Photochemical Aging of Dark Brown Carbon Aerosols Using a PAM-OFR
SHU-WEN YOU, Taveen S. Kapoor, Prabhav Upadhyay, Rajan K. Chakrabarty, Washington University in St. Louis
Abstract Number: 275
Working Group: Carbonaceous Aerosols
Abstract
Dark brown carbon (d-BrC) aerosols exhibit significant light absorption across the ultraviolet (UV), visible, and near-infrared (IR) spectra, rivaling that of black carbon. D-BrC has been shown to account for 10% of both the number and mass fractions of wildfire smoke during the 2019 FIREX-AQ. The intrinsic light absorption of a material is represented by the imaginary part (k) of the complex refractive index (RI, m = n + ik). Several factors—including the complex chemical nature, analytical techniques, and atmospheric transport—contribute to considerable uncertainties in the k of d-BrC. However, a knowledge gap remains regarding the magnitude and timescale of photobleaching for d-BrC. To systematically investigate the impact of photochemical aging on d-BrC’s optical properties over time, we plan to produce d-BrC in the laboratory using a recently developed method that involves aerosolization of dry distilled wood tar followed by heat-shock treatment, and photochemically aging it in a potential aerosol mass oxidation flow reactor (PAM-OFR, Aerodyne). The d-BrC aerosols will be exposed to various hydroxyl radical (OH) exposure levels to simulate 0, 1, 2, 5, and 7 days of equivalent aging. Four integrated photoacoustic nephelometers (IPN) will be operated to monitor the absorption and scattering coefficients of d-BrC across the UV, visible, and IR spectrum. Along with the number size distribution, which is measured by a scanning mobility particle sizer (SMPS, TSI), we will invert the effective RI using PyMieScatt to evaluate the effect of photochemical aging on the changes in d-BrC’s RI. This study will significantly benefit the community by providing the evolution of absorptivity for current climate models during photochemical aging, thereby more accurately estimating its radiative forcing effect.