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

MASAYUKI TAKEUCHI, Yuchen Wang, Nga Lee Ng, Georgia Institute of Technology

     Abstract Number: 374
     Working Group: Aerosol Chemistry

Abstract
Photolysis of gaseous organic nitrates plays an important role in the formation and fate of air pollutants, such as nitrogen oxides (NO_x) and ozone (O₃). Monoterpenes are prevalent biogenic volatile organic compounds, contributing to the formation of organic nitrates; however, there is currently a lack of experimental constraint on the photolysis chemistry of monoterpene nitrates. Here, we conducted laboratory chamber experiments to investigate photolysis of organic nitrates formed from hydroxyl and nitrate radical oxidations of α-pinene and β-pinene. We directly measured their photolysis rates by chemical ionization mass spectrometry (CIMS). The chamber photolysis rate constants vary depending not only on the molecular formulae of organic nitrates but also on the precursor type and oxidation condition. While the photolysis rate constants of many detected C10 organic nitrates are on the order of 10^-5 s^-1 or larger, the other C₁₀ organic nitrates exhibit little to no decay behavior. The most photo-labile organic nitrate is C₁₀H₁₇NO₅ formed from nitrate radical oxidation of α-pinene and β-pinene with a chamber photolysis rate constant of 1.1 (± 0.1) and 1.3 (± 0.3) × 10^-4 s^-1, respectively. We further determined that the ambient photolysis rate constant is as large as 6.4 (± 3.0) × 10^-4 s^-1 (corresponding photolysis lifetime of 0.43 ± 0.20 h). Compared to other loss processes (i.e., photooxidation and dry deposition) of gaseous organic nitrates, photolysis is a comparable or dominant loss process. These findings have important atmospheric implications regarding the role of monoterpene nitrates on the spatial distribution of NO_x and O₃ formation.