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Abstract
Abstract. 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 in 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 (WS) BrC fraction of laboratory-generated biomass burning particles from hardwood pyrolysis. Effects of direct UVB photolysis and OH oxidation in the aqueous phase on molecular-weight-separated BrC were studied. Results indicated that the majority of low-molecular-weight (MW) BrC (<400 Da) was rapidly photobleached by both direct photolysis and OH oxidation on an atmospheric timescale of approximately 1 h. High MW BrC (≥400 Da) underwent initial photoenhancement up to ∼15 h, followed by slow photobleaching over ∼10 h. 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 h. These ambient observations indicate that overall, biomass burning BrC across all molecular weights has an atmospheric lifetime of 15 to 28 h, 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 h) 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.
Highlights
Brown carbon (BrC), the fraction of organic aerosol that absorbs solar radiation in the UV and near-visible wavelengths, may potentially be an important climate warmer, with estimated direct radiative forcing that varies from +0.03 to +0.60 W m−2 (e.g., Park et al, 2010; Feng et al, 2013; Lin et al, 2014; Wang et al, 2014; Saleh et al, 2015; Jo et al, 2016) and a vertical distribution that can be distinctly different from black carbon and other climate warmers (Zhang et al, 2017)
Aqueous OH oxidation did not lead to additional loss of water-soluble organic carbon (WSOC), indicating that the loss of WSOC is only due to direct UVB photolysis
This initial increase in mass absorption coefficient (MAC) values is likely driven by the formation of more absorbing WS BrC, as MAC values increased by a factor of ∼ 2, while WSOC decreased only by a factor of less than 1.1
Summary
Brown carbon (BrC), the fraction of organic aerosol that absorbs solar radiation in the UV and near-visible wavelengths, may potentially be an important climate warmer, with estimated direct radiative forcing that varies from +0.03 to +0.60 W m−2 (e.g., Park et al, 2010; Feng et al, 2013; Lin et al, 2014; Wang et al, 2014; Saleh et al, 2015; Jo et al, 2016) and a vertical distribution that can be distinctly different from black carbon and other climate warmers (Zhang et al, 2017). Molecular identification of the chromophores is a challenging task, as there may be a multitude of light-absorbing compounds It remains unclear whether BrC is comprised of low concentrations of strongly absorbing chromophores or a large number of weakly absorbing chromophores in a complex organic matrix. Several classes of compounds have been identified as BrC in biomass burning organic aerosols (BBOA), such as nitroaromatic compounds (Desyaterik et al, 2013; Zhang et al, 2013; Mohr et al, 2013; Lin et al, 2016), humic-like substances (HULIS; Dinar et al, 2008; Hoffer et al, 2006; Fan et al, 2016; Wang et al, 2017), and other high-molecular-weight substances (i.e., compounds > 400 Da; Di Lorenzo and Young, 2016; Di Lorenzo et al, 2017; Wong et al, 2017)
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