Measurement Report: Evolving Sources and Composition of Urban Submicron Aerosols in Dublin: Impacts of Emission Reductions and Transboundary Transport

Abstract. Home heating remains a main driver of winter air pollution across many European cities, yet long-term evaluations of pollution trends and mitigation responses remain limited. Here we present continuous measurements of chemically-speciated PM₁ (particles with aerodynamic diameter < 1 μm) in Dublin, a temperate European city influenced by local residential heating and continental pollution, from 2016 to 2023 to assess pollution trends under solid fuel reduction efforts. Two typical pollution types were identified: intense short-lasting events (few hours, PM₁>100 µg m^-3) driven by heating emissions, and moderate long-lasting events (several days, PM₁<60 µg m^-3) originating from transboundary transport. Their interplay shapes seasonal pollution patterns: PM₁ peaks in winter, driven by local emissions, while transboundary transport dominates PM₁ in spring. Annual PM₁ declined from 6.5 to < 5.0 µg m^-3 over the years, mainly due to reductions in nitrate and ammonium (-0.11 and -0.09 µg m^-3 yr^-1), followed by solid fuel organic aerosols and black carbon (-0.07 and -0.08 µg m^-3 yr^-1). Although high pollution events were largely dominated by heating emissions, their intensity and frequency clearly declined. In contrast, limited reductions in locally-formed oxygenated organic aerosols (OOA_local), combined with increased transported OOA (+0.34 µg m^-3 yr^-1), raised their relative importance alongside rising ozone levels. This highlights the need for integrated strategies addressing PM₁ and ozone pollution. While declining nitrate and ammonium indicates regional precursor reductions, a rebound in local pollutants in 2023 highlighted the persistent vulnerability to heating emissions.