Accretion Product Formation from Ozonolysis and OH Radical Reaction of α-Pinene: Mechanistic Insight and the Influence of Isoprene and Ethylene

Abstract

α-Pinene (C₁₀H₁₆) represents one of the most important biogenic emissions in the atmosphere. Its oxidation products can significantly contribute to the secondary organic aerosol (SOA) formation. Here, we report on the formation mechanism of C₁₉ and C₂₀ accretion products from α-pinene oxidation, which are believed to be efficient SOA precursors. Measurements have been performed in a free-jet flow system. Detection of RO₂ radicals and accretion products was carried out by recent mass spectrometric techniques using different ionization schemes. Observed C₁₀-RO₂ radicals from α-pinene ozonolysis were O,O-C₁₀H₁₅(O₂) _xO₂ with x = 0, 1, 2, 3 and from the OH radical reaction HO-C₁₀H₁₆(O₂)_αO₂ with α = 0, 1, 2. All detected C₂₀ accretion products can be explained via the accretion reaction RO₂ + R'O₂ → ROOR' + O₂ starting from the measured C₁₀-RO₂ radicals. We speculate that C₁₉ accretion products are formed in an analogous way assuming CH₂O elimination. Addition of isoprene (C₅H₈), producing C₅-RO₂ radicals, leads to C₁₅ accretion products formed via cross-reactions with C₁₀-RO₂ radicals. This process is competing with the formation of C₁₉/C₂₀ products from the pure α-pinene oxidation. A similar behavior has been observed for ethylene additives that form C₁₂ accretion products. In the atmosphere, a complex accretion product spectrum from self- and cross-reactions of available RO₂ radicals can be expected. Modeling atmospheric conditions revealed that C₁₉/C₂₀ product formation is only reduced by a factor of 1.2 or 3.6 in isoprene-dominated environments assuming a 2- or 15-fold isoprene concentration over α-pinene, respectively, as present in different forested areas.