EC-18. Estimating the direct radiative forcing of wildfire smoke using aerosol optical properties measured during FIREX-AQ.

Abstract
Aerosol particles in the atmosphere can mask the climate warming effect of CO2 and other greenhouse gases by reflecting sunlight back into space. How much light these particles reflect back into space is commonly calculated using Mie theory. While Mie theory accurately describes directional scattering for homogenous, spherical particles, the particles in smoke can be irregularly shaped, and thus Mie theory may not be applicable. In this work, we compare measurements of smoke optical properties with those typically used in climate models, particularly the aerosol scattering asymmetry parameter. The asymmetry parameter describes how likely light scattered by a particle will be directed backwards (i.e. back into space if the sun is overhead), and thus is an important factor in determining the direct radiative effect of wildfire smoke. We measured the directional scattering of light at two wavelengths (405 and 660 nm) by fresh wildfire smoke during FIREX-AQ. The average asymmetry factor we measured at 405 nm is 12% lower (indicating more backscatter) than what would be predicted using Mie theory and a canonical refractive index for biomass burning aerosol. The average asymmetry factor measured at 660 nm is not statistically different from the Mie theory calculation. These changes in the asymmetry factor could result in an increase in the cooling efficiency of smoke from wildfires up to 16%.