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The reaction of the hydroxyl radical (HO) with the stable carbon isotopes of methane has been studied as a function of temperature from 273 to 353 K. The measured ratio of the rate coefficients for reaction with 12CH4 relative to 13CH4 (k12/k13) was 1.0054 (±0.0009 at the 95% confidence interval), independent of temperature within the precision of the measurement, over the range studied. The precision of the present value is much improved over that of previous studies, and this result provides important constraints on the current understanding of the cycling of methane through the atmosphere through the use of carbon isotope measurements.

Absolute rate constants and their temperature dependencies for the deactivation of O(1D) by five important atmospheric gases are reported. O(1D) atoms are produced by photolysis of ozone at 266 nm and the atoms are directly observed in time resolved decay of the O(1D) →O(3P) radiation at 630 nm. Gases which only quench O(1D) (O2, N2, and CO2) are observed to have a small negative temperature dependence while O3 and H2O, which also have a reactive channel, display no temperature dependence. Arrhenius expressions for the reactions measured are (A in units of 10−10 cm3/molecule⋅ s, E in cal/mole) O2(T=104–354 K) 0.29 exp(134/RT), N2(T=104–354 K) 0.20 exp(214/RT), CO2(T=139–200 K) 1.2 and (T=200–354 K) 0.68 exp(233/RT). The rate constants for O3 and H2O are 2.4×10−10 and 2.3×10−10 cm3/molecule⋅ s over ranges of 103–393 K and 253–353 K, respectively. The results are compared with other energy dependent measurements and with the theories reported in the literature.

The equilibrium constant for the reaction kl (1) NOS-+ HBr 2 Br-+ HNO, has been determined between 331 and 480'K using a variable-temperature flowing afterglow. These data give AH"(1) = -1.03 f 0.21 kcal/mol and AS"(1) = -4.6 f 1.0 cal/mol.'K. When combined with the known thermochemical values for HBr, Br-, and HNO,, this yields AH&,(NO,-) = -74.81 f 0.54 kcal/mol and S&(NO,-) = 59.4 cal/mol-'K. In addition AH,-1., and AS,-1,, for the gas-phase reactions NOS-. (HN03)n-i + HNO, --+ NOS-. (HNOa),were determined for n = 2 and 3. negative ion chemistry are discussed.The implications of these measurements to gas-phase

A redetermination of the temperature dependence of the absorption cross section (a) of NO 2 in the visible-ultraviolet region (264-649 nm) has been made in order to provide a more reliable data base for the calculation of NO,_ photolysis rates in the atmosphere. Experiments over a wide range of temperatures (--40.6 ø to 124øC) and NO 2 concentrations (3.4 x 10•3-7.3 x 10 • molecules ½m -3) were made at relatively low and high spectral resolutions, employing diode array and Fourier transform spectrometers, respectively. There are three aspects of the temperature dependence which were noted: (1) the integral of a plot of a versus 1/4 (264-649 nm) was essentially independent of temperature; (2) increasing temperature produced a shift of the spectrum toward longer wavelengths, resulting in a small negative temperature dependence of • over the 264-to 400-nm range and a small positive dependence over the 450-to 649-nm range; (3) increasing temperature produced broadening of individual spectral features, resulting in a systematic lowering of peaks and filling of valleys. Recommended cross sections, averaged over 5-nm wavelength intervals, are presented for use in tropospheric NO 2 photolysis rate calculations. EXPERIMENTAL METHODS Many of the experiments described here were carried out in a long-path, temperature-regulated, stainless steel cell, which has been described in detail previously [-Shetter et al., 1987]. The cell is constructed of three concentric tubes, the innermost of which (15.8-cm ID) houses multiple-reflection optics. In all of these studies the adjustable optical path was set at 48.6 m. The inner space between the tubes contains temperatureregulating fluid which is circulated from a temperaturecontrolled bath. The outer space between the tubes serves as an evacuable dewar. In all of the low-resolution studies re-7105