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Catechol (1,2-benzenediol), a common phenolic species emitted during biomass burning, is both redox active and metal chelating. When oxidized by OH radicals in the aqueous phase, it rapidly forms brown carbon (BrC). Here, we report chamber studies of the multiphase chemistry of catechol using HOOH as an OH radical source, soluble iron, simulated sunlight, and either deliquesced or solid-phase seed particles. BrC of remarkable similarity (MAC365 = 1.7 ±0.2 m2 g-1, “medium-BrC” category) was produced whenever gas-phase catechol was photolyzed in the chamber, with or without the presence of an OH radical source, soluble iron, or deliquesced aerosol. The speed and quantity of BrC formation varied, however. While BrC production was slower in the absence of an OH radical source, multiple lines of evidence suggest that OH generation via photosensitization by surface-adsorbed catechol can still generate BrC. Fenton chemistry actively occurred in surface-adsorbed water layers even below the seed particle deliquescence point, leading to significant production of gas-phase benzoquinone. Ratios of BrC and secondary organic aerosol (SOA) relative to catechol concentrations were highest in the presence of trace amounts of soluble iron, HOOH, and simulated sunlight, indicating that photo-Fenton chemistry contributed substantially to BrC and SOA formation by catechol. Finally, we observed that BrC and SOA formation by catechol / photo-Fenton chemistry can occur efficiently even at 40% RH. These results are consistent with catechol being a major source of secondary BrC in biomass burning plumes, even at moderate relative humidity. 

Guaiacol, present in wood smoke, readily forms secondary organic aerosol (SOA), and, in the aqueous phase, brown carbon (BrC) species. Here, BrC is produced in an illuminated chamber containing guaiacol(g), HOOH(g) as an OH radical source, and either deliquesced salt particles or guaiacol SOA at 50% relative humidity. BrC production slows without an OH source (HOOH), likely due to low levels of radical generation by photosensitization, perhaps involving surface-adsorbed guaiacol and dissolved oxygen. With or without HOOH, BrC mass absorption coefficients at 365 nm generated by the guaiacol + OH reaction reach a maximum at ~6 h of atmospheric OH exposure, after which photobleaching becomes dominant. In the presence of soluble iron but no HOOH, more BrC is produced, likely due to insoluble polymer production observed in previous studies. However, with both soluble iron and HOOH (enabling Fenton chemistry), significantly less SOA and BrC are produced due to very high oxidation rates, and the average SOA carbon oxidation state reaches 2, indicating carboxylate products like oxalate. These results indicate that SOA and BrC formation by guaiacol photooxidation can take place over a wider range of atmospheric conditions than previously thought, and that the effects of iron(II) depend on HOOH. Multiphase guaiacol photooxidation likely makes a significant contribution to producing highly oxidized SOA material in smoke plumes. 

Abstract. Alpha-dicarbonyl compounds are believed to form browncarbon in the atmosphere via reactions with ammonium sulfate (AS) in clouddroplets and aqueous aerosol particles. In this work, brown carbon formationin AS and other aerosol particles was quantified as a function of relativehumidity (RH) during exposure to gas-phase glyoxal (GX) in chamberexperiments. Under dry conditions (RH < 5 %), solid AS,AS–glycine, and methylammonium sulfate (MeAS) aerosol particles brown withinminutes upon exposure to GX, while sodium sulfate particles do not. When GXconcentrations decline, browning goes away, demonstrating that this drybrowning process is reversible. Declines in aerosol albedo are found to be afunction of [GX]2 and are consistent between AS and AS–glycineaerosol. Dry methylammonium sulfate aerosol browns 4 times more than dryAS aerosol, but deliquesced AS aerosol browns much less than dry AS aerosol.Optical measurements at 405, 450, and 530 nm provide an estimatedÅngstrom absorbance coefficient of -16±4. This coefficient andthe empirical relationship between GX and albedo are used to estimate anupper limit to global radiative forcing by brown carbon formed by 70 ppt GXreacting with AS (+7.6×10-5 W m−2). This quantity is< 1 % of the total radiative forcing by secondary brown carbonbut occurs almost entirely in the ultraviolet range.