OH-Initiated Photooxidation of Gas-Phase Atmospherically Relevant Monoterpene-Derived Organic Nitrates.

Monoterpene-derived organic nitrates (MT-ONs) can influence NOx recycling, secondary organic aerosol, and ozone formation. While OH-initiated photooxidation is considered a sink for MT-ONs, the rate constants and mechanisms remain poorly constrained. We investigate the gas phase photooxidation of three synthetic ONs derived from α-pinene, β-pinene, and d-limonene (3°_ApHN, 1°_BpHN, and 2°_LmHN) through chamber experiments. The photooxidation rate constants for MT-ONs range from (5.7 ± 0.5) to (11.0 ± 1.5) × 10-11 cm3 molecule-1 s-1, with corresponding lifetimes of 1.7-3.2 h under ambient OH concentrations. If we consider that products without a nitrooxy group (CHO) represent MT-ONs undergoing photooxidation to form NOx or nitric acid, our measurements suggest that up to 85%, 49%, and 23% of 3°_ApHN, 2°_LmHN, and 1°_BpHN, respectively, contribute to NOx recycling. The prevalence of CHO products is influenced by the molecular structure of MT-ONs and different peroxy radical chemistry. These results are different from prior studies that suggest that photooxidation leads to the complete degradation of MT-ONs into NOx or nitric acid. We propose photooxidation mechanisms, highlighting both OH addition and H abstraction as important processes. This study provides fundamental data to evaluate the contributions of MT-ON photooxidation to NOx recycling and offers new insights into the broader implications of ON chemistry in the atmosphere.