Formation and characteristics of atmospheric aerosols in an Alaskan boreal forest

Aerosols impact climate and human health by affecting radiative forcing, cloud formation, and air quality. Boreal forests emit large amounts of biogenic volatile organic compounds (BVOCs), which act as key precursors for aerosol formation. While European boreal forests are relatively well studied with respect to aerosol formation, observational constraints from the North American boreal forest remain sparse, despite its distinct species composition, climate, and emission characteristics.  To address this gap, continuous observations have been conducted since 2023 at a boreal forest site near Delta Junction, Alaska (63°52′58″N, 145°45′02″W), at a National Ecological Observatory Network (NEON) site and integrated within the long-term Atmospheric Science and Chemistry Measurement Network (ASCENT). From June to September 2025, the DENALI (Dynamics of Emissions, Nucleation and Aerosols to predict Land–atmosphere–climate Interactions) intensive field campaign deployed a comprehensive suite of state-of-the-art instruments to characterise the chemical composition and evolution of trace gases and aerosols in the Alaskan boreal forest. Preliminary results from the campaign show that approximately 40% of the days were influenced by wildfire smoke transported from parts of Alaska and Canada. During clean periods, PM2.5 concentrations were low with a mean organic aerosol concentration of less than 1.0 μg m-3. In contrast, wildfire periods showed a strong increase in aerosol loading, with organic mass fraction reaching up to 80 μg m-3. Periodic increases in sulphate were seen during wildfire events, with an additional episode potentially linked to a volcanic eruption in Russia in early August. Wind analyses show that air masses at the site mainly arrived from the south, especially during clean periods, indicating that air coming from southern regions could influence observed aerosol and trace gas properties. Despite having several days of relatively clean conditions, new particle formation events were observed only rarely during the campaign, unlike frequent NPF events observed in European boreal forests. Further analyses of the comprehensive dataset will help us give a more detailed assessment of the processes influencing aerosol formation at the site.