Brown carbon aerosols from burning of boreal peatlands: microphysical properties, emission factors, and implications for direct radiative forcing

The surface air warming over the Arctic has been almost twice as much as the global average in recent decades. In this region, unprecedented amounts of smoldering peat fires have been identified as a major emission source of climate-warming agents. While much is known about greenhouse gas emissions from these fires, there is a knowledge gap on the nature of particulate emissions and their potential role in atmospheric warming. Here, we show that aerosols emitted from burning of Alaskan and Siberian peatlands are predominantly brown carbon (BrC) - a class of visible light-absorbing organic carbon (OC) - with a negligible amount of black carbon content. The mean fuel-based emission factors for OC aerosols ranged from 3.8 to 16.6 gkg(-1). Their mass absorption efficiencies were in the range of 0.2-0.8m(2)g(-1) at 405nm (violet) and dropped sharply to 0.03-0.07m(2)g(-1) at 532 nm (green), characterized by a mean Angstrom exponent of approximate to 9. Electron microscopy images of the particles revealed their morphologies to be either single sphere or agglomerated tar balls. The shortwave top-of-atmosphere aerosol radiative forcing per unit optical depth under clear-sky conditions was estimated as a function of surface albedo. Only over bright surfaces with albedo greater than 0.6, such as snow cover and low-level clouds, the emitted aerosols could result in a net warming (positive forcing) of the atmosphere.