Measurement of size-dependent single scattering albedo of fresh biomassburning aerosols using the extinction-minus-scattering technique with acombination of cavity ring-down spectroscopy and nephelometry

摘要

Biomass burning (BB) aerosols have a significant effecton regional climate, and represent a significant uncertainty in ourunderstanding of climate change. Using a combination of cavity ring-downspectroscopy and integrating nephelometry, the single scattering albedo(SSA) and Ångstrom absorption exponent (AAE) were measured for severalNorth American biomass fuels. This was done for several particle diameters for thesmoldering and flaming stage of white pine, red oak, and cedar combustion.Measurements were done over a wider wavelength range than any previousdirect measurement of BB particles. While the offline sampling system usedin this work shows promise, some changes in particle size distribution wereobserved, and a thorough evaluation of this method is required. Theuncertainty of SSA was 6 %, with the truncation angle correction of thenephelometer being the largest contributor to error. While scattering andextinction did show wavelength dependence, SSA did not. SSA values rangedfrom 0.46 to 0.74, and were not uniformly greater for the smoldering stagethan the flaming stage. SSA values changed with particle size, and notsystematically so, suggesting the proportion of tar balls to fractal blackcarbon change with fuel type/state and particle size. SSA differences of0.15–0.4 or greater can be attributed to fuel type or fuel state for freshsoot. AAE values were quite high (1.59–5.57), despite SSA being lower thanis typically observed in wildfires. The SSA and AAE values in this work donot fit well with current schemes that relate these factors to the modifiedcombustion efficiency of a burn. Combustion stage, particle size, fuel type,and fuel condition were found to have the most significant effects on theintrinsic optical properties of fresh soot, though additional factorsinfluence aged soot.