The impact of shipping on the air quality in European port cities with a detailed analysis for Rotterdam

Air quality in cities with large maritime ports is considerably impacted by emissions from shipping activity which is of a growing relevance due to an increasing relative contribution. To explore the extent of shipping emissions to ambient air quality, simulations with the chemical transport model LOTOS-EUROS (LOng Term Ozone Simulation – EURopean Operational Smog model) were performed for the year 2018 at an approximate 1 × 1 km resolution for six European cities with large ports, i.e., Rotterdam, Antwerp, Hamburg, Amsterdam, Le Havre, and London. It was found that depending on the investigated city, 6.5%–62% of the nitrogen dioxide (NO₂) concentration in the city centres is attributable to shipping activities. This corresponds to contributions of 1.8–11.5 μg/m³ to the ambient air NO₂ concentrations. The average NO₂ contribution of shipping in these six cities was 28% (7.1 μg/m³). The largest relative contribution was found for Le Havre where 62% (10.8 μg/m³) of the annual average NO₂ concentration was caused by shipping emissions. The largest absolute contribution is found for the city centre of Hamburg with 11.5 μg/m³ (41%). The lowest absolute and relative contribution (respectively 1.8 μg/m³ and 6.5%) are found for London, also having the smallest port in terms of tonnage throughput, which is one of the influential factors that determine emission totals, investigated in this study. For the other investigated pollutants, i.e., PM_2.5, PM₁₀ and SO₂, contributions from shipping were less pronounced with average contribution for all cities of 10% (1.2 μg/m³) 7% (1.5 μg/m³) and 4% (0.16 μg/m³) respectively. To assess the effect of model choices on these results, this study also looked into the choice of simulation resolution and relations between meteorological parameters and NO₂ concentrations. Following simulations with varying chemical transport model resolutions (1 × 1 km to 24 × 24 km), it is found that a decrease in ambient air pollutant concentrations away from localized emission sources is more pronounced at higher (1 × 1 km) model resolutions and source contributions are influenced more significantly than total concentrations. Considering meteorology, generally low wind speeds (1–2 m/s) lead to high NO₂ concentration in city centres. For the cities where the port is much closer to the city centre (e.g., London, Le Havre, Hamburg and Antwerp) the absolute NO₂ concentrations as well as the contributions from shipping emissions become highest for windless conditions. The high concentrations (>60 μg/m³ NO₂) only occur when wind speeds fall below 6 m/s.