Enhanced Radiative Forcing Effects and Albedo Reduction from Brown Carbon Aerosol Deposition on Glacier Snow

GANESH CHELLUBOYINA, Benjamin Sumlin, Payton Beeler, Rajan K. Chakrabarty, Washington University in St. Louis

     Abstract Number: 623
     Working Group: Aerosol Physics

Abstract
As wildfire frequency and intensity have trended upward in recent years, the deposition of light-absorbing aerosols of biomass-burning origin on snow and ice has gained salience. These impurities, when dispersed in snow and ice, induce a reduction in surface albedo compared to pristine snow. Previous studies have extensively measured the concentrations of black carbon (BC) and mineral dust in snow, and attributed albedo reductions to these constituents. However, the influence of brown carbon (BrC), which is co-emitted with BC in wildfires, on albedo remains poorly constrained, and has not been examined as much on account of the wide variability in its optical properties, as well as its differing solubilities.

BrC can be further categorized based on its optical properties, with strongly absorbing BrC (S-BrC, k550 in the range 0.1-0.25) approaching the spectral imaginary refractive index of BC. We simulate aerosol dispersal scenarios in glacier snow to calculate spectral and broadband albedo responses to varying BrC optical properties and concentrations. These optical properties are drawn from parameterizations and FIREX field campaign data. Our results show that, at 1 ppm of aerosol in snow, the strongest absorbing BrC can produce a broadband albedo reduction of up to 0.12 between the wavelengths 400-1000 nm. Within the category of S-BrC, for the same concentration of BrC aerosol, the spread in albedo reductions can be as large as 0.11. Further, we show the results of a combined sensitivity analysis that includes snow microphysical properties such as ice grain size along with aerosol optical properties, and develop a parameterization to predict broadband albedo.

To validate these results, we compare the simulated spectral albedos with real-world measurements from mountain slope sites available in the literature. We also calculate the annual-mean radiative forcing for select locations, and an energy balance analysis is performed. These results may be of interest in better predicting glacier and snow melt in areas downwind of forest fires.