<p><strong>Abstract.</strong> Biomass burning is a major source of atmospheric brown carbon (BrC) and through its absorption of UV/VIS radiation, it can play an important role on the planetary radiative balance and atmospheric photochemistry. The considerable uncertainty of BrC impacts is associated with its poorly constrained sources, transformations and atmospheric lifetime. Here we report laboratory experiments that examined changes in the optical properties of the water-soluble BrC fraction of biomass burning particles. Effects of direct UVB photolysis and OH oxidation in the aqueous phase on molecular weight-separated BrC were studied. Results indicated that low molecular weight (MW) BrC (< 400 Da) was rapidly photobleached by both direct photolysis and OH oxidation on an atmospheric timescale of approximately 1 hour. High MW BrC (≥ 400 Da) underwent initial photoenhancement over a few hours, followed by slow photobleaching over ~ ten hours. The laboratory experiments were supported by observations from ambient BrC samples that were collected during the fire seasons in Greece. These samples, containing freshly emitted to aged biomass burning aerosol, were analyzed for both water and methanol soluble BrC. Consistent with the laboratory experiments, high MW BrC dominated the total light absorption at 365 nm for both methanol and water-soluble fractions of ambient samples with atmospheric transport times of 1 to 68 hours. These ambient observations indicate that overall, biomass burning BrC across all molecular weights have an atmospheric lifetime of 15 to 20 hours, consistent with estimates from previous field studies – although the BrC associated with the high MW fraction remains relatively stable and is responsible for light absorption properties of BrC throughout most of its atmospheric lifetime. For ambient samples of aged (> 10 hours) biomass burning emissions, poor linear correlations were found between light absorptivity and levoglucosan, consistent with other studies suggesting a short atmospheric lifetime for levoglucosan. However, a much stronger correlation between light absorptivity and total hydrous sugars was observed, suggesting that they may serve as more robust tracers for aged biomass burning emissions. Overall, the results from this study suggest that robust model estimates of BrC radiative impacts require consideration of the atmospheric aging of BrC and the stability of high-MW BrC.</p>

Figure S1. Measured photon flux in the photoreactor using chemical actinometry (dash lines) and spectroradiometer (solid line) for UVB (blue) and UVA lights (red; Wong et al., 2017). The actinic flux for a clear-sky summer day (black line) is included for comparison.