Rapid Photolysis of Gaseous Organic Nitrates Formed from Hydroxyl and Nitrate Radical Oxidation of α-Pinene and β-Pinene

Photolysis of gaseous organic nitrates is crucial for understanding the formation and fate of air pollutants such as nitrogen oxides (NO _<i>x</i> ) and ozone (O₃). Monoterpenes are prevalent biogenic volatile organic compounds (VOCs), greatly contributing to the formation of organic nitrates; however, there is currently a lack of experimental constraints on the photolysis chemistry of monoterpene organic nitrates. Here, we investigated the photolysis of monoterpene organic nitrates via novel dual chamber experiments, in which a large suite of organic nitrates was formed from hydroxyl and nitrate radical oxidation of α-pinene and β-pinene in one chamber and were introduced into another chamber to study photolysis. We directly measured their photolysis rates with chemical ionization mass spectrometry by minimizing the interferences of other types of chemical and/or physical reactions. The chamber photolysis rate constants vary depending not only on the molecular formulas of organic nitrates but also on the VOC type and oxidation condition. While the photolysis rate constants of 53.1% of the C₁₀ organic nitrates for which the rate constants were estimated are on the order of 1 × 10^-5 s^-1 or larger, the other C₁₀ organic nitrates exhibit little to no decrease in their measured signals. This highlights how differences in chemical structures can affect the photolability of organic nitrates. The most photolabile organic nitrate is C₁₀H₁₇NO₅ (either hydroxy carbonyl nitrate or hydroperoxy nitrate) formed from nitrate radical oxidation of α-pinene and β-pinene, with the chamber photolysis rate constant of 1.1 (±0.1) and 1.3 (±0.3) × 10^-4 s^-1, respectively. When extrapolated to ambient conditions (solar zenith angle of 28.14° in summer), the photolysis rate constant is as large as 6.4 (±3.0) × 10^-4 s^-1 (corresponding to a photolysis lifetime of 0.43 ± 0.20 h). Compared to other loss processes (i.e., OH oxidation and dry deposition) of gaseous organic nitrates formed from nitrate radical oxidation of β-pinene, photolysis serves as either a comparable or dominant sink depending on the molecular formulas of organic nitrates. These findings have important atmospheric implications regarding the role of monoterpene organic nitrates in the spatial distribution of NO _<i>x</i> and O₃ formation.