RATE CONSTANTS FOR THE REACTION BETWEEN OH AND CH3ONO2 AND C2H5ONO2 OVER A RANGE OF PRESSURE AND TEMPERATURE

A discharge-flow apparatus equipped with resonance-fluorescence detect ion for OH radicals has been used to measure rate coefficients for the two reactions OH + CH3ONO2 --> products (1) OH + C2H5ONO2 --> product s (2) 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 increas e with increasing temperature. In light of these findings, we conclude that at low pressure the reaction mechanism is consistent with an abs traction process. The rate constants derived are k(1) = (4.1 +/- 0.8) x 10(-13) exp(-604 +/- 121/T) cm(3) molecule(-1)s(-1) and k(2) = (3.30 +/- 0.56) x 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 d ominant loss process in the atmosphere for CH3ONO2 and C2H5ONO2. Lifet imes of CH3ONO2 and C2H5ONO2 are derived for the lower atmosphere; the y vary from a few years to several days depending upon season and loca tion. Possible sources of these alkyl nitrates in the atmosphere are d iscussed.