Rate coefficients for the IO self-reaction, IO + IO f products (4), were determined over the range 250-320 K and were found to be independent of temperature with an average value of (9.9 ( 1.5) × 10 -11 cm 3 molecule -1 s -1 . Absorption cross sections for the IO radical were measured between 340 and 480 nm. The absorption cross section at the peak of the (4,0) band was determined as a function of temperature. The cross section is essentially independent of temperature with an average value between 373 and 203 K of (3.6 ( 0.5) × 10 -17 cm 2 molecule -1 . Comparison with previous measurements and a discussion of the atmospheric implications of these results are also presented.

The flash photolysis/UV absorption technique has been used to study the self-reaction of BrO radicals over the temperature range 222-298 K and the pressure range 100-760 Torr of N 2 or O 2 . Two chemical sources of BrO radicals were used: photolysis of Br 2 in the presence of excess ozone and photolysis of O 2 in the presence of excess Br 2 . The overall rate constant, k 1 , for the BrO self-reaction (defined by -d[BrO]/dt ) 2k 1 [BrO] 2 ) was found to be temperature and pressure independent at T g 250 K, with k 1 ) (2.88 ( 0.20) × 10 -12 cm 3 molecule -1 s -1 . At temperatures below 250 K, k 1 was found to be pressure dependent, due to the emergence of a new termolecular channel of the BrO self-reaction 1c, -1c forming the BrO dimer, Br 2 O 2 (BrO + BrO + M h Br 2 O 2 + M). Channel-specific rate constants were determined for the two bimolecular channels of the BrO self-reaction above 250 K, giving for (1a) (BrO + BrO f 2Br + O 2 ) k 1a ) (5.31 ( 1.17) × 10 -12 exp{(-211 ( 59)/T} cm 3 molecule -1 s -1 and for (1b) (BrO + BrO f Br 2 + O 2 ) k 1b ) (1.13 ( 0.47) × 10 -14 exp{(983 ( 111)/T} cm 3 molecule -1 s -1 . Below 250 K, the overall rate coefficient of the two bimolecular channels is reduced as the dimer forming channel emerges. At 235 and 222 K, rate constants for the formation (k 1c ) and decomposition (k -1c ) of Br 2 O 2 were recorded. Using the values for K 1c , ∆H r for reaction 1c was estimated as -58.6 ( 0.1 kJ mol -1 . A UV absorption spectrum attributed to Br 2 O 2 was also recorded over the wavelength range 300-390 nm. The cross section of the smooth Br 2 O 2 spectrum was found to be 1.2 × 10 -17 cm 2 molecule -1 at 320 nm. These results are rationalized in terms of a mechanism of the BrO self-reaction that shows competition, at low temperatures, between collisional quenching and unimolecular dissociation of an excited BrOOBr* intermediate. The rate constant for the reaction of oxygen atoms with molecular bromine was also determined in the course of these experiments [O + Br 2 f BrO + Br (5)], giving k 5 ) (5.12 ( 1.86) × 10 -13 exp{(989 ( 91)/T} cm 3 molecule -1 s -1 . All errors are 1σ. The atmospheric implications of these results are discussed.

Quantum yields for NO 3 production in the photolysis of BrONO 2 and N 2 O 5 were measured at 248, 308, and 352.5 nm. The measured values for BrONO 2 were found to be independent of pressure over the range 150-600 Torr and bath gas (N 2 or O 2 ) and are 0.28 ( 0.09, 1.01 ( 0.35, and 0.92 ( 0.43 at 248, 308, and 352.5 nm, respectively. Quantum yields of Br and BrO in the photolysis of BrONO 2 were also estimated. The measured values for NO 3 production in the photolysis of N 2 O 5 were 0.64 ( 0.10, 0.96 ( 0.15, and 1.03 ( 0.15 at 248, 308, and 352.5 nm, respectively. Rate coefficients for the reactions Br + BrONO 2 f Br 2 + NO 3 (18) and Cl + BrONO 2 f ClBr + NO 3 (19) were measured at 298 K to be k 18 ) (6.7 ( 0.7) × 10 -11 cm 3 molecule -1 s -1 and k 19 ) (1.27 ( 0.16) × 10 -10 cm 3 molecule -1 s -1 . The NO 3 product yields for reactions 18 and 19 were measured to be 0.88 ( 0.08 and 1.04 ( 0.24, respectively. The absorption cross sections for N 2 O 5 between 208 and 398 nm are also reported. All quoted uncertainties are 2σ and include estimated systematic errors. On the basis of the measured quantum yields of NO 3 , the atmospheric photolysis rate of BrONO 2 is discussed.

A study of the temperature dependence of the ultraviolet‐visible absorption cross sections for NO2 has been made in the temperature range 213–298 K and between 310 and 570 nm using a diode array spectrometer. Analysis of the experimental data allowed the simultaneous measurement of the NO2 and N2O4 cross sections and the equilibrium constant for the dimerization of NO2. The absorption cross sections were measured at a spectral resolution of 0.54 nm full width at half maximum (FWHM) and are presented for use in the analysis of differential absorption spectra of atmospheric NO2. Averaged cross section data for NO2 are presented for use in photolysis rate calculations. Values of cross sections for N2O4 and equilibrium constant for the association of NO2 are presented in the range 213 to 263 K. No temperature effect was observed on the overall shape of the NO2 absorption spectrum or on the averaged cross section values used for calculating atmospheric photolysis rates. Changes in the fine structure of the NO2 spectrum with temperature are observed and this has important implications for measurements of atmospheric NO2 made by ultraviolet‐visible absorption techniques.