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Measurements of the Rate Constant of HO2 + NO2 + N2 f HO2NO2 + N2 Using...

Christensen, L., M. Okumura, S. P. Sander, R. R. Friedl, C. E. Miller, and J. J. Sloan (2004), Measurements of the Rate Constant of HO2 + NO2 + N2 f HO2NO2 + N2 Using Near-Infrared Wavelength-Modulation Spectroscopy and UV-Visible Absorption Spectroscopy, J. Phys. Chem. A, 108, 80-91, doi:10.1021/jp035905o.

Rate coefficients for the reaction HO2 + NO2 + N2 f HO2NO2 + N2 (reaction 1) were measured using simultaneous near-IR and UV spectroscopy from 220 to 298 K and from 45 to 200 Torr. Using the data acquired in the present experiment, the low-pressure and high-pressure limit rate constants for reaction 1 were determined to be ko ) (2.1 ( 0.1) × 10-31 × (T/300)-(3.1(0.3) cm6 molecule-2 s-1 and k∞ ) (2.4 ( 0.1) × 10-12 × (T/300))-(1.9(0.5) cm3 molecule-1 s-1, using the expressions for rate constants adopted by the NASA data evaluation panel (Fc ) 0.6). The reaction rate was significantly enhanced in the presence of methanol due to a chaperone effect involving an HO2‚CH3OH complex. Enhancement parameters for this process were quantified as a function of temperature. During the course of our studies, we observed an unexpected timedependent UV absorption unaccounted for in previous examinations of reaction 1 that employed UV spectroscopy to monitor HO2. We show that this absorption, which may have led to errors in those prior studies, is due to the process NO2 + NO2 a N2O4 (reaction 3). Using UV-visible spectroscopy, we determine k-3 to be (36 ( 10) s-1 at 231 K and 100 Torr using the NASA-recommended equilibrium constant for the dimerization of NO2. This represents the first measurement of k-3 at T < 250 K.

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Tropospheric Composition Program (TCP)
Upper Atmosphere Research Program (UARP)