Investigating multiannual NO2 trends over Malta’s main harbour area

Abstract

The study investigated multi-annual nitrogen dioxide (NO2) trends across the Maltese Islands between 2019 and 2023, with a focused case study on the main harbour area. The research aimed to investigate spatial and temporal variations in NO2 concentrations by integrating satellite-derived data from the Sentinel-5P TROPOMI instrument with ground-based monitoring from the Environment and Resources Authority (ERA). The complementary strengths and weaknesses of both monitoring approaches were evaluated. Both satellite and ground station monitoring analyses revealed substantial annual and seasonal fluctuations in NO2 levels, strongly influenced by societal disruptions such as the COVID-19 pandemic. Concentrations markedly declined in 2020 and 2021 due to reduced anthropogenic activity, before rebounding in 2022 and 2023, with notable hotspots observed in Luqa, Qormi, and the main harbour area. NO2 levels in the summer months were consistently elevated, likely tied to increased tourism, aviation, and energy demands. The study also explored the influence of cruise liner traffic on local NO2 levels. A statistically significant correlation between annual cruise liner traffic and ground station data at Msida was identified. Satellite data lacked the resolution to detect these localised emissions. Comparative analyses showed moderate agreement between satellite and ground-based measurements, with discrepancies being attributed to differences in vertical resolution and limitations in the unit conversion methodology. By combining Earth Observation (EO) and in-situ data, this study demonstrated the value of hybrid monitoring approaches in capturing air quality dynamics, particularly in small, densely populated island settings. These findings and the use of a hybrid monitoring approach have the potential to improve monitoring and guide targeted mitigation of urban and maritime pollution, as well as informing policy interventions.