Measurements of brown carbon and its optical properties from boreal forest fires in Alaska summer

Brown carbon (BrC) plays an important role in global radiative budget but there have been few studies on BrC in Arctic despite rapid warming and increasing wildfires in this region. Here we investigate the optical properties of BrC from boreal fires in Alaska summer, with two sets of measurements from PILS-LWCC-TOC (Particle-IntoLiquid-Sampler - Liquid-Waveguide Capillary flow-through optical Cell - Total-Organic-Carbon analyzer) and filter measurements. We show that during intense wildfires, the mass absorption coefficient at 365 nm (MAC365) from water soluble organic carbon (WSOC) remained stable at -1 m2 g-1. With all plumes sampled and derived transport time, we show a decrease of MAC365 with plume age, with a shorter photobleaching lifetime (-11 h) at 365 nm compared to 405 nm (-20 h). The total absorption by organic aerosols measured from filters at 365 nm is higher than the absorption by WSOC by a factor 2-3, suggesting a dominant role of insoluble organic carbon. Overall BrC dominates absorption in the near-ultraviolet and visible radiation during wildfire season in Alaska summer.