Characterizing Winter-Time Brown Carbon: Insights into Chemical and Light-Absorption Properties in Residential and Traffic Environments

Brown carbon (BrC) is an organic aerosol (OA) component that possesses light-absorbing properties in the UV-vis spectrum, impacting climate change. However, the current understanding of climate repercussions stemming from BrC emissions remains insufficient due to a lack of comprehensive knowledge regarding its chemical makeup, light-absorption, and the role of atmospheric aging in shaping BrC properties. This study investigates BrC in PM1 (particulate matter < 1 µm) during winter in Helsinki, Finland, in a street canyon and a residential area with wood combustion. The aim was to ascertain BrC sources, chemical composition, and contribution to UV-Vis light absorption. The study utilized a seven-wavelength aethalometer (AE33) to measure black carbon (BC) and BrC light absorptions, and a soot particle aerosol mass spectrometer (SP-AMS) to determine OA composition. An OA source apportionment using positive matrix factorization followed by a multiple regression analysis between each factor and BrC absorption were performed to determine the mass absorption coefficients of BrC (MACBrC) and light absorption contributions of distinct sources across 370-660 nm wavelengths. The BrC UV-Vis absorption relative to BC was up to 20.4±9.4% in the residential area and up to 18.9±8.6% in the traffic site, with a higher contribution observed at lower wavelengths. Residential BrC absorption showed sporadic peaks, while street canyon absorption was lower but consistent. MACBrC was higher for biomass burning organic aerosol but still significant for long-range transported (LRT) and traffic-related aerosols. Hydrocarbon-like organic aerosol exhibited higher MACBrC at 470 nm than 370 nm at both sites. Combined with particulate mass concentrations, biomass burning and LRT particles contributed the most to light absorption. The chemical composition analysis revealed stronger correlations between BrC absorption and SP-AMS-measured ions, especially in residential areas and for polycyclic aromatic hydrocarbons, oxidized aromatics, and nitrogen-containing aromatics. The study emphasizes the importance of anthropogenic sources in BrC light absorption.