Humidity-Driven Changes in the Light Absorption Properties of Biomass Burning Secondary Brown Carbon

MUHAMMAD ABDURRAHMAN, Rawad Saleh, University of Georgia

     Abstract Number: 568
     Working Group: Carbonaceous Aerosols

Abstract
The effect of secondary brown carbon (SBrC) from biomass burning on atmospheric radiative balance is poorly constrained, partly due to the evolution of its optical properties during photochemical aging, which is influenced by environmental conditions such as relative humidity (RH). While previous studies have studied RH effects on SBrC absorption, they often focused on SBrC formation from surrogate species rather than actual combustion emissions. Here, we performed experiments to investigate the evolution of SBrC absorption due to photochemical aging, using emissions from two common forest fuels: pine needles and duff (organic forest floor layer). Smoldering combustion was conducted in a 7.5 m³ smog chamber pre-conditioned at either low (20%) or high (80%) RH. After allowing primary particles to deposit onto chamber walls, precursor gases were exposed to UV light to form secondary organic aerosol. Absorption at 422 nm and 532 nm was measured using a photoacoustic spectrometer, and particle size distributions were monitored using a scanning mobility particle sizer. The imaginary part of the refractive index (k) was retrieved using optical closure. For pine-derived SBrC, absorption under low RH initially increased and then gradually declined, losing ~17% and ~23% of its initial value at 422nm and 532 nm, respectively. At high RH, absorption exhibited a similar trend but at a much faster rate, with over 90% loss at both wavelengths, indicating accelerated photobleaching. The duff-derived SBrC also exhibited a steady decline in absorption at low RH, leading to an overall 67% loss. Under high RH, absorption initially dropped sharply but then gradually increased, suggesting a dynamic interplay between photobleaching and chromophore formation. These findings demonstrate that RH significantly influences the evolution of SBrC absorption in a fuel-specific manner, with higher RH enhancing SBrC reactivity by promoting greater interaction between oxidants and aerosol constituents.