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The reactions at 298 K of gaseous N205 with NaBr(s) or with BrNO(g) in 1 atm of helium were followed by using Fourier transform infrared spectroscopy. In both cases, the formation of infrared absorption bands at 787, 1292, and approximately 1660 cm"1 11, which could not be assigned to known reactant or product species, was observed. Based in part on published low-temperature matrix spectra, we assign these bands to gaseous nitryl bromide (BrN02). This appears to be the first gas-phase infrared spectroscopic observation of this nitryl halide.

The absorption cross sections for gaseous ClNO2 in the 200–370 nm region and for Cl2 in the 270–400 nm region have been determined at 298 K. The cross sections for Cl2 are in excellent agreement with the literature. At λ < 300 nm the absorption cross sections for ClNO2 are in good agreement with those of lilies and Takacs (1976/1977) and Nelson and Johnston (1981) but are higher than those of Martin and Gareis (1956) from 220 to 240 nm. In the tropospherically important region beyond 290 nm our results, which have been corrected for impurity Cl2 using a combination of mass spectrometry and UV absorption, fall between those of Martin and Gareis (1956) and lilies and Takacs (1976/1977) and are in excellent agreement with the values of Nelson and Johnston (1981). Estimates of the photolysis rate constants and photolytic lifetimes for ClNO2 for various solar zenith angles at the Earth's surface are reported. It is shown that atomic chlorine from ClNO2 photolysis may be a significant initiator of organic photooxidation in both moderately polluted and remote marine atmospheres.

A new dark source of O H for relative rate studies is described. This technique, which is simple to apply and yields results of high precision, is applicable to organics which react very slowly with 0 3 . This method involves measuring the relative rates of decay of pairs of simple alkanes (ethane, propane, n-butane, isobutane, and n-hexane) or toluene in the presence of relatively high concentrations of 0 3 / 0 2 (4-36 Torr 03) at 298 K and 1 atm total pressure in the dark. The conclusion that O H is the reactant is reached by comparison of the measured relative rates of decay of the organics to literature values for the O H reactions and to experiments reported here in which known photolytic sources of OH, specifically O3-H2O or CH3ONO-NO-air, were used.The same technique was applied to measure the temperature dependence of the relative rate constants for the ethane/propane reactions from 298 to 373 K. The value of (El -E2)/R, where El is the activation energy for the OH-ethane reaction and E2 that for the OH-propane reaction, was found to be (353 f 36) K (f2a), compared to a literature value2J2 of (380 f 180) K based on the absolute rate constants. Similarly, our ratio of the preexponential factors, A I / & , was (0.82 f 0.09), compared to 0.80 from recent literature evaluations.2J2 Thus this dark reaction provides a new means of measuring relative rate constants of O H reactions for those organics which do not themselves react with 0 3 at a significant rate. From the absolute rates of decay of the organics, the average concentration of O H generated in these dark reactions was found to be comparable at 298 K to that from the photolysis of CH30NO-NO-air mixtures, which is approximately 2 orders of magnitude less than during photolysis of the O3-H2O mixtures at 254 nm. In the dark system, the average O H concentration increases with the pressure of 0 3 and with the temperature. Potential mechanisms of formation of O H in this system arediscussed. This technique could prove to be particularly useful for studying the kinetics of compounds which themselves photolyze and hence for which application of photolytic sources of OH is not appropriate.

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