Abstract
Abstract. We quantify the hygroscopic properties of particles freshly emitted from biomass burning and after several hours of photochemical aging in a smog chamber. Values of the hygroscopicity parameter, κ, were calculated from cloud condensation nuclei (CCN) measurements of emissions from combustion of 12 biomass fuels commonly burned in North American wildfires. Prior to photochemical aging, the κ of the fresh primary aerosol varied widely, between 0.06 (weakly hygroscopic) and 0.6 (highly hygroscopic). The hygroscopicity of the primary aerosol was positively correlated with the inorganic mass fraction of the particles. Photochemical processing reduced the range of κ values to between 0.08 and 0.3. The changes in κ were driven by the photochemical production of secondary organic aerosol (SOA). SOA also contributed to growth of particles formed during nucleation events. Analysis of the nucleation mode particles enabled the first direct quantification of the hygroscopicity parameter κ for biomass burning SOA, which was on average 0.11, similar to values observed for biogenic SOA. Although initial CCN activity of biomass burning aerosol emissions are highly variable, after a few hours of photochemical processing κ converges to a value of 0.2 ± 0.1. Therefore, photochemical aging reduces the variability of biomass burning CCN κ, which should simplify analysis of the potential effects of biomass burning aerosol on climate.
Highlights
Biomass burning is a major source of aerosol emissions globally (Andreae et al, 2004; Crutzen and Andreae, 1990)
Smog chamber experiments were carried out to investigate the effects of photochemical aging on emissions from biomass fires simulated at the US Forest Service Fire Science Laboratory (FSL) in Missoula, MT as part of the Third Fire Lab at Missoula Experiment (FLAME-III) study
During the photo-oxidation period the suspended mass increased slightly due to production of secondary organic aerosol (SOA), despite the loss of particles to the walls
Summary
Biomass burning is a major source of aerosol emissions globally (Andreae et al, 2004; Crutzen and Andreae, 1990). These aerosols have the potential to activate to form cloud droplets and impact cloud properties and climate. Biomass burning emits a large amount of water vapor and has the potential to form pyro-cumulus clouds with relatively high supersaturation conditions (Reutter et al, 2009). Atmospheric modeling has shown that biomass burning is an important global source of CCN (Pierce et al, 2007; Spracklen et al, 2011). More recent research has shown that some biomass burning aerosols can activate at supersaturations as low as 0.05 % (Rogers et al, 1991), which would be more typical of a stratiform cloud
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