Brown carbon emissions from laboratory combustion of Eurasian arctic-boreal and South African savanna biomass

Abstract

Warming climate is predicted to increase forest fires which can be a major source of black and brown carbon (BC and BrC) into the atmosphere. Unlike North American forest fires, very limited studies have characterized North Eurasian biomass burning (BB) emissions. In this work, we determined the emission factors (EF) of carbonaceous aerosols and characterized light absorption of BrC emitted from boreal and peat burning through offline filter extraction method. The results were compared to African savanna emissions. Effects of atmospheric dilution and oxidative aging on BrC absorptivity were investigated for selected BB emissions sampled into an environmental chamber. Organic carbon (OC) and elemental carbon (EC) EFs of fresh BB emissions ranged between 1.30–89.9 and 0.01–4.80 g kg−1 respectively. Methanol soluble OC (MSOC) represented more than 92 % of fresh BB emissions, irrespective of fuel type, and consisted of weakly absorbing BrC with imaginary refractive index at 550 nm (kMSOC_550) ranging from 0.002 to 0.011. Water soluble OC (WSOC) fractions varied among fresh BB emissions but overall exhibited higher mass absorption efficiencies at 365 nm (MAE365) than MSOC. Dilution-related evaporative loss in environmental chamber resulted in less volatile OC, making them less soluble in methanol. Photochemical and dark oxidative aging further increased the low volatility OC fractions of the organics along with its oxidation state. Our estimated OC-EC emission factors and kMSOC for fresh BB emissions can be used for future modelling purposes. Further online measurements are needed to account for non-soluble strong BrC in aged BB emissions.