developed on 2019–2022 data and tested on 2023–2024, two years set aside in advance. In this test, regional NO2 was by far the strongest predictor, raising average precision (random guessing ≈ 0.11) from 0.17 to 0.39 (logistic) and from 0.25 to 0.67 (gradient-boosted). Previous-day shipping activity added much less; after accounting for regional NO2, it gave a small gain only in the logistic model, mainly at Bab el-Mandeb and the Cape of Good Hope, where the Red Sea crisis sharply changed traffic. Reference areas 10–50 km from the corridor gave the same pattern. Since the models use regional NO2 from the same satellite overpass and concurrent meteorology, they are conditional predictions of same-day extremes, not advance forecasts or early warnings. Interpreting extreme NO2 at maritime chokepoints therefore requires explicit account of the regional atmospheric state.