Photochemical evolution of atmospheric brown carbon in Arctic Svalbard

Abstract

Atmospheric brown carbon (BrC) is a light-absorbing organic aerosol that contributes to positive radiative forcing. BrC evolves substantially with atmospheric photochemistry, which remains poorly understood in the rapidly warming Arctic, leading to uncertainties in assessing BrC radiative forcing. In this study, we characterize the photochemical evolution of BrC subgroups by measuring optical absorption and fluorescence of methanol-soluble BrC (MeS-BrC) and water-soluble BrC (WS-BrC) in Arctic Svalbard from May 2022 to April 2023. Parallel factor analysis identified one fluorescent humic-like substances (HULIS) component in MeS-BrC and three fluorescent HULIS components in WS-BrC, which were further classified as one less-oxygenated HULIS (LO-HULIS) and two more-oxygenated HULIS (MO-HULIS) components in WS-BrC. Long-range transport elevated mass absorption efficiency at 365 nm (MAE365) and mass-normalized fluorescence intensity at the peak wavelength (Fmax) of all HULIS components in MeS-BrC and WS-BrC in December-March. Following polar sunrise in mid-February, MeS-BrC MAE365, MeS-BrC HULIS Fmax, and WS-BrC LO-HULIS Fmax decreased, whereas WS-BrC MAE365 and WS-BrC MO-HULIS Fmax increased. This indicates photobleaching in less-polar, less-oxygenated BrC and photo-enhancement in more-polar, more-oxygenated BrC during initial photochemistry. Our findings imply that photobleaching and photo-enhancement are essential processes in the evolution of BrC in the Arctic, which help to better constrain BrC properties in assessing aerosol direct radiative forcing and broader aerosol-radiation and climate interactions.