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

Abstract

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 lightabsorbing 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 g kg −1 . Their mass absorption efficiencies were in the range of 0.2-0.8 m 2 g −1 at 405 nm (violet) and dropped sharply to 0.03-0.07 m 2 g −1 at 532 nm (green), characterized by a mean Ångström exponent of ≈ 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.Published by Copernicus Publications on behalf of the European Geosciences Union.

Relative importance of black carbon, brown carbon, and absorption enhancement from clear coatings in biomass burning emissions

“…The range of E AbsDen at 405 nm observed in this study is similar to previous studies (McMeeking et al, 2014;Lack et al, 2012a), except for peat emissions where much higher absorption enhancement is observed. The peat burns give a very high value (5.65 ± 1.43) of E AbsDen at 405 nm because smoldering emissions from peat are predominantly BrC with a negligible amount of BC content (Chakrabarty et al, 2016;Pokhrel et al, 2016;Stockwell et al, 2016). It is evident from Fig.…”

Brown carbon absorption in the red and near-infrared spectral region

“…The absorption Ångström exponent (AAE) of BrC varies between 2 and 11, and the mass absorption coefficient of the water-insoluble fraction is generally much higher than that of the water-soluble fraction (Laskin et al, 2015). Smoldering combustion of peat yielded a variety of particles including spherical ones and agglomerates typically having a higher bulk oxygen-to-carbon ratio (Chakrabarty et al, 2016) than that reported by Pósfai et al (2004). These compositional differences likely resulted in a significantly higher Ångström exponent for the various particles formed in slow-burning processes at low temperature than that for laboratory-generated "pure" tar balls with a C / O ratio of about 10 and distinctive morphology (Hoffer et al, 2016).…”

Radiative characteristics of aerosol during extreme fire event over Siberia in summer 2012

“…Recent studies showed that OA components can contribute substantially to light absorption. In contrast to BC, which absorbs light throughout the UV-visible spectrum, an OA component such as brown carbon (BrC) absorbs mostly at the ultraviolet wavelengths and less significantly in the visible spectral range (Kirchstetter et al, 2004;Bergstrom et al, 2007;Chen and Bond, 2010;Zhong and Jang, 2014;Chakrabarty et al, 2016). The majority of BrC is emitted to the atmosphere through low temperature, incomplete combustion of biomass, bio-and fossil fuel (Bond, 2001;Kirchstetter et al, 2004;Bergstrom et al, 2007;Lewis et al, 2008;Chen and Bond, 2010;Zhong and Jang, 2014).…”