Retrieval of ultra-violet aerosol absorption from radiation measurements in young wildfire plumes

Abstract. Aerosols play an important role for atmospheric radiative transfer in biomass burning (BB) plumes, where they control photochemistry, direct radiative forcing, and radiation-induced atmospheric dynamics. The optical properties of BB aerosol, however, remain poorly constrained, with respect to their absorptive properties at ultraviolet and visible wavelengths. In-situ observations show considerable variability due to heterogeneity in BB plumes, and different measurement methods do not agree with each other. To overcome this challenge, we have developed an algorithm based on the VLIDORT for photochemistry (VPC) radiative transfer model to retrieve the imaginary refractive index k(λ) from airborne actinic flux observations at wavelengths λ of 310–440 nm. Using three flights from NASA/NOAA’s Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) airborne experiment, we obtain values k(387 nm) between 0.02 and 0.03 for different transects, while the absorption Angstrom exponent α_k is 4±1. Volume absorption cross section (VAC) and single scattering albedo generally agree with in situ observations, but show less variability, most likely because of the inherent spatial averaging of our observations. k(λ), VAC and single-scattering co-albedo decrease with physical plume age, with half-lives τ_1/2 of 13±3, 16±4 and 17±4 hours, respectively. Based on our observations, we present a parameterization of the absorptive properties of BB aerosol from western US wildfires as a function of wavelength and plume age, which will help to improve the representation of BB aerosol in models.