Satellite unravels recent changes in atmospheric nitrogen oxides emissions from global ocean shipping

Nitrogen dioxide (NO₂) in the lower marine atmosphere, mainly emitted by maritime shipping, plays a crucial role in air pollution formation and global human health. However, few measurements of marine atmospheric NO₂ hinder knowledge of trace gas trends and atmospheric chemistry evolution due to shipping emissions. In this study, we use long-term satellite observations of tropospheric NO₂ column from the European TROPOspheric Monitoring Instrument (TROPOMI) and the Chinese Environmental trace gases Monitoring Instrument (EMI) to analyze marine atmospheric NO₂ variations, especially during the global COVID-19 pandemic and escalating geopolitical crises. First, we demonstrate the detection of NO₂ enhancements along shipping routes, including the North Atlantic route, the North Pacific route, and the Cape route, indicating significant emissions of atmospheric NO₂ from on-ocean shipping. Second, we observe and quantify the response of marine atmospheric NO₂ concentrations to major shipping events, such as the Suez Canal blockage, the Los Angeles-Long Beach port congestion, and the Russia-Ukraine war, resulting in local NO₂ concentration variations of approximately 40% decrease to 70% increase. Long-term analysis reveals reduced NO₂ concentrations in most coastal ports and maritime shipping routes during the COVID-19 lockdown, with reductions exceeding 50% or durations lasting up to 200 days. However, some rapidly developing ports, such as Beibu Gulf (China) and Dakar (Senegal), did not experience a decrease in NO₂ concentrations, suggesting that local authorities need to pay more attention to these fast-growing yet underestimated emission sources. In addition, by excluding the impact of meteorology using statistical models, we find that the current Emission Control Area (ECA) policies have effectively reduced NO₂ concentrations in Chinese coastal ports. These results contribute to understanding spatiotemporal characteristics of marine atmospheric NO₂, including ports and open-sea shipping routes, and guide further ECA policies to control marine NO₂ pollution.