Heterogeneous processing of biomass burning aerosol proxies by OH radicals for a wide range of OH concentrations and detection of volatilization products

Biomass burning aerosol (BBA) constitutes the majority of primary organic aerosol found in the atmosphere, with emission rates comparable to fossil-fuel burning. BBA affects earth's radiative budget directly through absorption and scattering of radiation or indirectly by modifying cloud radiative properties, and impacts air quality. Quantifying BBA source strength and thus its effects on air quality, human health, and climate can be difficult since these organic particles can chemically transform during atmospheric transport, a process also termed aging, due to heterogeneous reactions with oxidants and radicals such as OH. In this work we investigate the reactive uptake of OH radicals by typical BBA compounds that also serve as molecular markers for source apportionment studies. Organic substrates of cellulose pyrolysis products such as levoglucosan (1,6-anhydro-β-glucopyranose, C₆H₁₀O₅), resin acids such as abietic acid (1-phenanthrenecarboxylic acid, C₂₀H₃₀O₂), and lignin decomposition products such as 5-nitroguaiacol (2-methoxy-5-nitrophenol, C₇H₇NO₄) have been exposed to a wide range of OH concentrations (~10⁷-10¹¹ cm^-3), in presence of O₂ in a rotating wall flow reactor operated at 2-6 mbar coupled to a custom built chemical ionization mass spectrometer (CIMS). OH radicals were generated through H₂ dissociation in an Evenson microwave resonant cavity operated at 2.45 GHz followed by reaction with O₂ or NO₂. In addition, potential volatilization of organic material due to heterogeneous oxidation by OH has been determined in-situ by monitoring the volatile organic compounds using a high resolution-proton transfer reaction-time of flight-mass spectrometer (HR-PTR-ToF-MS). The volatilization studies are conducted at 1 atm and OH is generated by O₃ photolysis in the presence of H₂O vapor and quantified using a photochemical box model as well as through reaction with a known concentration of isoprene (2-methyl-1,3-butadiene, C₅H₈). Reactive uptake validation experiments show good agreement with previously derived uptake coefficients for similar OH concentrations including levoglucosan. However, changes in OH concentration by ~4 orders of magnitude results in OH uptake coefficient variations of ~2 orders of magnitude. Higher OH concentration yields lower OH uptake coefficients. Our experiments strongly suggest that the highly reactive OH uptake follows a Langmuir-Hinshelwood type uptake mechanism, i.e. adsorption of OH is followed by reaction with the organic substrate, instead of an Eley-Rideal mechanism in which gas-to-surface collision results in reaction. In other words, surface saturation may play a role at high OH concentrations. Oxidation lifetime estimates for each investigated organic substrate are ~4 days commensurate with wet deposition (~5-10 days). Initial volatilization results indicate the formation of short-chained hydrocarbon species such as acetaldehyde (C₂H₄O), formic acid (CH₂O₂), and acetic acid (C₂H₄O₂).