Abstract
Abstract. Biomass burning (BB) aerosols have a significant effect on regional climate, and represent a significant uncertainty in our understanding of climate change. Using a combination of cavity ring-down spectroscopy and integrating nephelometry, the single scattering albedo (SSA) and Ångstrom absorption exponent (AAE) were measured for several North American biomass fuels. This was done for several particle diameters for the smoldering and flaming stage of white pine, red oak, and cedar combustion. Measurements were done over a wider wavelength range than any previous direct measurement of BB particles. While the offline sampling system used in this work shows promise, some changes in particle size distribution were observed, and a thorough evaluation of this method is required. The uncertainty of SSA was 6 %, with the truncation angle correction of the nephelometer being the largest contributor to error. While scattering and extinction did show wavelength dependence, SSA did not. SSA values ranged from 0.46 to 0.74, and were not uniformly greater for the smoldering stage than the flaming stage. SSA values changed with particle size, and not systematically so, suggesting the proportion of tar balls to fractal black carbon change with fuel type/state and particle size. SSA differences of 0.15–0.4 or greater can be attributed to fuel type or fuel state for fresh soot. AAE values were quite high (1.59–5.57), despite SSA being lower than is typically observed in wildfires. The SSA and AAE values in this work do not fit well with current schemes that relate these factors to the modified combustion efficiency of a burn. Combustion stage, particle size, fuel type, and fuel condition were found to have the most significant effects on the intrinsic optical properties of fresh soot, though additional factors influence aged soot.
Highlights
Biomass burning (BB) is recognized as one of the largest sources of absorbing aerosols in the atmosphere (Bond et al, 2013; Jacobson, 2014; Ramanathan and Carmichael, 2008; Moosmüller et al, 2009)
A comparison of the size distributions of white pine during combustion and after renebulization showed a change in the particle size distribution
It is not known whether the optical properties of size-selected particles change due to this sampling process
Summary
Biomass burning (BB) is recognized as one of the largest sources of absorbing aerosols in the atmosphere (Bond et al, 2013; Jacobson, 2014; Ramanathan and Carmichael, 2008; Moosmüller et al, 2009). Smoke from BB is composed of gaseous and aerosol constituents, including black carbon (BC), brown carbon (BrC), organic carbon (OC), and mineral dust, all of which have critical climate and health impacts. BB aerosols have significant impacts, on local climate, and on regional climate, air quality, and hydrological cycles (Alonso-Blanco et al, 2014; Haywood et al, 2003, 2008; Fu et al, 2012; Lin et al, 2013; Yen et al, 2013; Reid et al, 2005, 2013). Depending on surface albedo and the relative amounts of OC and BC/BrC, BB smoke can heat or cool the atmosphere, provide condensation nuclei for ice and water reduce visibility, and affect air quality. The recent estimate (IPCC, 2013) of biomass aerosol radiative forcing is 50 % larger than earlier estimates
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