Heterogeneous oxidation of saturated organic aerosols by hydroxyl radicals: uptake kinetics, condensed-phase products, and particle size change

[摘要] The kinetics and reaction mechanism for the heterogeneous oxidation ofsaturated organic aerosols by gas-phase OH radicals were investigated underNO_x-free conditions. The reaction of 150 nm diameter Bis(2-ethylhexyl)sebacate (BES) particles with OH was studied as a proxy for chemical agingof atmospheric aerosols containing saturated organic matter. An aerosolreactor flow tube combined with an Aerodyne time-of-flight aerosol massspectrometer (ToF-AMS) and scanning mobility particle sizer (SMPS) was usedto study this system. Hydroxyl radicals were produced by 254 nm photolysisof O₃ in the presence of water vapour. The kinetics of theheterogeneous oxidation of the BES particles was studied by monitoring theloss of a mass fragment of BES with the ToF-AMS as a function of OHexposure. We measured an initial OH uptake coefficient of γ₀=1.3 (±0.4), confirming that this reaction is highly efficient. Thedensity of BES particles increased by up to 20% of the original BESparticle density at the highest OH exposure studied, consistent with theparticle becoming more oxidized. Electrospray ionization mass spectrometryanalysis showed that the major particle-phase reaction products aremultifunctional carbonyls and alcohols with higher molecular weights thanthe starting material. Volatilization of oxidation products accounted for amaximum of 17% decrease of the particle volume at the highest OH exposurestudied. Tropospheric organic aerosols will become more oxidized fromheterogeneous photochemical oxidation, which may affect not only theirphysical and chemical properties, but also their hygroscopicity and cloudnucleation activity.

[学科分类] 大气科学