In situ aerosol optics in Reno, NV, USA during and after the summer 2008 California wildfires and the influence of absorbing and non-absorbing organic coatings on spectral light absorption

摘要

Hundreds of wildfires in Northern California were sparked by lightningduring the summer of 2008, resulting in downwind smoke for the months ofJune and July. Comparisons are reported for aerosol optics measurements inReno, Nevada made during the very smoky month of July and therelatively clean month of August. Photoacoustic instruments equipped withintegrating nephelometers were used to measure aerosol light scattering andabsorption coefficients at wavelengths of 405 nm and 870 nm, revealing a strong variationof aerosol light absorption with wavelength. Insight on fuels burned isgleaned from comparison of Ångström exponents of absorption (AEA)versus single scattering albedo (SSA) of the ambient measurements withlaboratory biomass smoke measurements for many fuels. Measurements duringthe month of August, which were largely unaffected by fire smoke, exhibitsurprisingly low AEA for aerosol light absorption when the SSA is highest,again likely as a consequence of the underappreciated wavelength dependenceof aerosol light absorption by particles coated with non-absorbing organicand inorganic matter. Coated sphere calculations were used to show that AEAas large as 1.6 are possible for wood smoke even with non-absorbing organiccoatings on black carbon cores, suggesting care be exercised when diagnosingAEA.