Investigating the representativeness of the two Maltese urban background stations

Abstract

Nitrogen dioxide (NO2) is a key air pollutant in urban environments, closely associated with vehicular emissions and linked to adverse health outcomes, including respiratory illnesses and increased cardiovascular risk. Within the framework of the Ambient Air Quality Directive (Directive 2008/50/EC and its 2024 recast, Directive 2024/2881), Member States are required to ensure that monitoring stations are spatially representative of the areas they are intended to assess. This study investigates whether the urban background monitoring stations in Attard and Żejtun, Malta, fulfil this requirement and accurately reflect population exposure to NO2. Data of the Environment and Resources Authority’s real-time and passive monitoring networks were used to assess representativeness. The study focused on NO2 data collected between 2018 and 2023, excluding 2020 due to COVID-19-related anomalies. Using Principal Component Analysis and Ward’s hierarchical clustering, the Attard and Żejtun stations were found to group with other sites delineated as urban background sites, supporting their classification. Spatial representativeness was further analysed by grouping background sites within the agglomeration into NO2 concentration bands, with ±15% of the annual average concentration used as the tolerance threshold based on FAIRMODE guidance. The analysis revealed that while Attard and Żejtun clustered with background sites located at the edge of the Valletta-Sliema agglomeration, most stations within the agglomeration itself exhibited higher NO2 concentration ranges. This indicates that both stations are not representative of broader urban background exposure.