Theoretical study on gas-phase reactions of nitrate radicals with methoxyphenols: Mechanism, kinetic and toxicity assessment.

Creosol and 4-ethylguaiacol are two important methoxyphenols, lignin pyrolysis products, which are discharge into the atmosphere in large quantities. In this work, theoretical calculations of the reaction mechanism towards the two compounds with NO3 radicals was performed using DFT method. The rate constants and toxicity assessment were also investigated. The atmospheric lifetime for creosol and 4-ethylguaiacol were 0.82 and 0.19 h, respectively. A new reaction pathway was proposed for the transformation of methoxyl into hydroxyl, which has not yet been clarified in previous studies. The toxicity of methoxyphenols and their degradation products is closely related to their hydrophobicity. Although most degradation products are less toxic, they also should be pay more attention, especially for nitro-substituents. A new reaction pathway was proposed for the transformation of methoxyl into hydroxyl. The toxicity is closely related to their hydrophobicity.

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] ∼10(7)-10(11) 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 × 10(7)-3 × 10(9) molecule cm(-3) for all investigated organic compounds, but decrease to 0.008-0.034 for [OH] = 4.1 × 10(10)-6.7 × 10(10) 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 K(OH) = 3.81 × 10(-10) cm(3) molecule(-1) and k(s) = 9.71 × 10(-17) cm(2) 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.