Rapid Evolution of Aerosol Particles and their Optical Properties Downwind of Wildfires in the Western U.S.

Abstract. During the first phase of the Biomass Burn Operational Project (BBOP) field campaign, conducted in the Pacific Northwest, the DOE G-1 aircraft was used to follow the time evolution of wildfire smoke from near the point of emission to locations 2–3.5 hours downwind. In nine flights we made repeated transects of wildfire plumes at varying downwind distances and could thereby follow the plume's time evolution. On average there was little change in dilution-normalized aerosol mass concentration as a function of downwind distance. This consistency hides a dynamic system in which primary aerosol particles are evaporating and secondary ones condensing. Organic aerosol is oxidized as a result. On all transect more than 90 % of aerosol is organic. In freshly emitted smoke aerosol, NH4+ is approximately equivalent to NO3−. After two hours of daytime aging, NH4+ increased and is approximately equivalent to the sum of Cl−, SO42− and NO3−. Particle size increased with downwind distance causing particles to be more efficient scatters. Averaged over nine flights, mass scattering efficiency increased in ~ two hours by 56 % and in one fight doubled. Coagulation and material transport from small to large particles are discussed as mechanisms for increasing particle size. As absorption remained nearly constant with age the time evolution of single scatter albedo was controlled by age-dependent scattering. Near-fire aerosol had a single scatter albedo (SSA) of 0.8–0.9. After one to two hours of aging SSAs were typically 0.9 and greater. Assuming global-average surface and atmospheric conditions, the observed age-dependence in SSA would change the direct radiative effect of a wildfire plume from near zero near the fire to a cooling effect downwind.