Heterogeneous OH oxidation of biomass burning organic aerosol surrogate compounds: assessment of volatilisation products and the role of OH concentration on the reactive uptake kinetics†
The reactive uptake coefficients (γ) of OH by levoglucosan, abietic acid, and nitroguaiacol serving as surrogate compounds for biomass burning aerosol have been determined employing a chemical ionisation mass spectrometer coupled to a rotating-wall flow-tube reactor over a wide range of [OH] ∼107–1011 molecule cm−3. Volatilisation products of these organic substrates due to heterogeneous oxidation by OH have been determined at 1 atm using a high resolution proton transfer reaction time-of-flight mass spectrometer (HR-PTR-ToF-MS). γ range within 0.05–1 for [OH] = 2.6 × 107–3 × 109 molecule cm−3 for all investigated organic compounds, but decrease to 0.008–0.034 for [OH] = 4.1 × 1010–6.7 × 1010 molecule cm−3. γ as a function of [OH] can be described by a Langmuir–Hinshelwood model, neglecting bulk processes, suggesting that despite its strong reactivity, OH is mobile on surfaces prior to reaction. The best fit Langmuir–Hinshelwood parameters on average are KOH = 3.81 × 10−10 cm3 molecule−1 and ks = 9.71 × 10−17 cm2 molecule−1 s−1 for all of the investigated organic compounds. Volatilised products have been identified indicating enhancements over background of 50% up to a factor of 15. Amongst the common volatile organic compounds (VOCs) identified between levoglucosan, abietic acid, and nitroguaiacol were methanol, acetaldehyde, formic acid, and acetic acid. VOCs having the greatest enhancement over background were glucic acid from levoglucosan, glycolic acid from abietic acid, and methanol and nitric acid from nitroguaiacol. Reaction mechanisms leading to the formation of glucic acid, glycolic acid, methanol, and nitric acid are proposed. Estimated lower limits of atmospheric lifetimes of biomass burning aerosol particles, 200 nm in diameter, by heterogeneous OH oxidation under fresh biomass burning plume conditions are ∼2 days and up to ∼2 weeks for atmospheric background conditions. However, estimated lifetimes depend crucially on [OH] and corresponding γ, emphasising the need to determine γ under relevant conditions.