Atmospheric Lifetime of Primary Brown Carbon Can Be Increased by Ultraviolet Irradiation

Habeeb Al-Mashala, Katrina Betz, Colton Calvert, Jace Barton, Elevia Bruce, ELIJAH SCHNITZLER, Oklahoma State University

     Abstract Number: 474
     Working Group: Carbonaceous Aerosol

Abstract
Biomass burning organic aerosol (BBOA) impacts climate directly by scattering and absorbing solar and terrestrial radiation. The fraction of BBOA that absorbs visible sunlight is called brown carbon (BrC), and its absorptivity can change during its atmospheric residence time, in part due to multiphase oxidation. For example, multiphase ozonolysis leads to whitening of primary BrC constituents. Irradiation, in addition to multiphase processing, can also change the properties of BrC. Here, we investigate the interplay between irradiation and multiphase processing by measuring the reactive uptake of ozone to thin films of BBOA before and after their exposure to UV radiation in a photoreactor, with peak emission at 310 nm. The thin films were prepared from the lower volatility fraction of BBOA collected from the smoldering sapwood of eastern red cedar, a species that is associated with wild- and prescribed fires in the southern Great Plains of the United States. Irradiation increased the mass absorption coefficient of the BrC at near-UV and visible wavelengths. It also significantly decreased the reactive uptake of ozone, a result attributed to increased viscosity of the BBOA material. These changes in absorptivity and viscosity are consistent with results of mass spectrometry and volatility tandem differential mobility analysis, which show that high-molecular-weight species constitute a greater fraction of the total mass after irradiation. The mass loss during irradiation is negligible, so this change in fraction likely occurs from the formation of oligomers from monomers and dimers of phenolic species, which initially originate from the thermal degradation of lignin. Our results have significant implications on the warming effect contributed by BrC, since UV irradiation both darkens BBOA material and makes it more resistant to whitening upon multiphase processing by oxidants, including ozone.