Aerosol Uptake Coefficients of Isoprene Epoxides: Determination and Parameter Estimation from Online Field Measurements of Organic Molecular Markers

This study introduces a new approach that combines online field measurements and photochemical box modeling to estimate summer daytime heterogeneous reaction rates ( k het ) and uptake coefficients (γ) for isoprene epoxydiols (IEPOX) and hydroxymethyl-methyl-α-lactone (HMML)/methacrylic acid epoxide (MAE) under ambient conditions. The k het and γ values are highly acidity-dependent. For IEPOX, γ increases from (2.1 ± 1.5) × 10 –3 under pH > 4 to (6.7 ± 6.0) × 10 –3 under pH < 2, and k het increases from (2.2 ± 1.3) × 10 –5 under pH > 4 to (7.3 ± 6.7) × 10 –5 under pH < 2. For HMML/MAE, γ increases from (2.2 ± 1.9) × 10 –5 under pH > 4 to (1.3 ± 1.1) × 10 –4 under pH < 2, and k het increases from (2.3 ± 1.9) × 10 –7 under pH > 4 to (1.4 ± 1.3) × 10 –6 under pH < 2. These estimates are comparable to those derived from chamber experiments under similar conditions but higher than those from field studies estimated by reactive uptake parametrization. Furthermore, multilinear regression is employed to determine three aqueous phase reaction parameters (HaqkH2O,H+, HaqkH2O,HSO4− and HaqkSO42−,H+). These parameters can be used to estimate γ in chemical transport models, and their values approach the upper limit of previously reported data. The study demonstrates the feasibility of deriving kinetic parameters for isoprene SOA intermediates under ambient conditions using bihourly organic molecular tracer data.