Properties and Atmospheric Oxidation of Terebic Acid Aerosol

Abstract

Terebic acid (C₇H₁₀O₄) is a biogenic secondary organic aerosol constituent, produced by the oxidation of first- and second-generation products of monoterpenes such as α-pinene, β-pinene, and Δ³-carene. It is a processed derivative of terpenylic acid and has been identified in aerosol samples from terrestrial and forest environments. The physicochemical properties of pure terebic acid aerosol were characterized using two different atmospheric simulation chambers and a suite of online particle and gas-phase instrumentation. Its mass spectrum, obtained by a high-resolution time-of-flight mass spectrometer, had characteristic peaks at mass-to-charge (m/z) ratios 81, 96, 100, 115, and 143, mainly related to oxygenated fragment ions. The density of terebic acid aerosol was 1.33 ± 0.20 g cm^–3, and its vaporization enthalpy was 85 kJ mol^–1. The estimated saturation concentration at 298 K of 2.6 ± 1.2 μg m^–3 places terebic acid in the semivolatile organic compound category. Oxidation of terebic acid aerosol by hydroxyl (OH) radicals resulted in a substantial reduction in organic aerosol (OA) mass concentration (up to 80%), with no significant alteration in the OA spectrum or aerosol O:C ratio, indicating negligible production of secondary OA. Gas-phase analysis detected the production of smaller compounds, such as acetone. The terebic acid oxidation products were mostly in the gas phase as fragmentation appears to dominate its reaction with OH radicals. The gas-phase reaction rate constant with OH was estimated to be 3 × 10^–12 cm³ molecule^–1 s^–1.