Rate constants for the reaction between OH and CH3ONO2 and C2H5ONO2 over a range of pressure and temperature

Abstract

A discharge-flow apparatus equipped with resonance-fluorescence detection for OH radicals has been used to measure rate coefficients for the two reactions over the pressure range 1–20 Torr at T=298 K and ca. 1–3 Torr over the temperature range 300–400 K. The rate constants, k ₁ and k ₂ , were found to be invariant with pressure, but were found to increase with increasing temperature. In light of these findings, we conclude that at low pressure the reaction mechanism is consistent with an abstraction process. The rate constants derived are k ₁ =(4.1±0.8)×10 ^-13 exp(-604±121/T) cm ³ molecule ^-1 s ^-1 and k ₂ =(3.30±0.66)×10 ^-12 exp(-699±140/T) cm ³ molecule ^-1 s ^-1 . The existence of pressure-dependent (association) channels for reactions (1) and (2) is briefly discussed. Model calculations are presented that imply that photolysis, and not reaction with the OH radical, is the dominant loss process in the atmosphere for CH ₃ ONO ₂ and C ₂ H ₅ ONO ₂ . Lifetimes of CH ₃ ONO ₂ and C ₂ H ₅ ONO ₂ are derived for the lower atmosphere; they vary from a few years to several days depending upon season and location. Possible sources of these alkyl nitrates in the atmosphere are discussed.