Quantifying the impact of maritime AIS-based emissions on Gulf of Mexico coastal air quality using high-resolution modelling

Air quality is a critical factor in both public health and environmental protection, particularly in coastal regions that are heavily influenced by maritime activity. This study quantifies the contribution of shipping emissions to atmospheric pollutant levels in coastal areas along the Gulf of Mexico, an important international trade hub.

The WRF-Chem v4.5.2 model was used with a horizontal resolution of 5x5 km and 38 vertical levels. The MOZART chemistry mechanism was coupled with the MOSAIC aerosol scheme, and the model was run with an hourly temporal resolution. Maritime emissions were derived from Automatic Identification System (AIS) data, while terrestrial emissions were represented using the CAMS inventory. Representative case studies were identified through long-term synoptic analysis based on 30 years using HYSPLIT. This analysis applied frequency and cluster analysis to air mass transport patterns enabled the dominant synoptic transport components over the Gulf of Mexico to be identified: northerly (N), northeasterly (NE), easterly (E), southeasterly (SE) and northwesterly (NW) flows. These collectively represent over 85% of prevailing atmospheric circulation conditions in the region. For each synoptic component, a representative case study was selected consisting of a four-day simulation period, excluding model spin-up time. Two numerical experiments were conducted for each synoptic component: one including ship emissions, and one excluding them while keeping all other emission sources constant.

The model results were evaluated using observations from 116 air quality monitoring stations (AQMS) located no more than 10 km from the coastline. The results show that shipping emissions have a significant impact on coastal air quality, which varies depending on the pollutant. Of the primary pollutants, nitrogen dioxide (NO₂) was found to be the most sensitive to maritime emissions. Maximum contributions were found to be 37% under SE flow and 29% under E conditions, reflecting the efficient transport of emissions from major shipping corridors onto land. Sulfur dioxide (SO₂) contributions peaked at around 13% under SE flow, highlighting the impact of fuel sulfur content and shipping density. Particulate matter concentrations were also notably affected, with PM₁₀ contributions exceeding 20% under NE and SE regimes, while PM_2.5 exhibited maximum increases of around 15% under E transport. For secondary pollutants, ozone (O₃) formation showed positive contributions of up to 15% under E flow, highlighting the role of NO_X derived from ships in photochemical processes. In contrast, carbon monoxide (CO) had comparatively smaller impacts, with maximum contributions below 9%. While observed concentrations generally remain within air quality limits, the relative contribution of shipping emissions is significant and represents an important emerging pressure on coastal air quality.

These findings demonstrate that maritime emissions significantly influence pollutant levels in the coastal areas of the Gulf of Mexico, particularly under dominant synoptic regimes. The results emphasize the importance of including shipping emissions in regulatory and mitigation strategies and highlight the need to strengthen the regional implementation and enforcement of MARPOL (Annex VI) regulations to protect air quality and public health in coastal environments.