Heterogeneous oxidation kinetics of organic biomass burning aerosol surrogates by O3, NO2, N2O5, and NO3

Abstract

The reactive uptake coefficients (γ) of O₃, NO₂, N₂O₅, and NO₃ by levoglucosan, abietic acid, nitroguaiacol, and an atmospherically relevant mixture of those species serving as surrogates for biomass burning aerosol have been determined employing a chemical ionization mass spectrometer coupled to a rotating-wall flow-tube reactor. γ of O₃, NO₂, N₂O₅, and NO₃ in the presence of O₂ are in the range of 1−8 × 10⁻⁵, <10⁻⁶−5 × 10⁻⁵, 4−6 × 10⁻⁵, and 1−26 × 10⁻³, respectively, for the investigated organic substrates. Within experimental uncertainties the uptake of NO₃ was not sensitive to relative humidity levels of 30 and 60%. NO₃ uptake experiments involving substrates of levoglucosan, abietic acid, and the mixture exhibit an initial strong uptake of NO₃ followed by NO₃ gas-phase recovery as a function of NO₃ exposure. In contrast, the uptake of NO₃ by nitroguaiacol continuously proceeds at the same efficiency for investigated NO₃ exposures. The derived oxidative power, i.e. the product of γ and atmospheric oxidant concentration, for applied oxidants is similar or significantly larger in magnitude than for OH, emphasizing the potential importance of these oxidants for particle oxidation. Estimated atmospheric lifetimes for the topmost organic layer with respect to O₃, NO₂, N₂O₅, and NO₃ oxidation for typical polluted conditions range between 1–112 min, indicating the potential for significant chemical transformation during atmospheric transport. The contact angles determined prior to, and after heterogeneous oxidation by NO₃, representative of 50 ppt for 1 day, do not decrease and thus do not indicate a significant increase in hygroscopicity with potential impacts on water uptake and cloud formation processes.