Measurement report: Comprehensive seasonal study of the composition and sources of submicron aerosol during the JULIAC campaign in Germany

The seasonal variations of aerosol sources and their atmospheric evolution are investigated using observations from the year-long JULIAC (Jülich Atmospheric Chemistry Project) campaign (January-November 2019) in Jülich, Germany. Non-refractory submicron aerosol components were continuously measured alongside oxidants (OH, O3, NO3), trace gases, and meteorological conditions. Organic aerosols (OA) dominated the aerosol composition throughout the year (39 %-58 %), with secondary formation being the major source. OA, including organic nitrate and organosulfate, peaked during a summer heatwave event due to enhanced daytime and nighttime secondary OA formation driven by elevated concentrations of atmospheric oxidants. Changes in the OA composition during the heatwave suggest a shift in the formation pathways, where isoprene may play an important role. Biomass-burning, mainly wildfires and anthropogenic activities (e.g., heating, industry), is the dominant primary OA source (45 %-83 %), which may grow in influence due to climate change and the expected energy transition. Air masses containing OA from regional transport from marine and wildfire sources are identified through source apportionment. Analysis and modeling prove this method to be more reliable than traditional tracer-based methods. Regional transport to this study site typically shows a cleansing effect on the aerosol concentration, except in winter. Furthermore, seasonal variations in the effects of regional transport are seen, where identical transport pathways led to different influences on aerosol properties, driven by seasonal differences in biogenic and anthropogenic emissions. This study enhances understanding of seasonal variation in submicron aerosol properties in response to their sources, atmospheric evolution, and transport.