Second-Order Kinetic Rate Coefficients for the Aqueous-Phase Hydroxyl Radical (OH) Oxidation of Isoprene-Derived Secondary Organic Aerosol Compounds at 298 K

Abstract

The hydroxyl radical (OH) oxidation of the most abundant nonmethane volatile organic compound emitted to the atmosphere, isoprene (C₅H₈), produces a number of chemical species that partition to the condensed phase via gas-particle partitioning or form condensed-phase compounds via multiphase/heterogeneous chemistry to generate secondary organic aerosols (SOA). The SOA species in aerosol water or cloud/fog droplets may oxidize further via aqueous reaction with OH radicals, among other fates. Rate coefficients for compounds in isoprene's photochemical cascade are well constrained in the gas phase; however, a gap of information exists for the aqueous OH rate coefficients of the condensed-phased products, precluding the atmospheric modeling of the oxidative fate of isoprene-derived SOA. This work investigated the OH-initiated oxidation kinetic rate coefficients (k_OH) for six major SOA compounds formed from the high-NO and low-NO channels of isoprene's atmospheric oxidation and one analog, most of which were synthesized and purified for study: (k₁) 2-methyltetrol [MT: 1.14 (±0.17) × 10⁹ M^-1 s^-1], (k₂) 2-methyl-1,2,3-trihydroxy-4-sulfate [MT-4-S: 1.52 (±0.25) × 10⁹ M^-1 s^-1], (k₃) 2-methyl-1,2-dihydroxy-3-sulfate [MD-3-S: 0.56 (±0.15) × 10⁹ M^-1 s^-1], (k₄) 2-methyl-1,2-dihydroxy-but-3-ene [MDE: 4.35 (±1.16) × 10⁹ M^-1 s^-1], (k₅) 2-methyl-2,3-dihydroxy-1,4-dinitrate [MD-1,4-DN: 0.24 (±0.04) × 10⁹ M^-1 s^-1], (k₆) 2-methyl-1,2,4-trihydroxy-3-nitrate [MT-3-N: 1.12 (±0.15) × 10⁹ M^-1 s^-1], and (k₇) 2-methylglyceric acid [MGA: pH 2:1.41 (±0.49) × 10⁹ M^-1 s^-1; pH 5:0.97 (±0.42) × 10⁹ M^-1 s^-1]. The second-order rate coefficients are determined against the known k_OH of erythritol in pure water. The decays of each reagent were measured with nuclear magnetic resonance (NMR) and high-performance liquid chromatography-high resolution mass spectrometry (HPLC-HRMS). The aqueous photooxidation fates of isoprene-derived SOA compounds are substantial and may impact the SOA budget when implemented into global models.

Keywords: OH radical oxidation; aqueous photooxidation; isoprene derived compounds; methyltetrol; organonitrate; organosulfate.