Heterogeneous Kinetics of cis-Pinonic Acid with Hydroxyl Radical under Different Environmental Conditions.

To understand the atmospheric fate of secondary organic aerosol (SOA), heterogeneous degradation behaviors of a specific tracer derived from α-pinene-cis-pinonic acid (CPA), initiated by hydroxyl radicals (OH), were investigated under different environmental conditions using a flow reactor. The second-order rate constant (k2) of the CPA-OH reaction was determined to be (6.17 ± 1.07) × 10(-12) cm(3)·molecule(-1)·s(-1) at 25 °C and 40% relative humidity (RH). Higher temperature promoted this reaction, while relative humidity had a little inhibiting effect on it. The atmospheric lifetime of CPA varied from 2.1 to 3.3 days under different environmental conditions. Infrared spectrometry (IR), density functional theory (DFT) calculation and gas chromatography coupled mass spectrometry (GC-MS) results indicated that the oxidation products should be ascribed to poly(carboxylic acid)s. This study shows that the heterogeneous degradation of CPA initiated by OH radical is appreciable, and the concentrations of CPA measured in field measurements may underestimate the corresponding precursors of SOA.