Abstract. To gain insight into the spatial and chemical variation in submicron aerosol, a nationwide characterization of wintertime PM1 was performed using an aerosol chemical speciation monitor (ACSM) and aethalometer at four representative sites across Ireland. Dublin, the capital city of Ireland, was the most polluted area with an average PM1 concentration of 8.6 µg m−3, ranging from < 0.5 to 146.8 µg m−3 in December 2016. The PM1 in Dublin was mainly composed of carbonaceous aerosol (organic aerosol (OA) + black carbon (BC)), which, on average, accounted for 80 % of total PM1 mass during the monitoring period. Birr, a small town in the midlands area of Ireland with a population < 1 % of that in Dublin, showed an average PM1 concentration (4.8 µg m−3, ranging from < 0.5 to 63.0 µg m−3 in December 2015) of around half that (56 %) in Dublin. Similarly, the PM1 in Birr was also mainly composed of carbonaceous aerosol, accounting for 77 % of total PM1 mass. OA source apportionment results show that local emissions from residential heating were the dominant contributors (65 %–74 % of the OA) at the two sites, with solid fuel burning, on average, contributing 48 %–50 % of the total OA. On the other hand, Carnsore Point and Mace Head, which are both regional background coastal sites, showed lower average PM1 concentrations (2.2 µg m−3 for Carnsore Point in December 2016 and 0.7 µg m−3 for Mace Head in January 2013) due to the distance from emission sources. Both sites were dominated by secondary aerosol comprising oxygenated OA (OOA), nitrate, sulfate, and ammonium. This nationwide source apportionment study highlights the large contribution of residential solid fuel burning to urban air pollution and identifies specific sources that should be targeted to improve air quality. On the other hand, this study also shows that rural and coastal areas are dominated by secondary aerosol from regional transport, which is more difficult to tackle. Detailed characterization of the spatial and chemical variation in submicron aerosol in this relatively less studied western European region has significant implications for air quality policies and mitigation strategies, as well as for regional-transport aerosol modeling.