American Association for Aerosol Research - Abstract Submission

AAAR 39th Annual Conference
October 18 - October 22, 2021

Virtual Conference

Abstract View


Humidity-Dependent Brown Carbon Light Absorption and Photobleaching: Laboratory and Model Synthesis of Organic Dyes

KYLE GORKOWSKI, Katherine Benedict, James E. Lee, Allison Aiken, Christian Carrico, Tyler Capek, Manvendra Dubey, Los Alamos National Laboratory

     Abstract Number: 395
     Working Group: Carbonaceous Aerosol

Abstract
Non-refractory light-absorbing molecules can photochemically oxidize to become more hygroscopic and less absorbing on a per molecule basis. This dynamic process occurs during aerosol transport in the atmosphere and is particularly important during the aging of biomass-burning plumes. We will discuss both laboratory measurements and modeling comparisons for this process using brown carbon mimics (photobleaching, hygroscopicity, and light scattering).

We performed aging experiments on mono-disperse surrogate brown carbon aerosol using an Aerodyne oxidation flow reactor. The humidity-dependent optical absorption was measured at 450 nm using a newly developed humidity-controlled single scattering albedometer. We observed a decrease in absorption and increased single scatter albedo as the days of equivalent OH exposure increased (1 to 10 days). We interpret our observations using an integrated modeling framework that combines thermodynamics and optical modeling. Specifically, we used a binary activity thermodynamics model to simulate water uptake. We predicted the complex-refractive index from the molecular structures using a newly developed molecular-optical model. We then combined these two steps in a hygroscopic-Mie model to compare our simulation to the observed humidity-dependent optical absorption.

Our synthesis combines the multiple processes by which brown carbon undergoes photo-bleaching in the atmosphere that we will develop into a simplified parameterization for use in climate models.