Constraining Gas Phase Yields and Reactive Uptake Coefficients of Oxidation Products from the Hydroxyl Radical-Isoprene Reaction onto Acidic Particles by Vocus Ammonia-Adduct Chemical Ionization Mass Spectrometry (Vocus NH4+ CIMS)

Abstract

Isoprene, the most abundant nonmethane volatile organic compound in the atmosphere, undergoes photochemical reactions with hydroxyl radical (^•OH), a major sink for isoprene, leading to the formation of secondary organic aerosol (SOA). Using a Vocus Chemical Ionization Mass Spectrometer with ammonium-adduct ions (Vocus NH₄⁺ CIMS), this study used the positive ion mode to quantify the yields and time-dependent reactive uptake of oxidized volatile organic compounds (OVOCs) produced from ^•OH-initiated oxidation of isoprene under dry conditions. Molar gas-phase yields of key oxidation products were constrained using sensitivities derived from a voltage scan of the front and back end of the Vocus ion–molecule reactor region. Carefully designed chamber experiments measured uptake coefficients (γ) for key isoprene-derived oxidation products onto acidic sulfate particles. The γ values for both C₅H₁₀O₃ isomers (IEPOX/ISOPOOH) and C₅H₈O₄, another epoxy species from isoprene photo-oxidation, rapidly decreased as the SOA coating thickness increased, demonstrating a self-limiting effect. Despite ISOPOOH/IEPOX contributing around 80% to total reactive uptake, other oxidation products from isoprene photooxidation were estimated to contribute 20% of the total SOA formation. These findings highlight the importance for future models to consider the self-limiting effects of ISOPOOH/IEPOX and SOA formation through non-IEPOX pathways.