A218-0010
Molecular Foundations of Humidity Dependent Absorption Enhancements by Brown Carbon Surrogates: Laboratory Observations and Mixing Rules

Wednesday, 16 December 2020
Poster
Kyle Gorkowski1, Tyler Capek2, Christian Carrico3, Allison C Aiken1, James Lee4, Claudio Mazzoleni2 and Manvendra Krishna Dubey1, (1)Los Alamos National Laboratory, Los Alamos, NM, United States, (2)Michigan Technological University, Physics, Houghton, MI, United States, (3)New Mexico Institute of Mining and Technology, Socorro, NM, United States, (4)Los Alamos National Laboratory, Los Alamos, United States
Abstract:
Large uncertainties persist in aerosol optical properties, particularly their relative humidity (RH) dependence that impacts atmospheric radiative transfer significantly. These uncertainties are especially challenging for the humidity dependence of light absorption by brown carbon aerosols emitted by biomass burning. To tackle these challenges, we report laboratory optical measurements of surrogate brown carbon species as a function RH. We target single molecular species that span a range of electronic structures, absorption spectra, and solubilities, as well as their mixtures with scattering and absorbing aerosols. Our observations are used to build a fundamental framework to treat the effects of humidity and cloud processing on brown carbon radiative forcing in climate models.

We used a Cavity-Attenuated Phase-shift Single Scattering Albedo PM monitor (CAPS-PMSSA, Aerodyne, Inc.) with custom built RH-control to measure in-situ aerosol light extinction and scattering at wavelength 450nm. We sampled three brown carbon surrogates (sodium fluorescein, Sunset Yellow, and Para Red); each was size selected, comprised of a single molecular species, and had a different hygroscopicity. For each species, we performed self-consistent optical and hygroscopic closure analysis using Mie and k-Kohler theory. We will contrast sodium fluorescein with Sunset Yellow (or Para Red), which have a similar number of π bonds, 12 and 13 respectively, but have substantial differences in single scatter albedo (SSA, 0.23 vs. 0.4). This difference is attributed to the configuration of the π bonds, which in sodium fluorescein there are more conjugated π bonds (11) than in Sunset Yellow or Para Red (8). Sunset Yellow and Para Red have sulfate and nitrate functional groups, while sodium fluorescein is oxygenated leading to different solubilities. All three systems increase in aerosol absorption by a factor of 1.2-1.4 at 80% RH. The SSA also increases as a function of RH since the additional water increases the scattering by particle growth and reduces the imaginary part of the refractive index of the mixed particle. We conducted mixing experiments, of these brown carbon surrogates, with ammonium sulfate, Nigrosin, and fullerene soot to mimic the complexity of ambient aerosol and to help develop mixing rules for climate models.

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