Direct Measurements and Implications of the Aerosol Asymmetry Parameter in Wildfire Smoke During FIREX‐AQ

Abstract We present direct measurements of the asymmetry parameter ( g ) from biomass burning aerosol at two wavelengths using the Laser Imaging Nephelometer. We compare the measurements with Mie theory calculations based on optically measured size distributions and with g values derived from hemispheric backscatter ( b ) measurements using both an integrating and an imaging nephelometer. During the FIREX‐AQ field mission, we measured the optical and microphysical properties of smoke plumes that had been emitted between 0.5 and 8.5 hr earlier. We find that the measured g can only be reproduced from particle size distribution measurements using a higher refractive index than is typically retrieved from remote measurements and assumed in some models. Retrievals performed using the GRASP algorithm suggest the refractive index is wavelength‐dependent with n = 1.55 ± 0.03 at λ = 660 nm and (1.63 ± 0.04) at λ = 405 nm. Using a simple radiative transfer equation, we show that the instantaneous aerosol cooling of the planet by fresh smoke is increased by 20% when evaluated using the measured g values instead of assuming n = 1.52. Besides improving model representations of radiative cooling by fresh smoke, using a more accurate aerosol optical model can improve retrievals of aerosol microphysical properties from remote sensing techniques. Better retrievals will provide a more accurate constraint on the emissions inventories used in global and regional models. This will ultimately reduce the uncertainty in radiative forcing associated with the increasing frequency and magnitude of wildfires.