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Published January 8, 2004 | public
Journal Article

Measurements of the Rate Constant of HO_2 + NO_2 + N_2 → HO_2NO_2 + N_2 Using Near-Infrared Wavelength-Modulation Spectroscopy and UV−Visible Absorption Spectroscopy


Rate coefficients for the reaction HO_2 + NO_2 + N_2 → HO_2NO_2 + N_2 (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 k_o = (2.1 ± 0.1) × 10^(-31) × (T/300)^(-(3.1±0.3)) cm^6 molecule^(-2) s^(-1) and k∞ = (2.4 ± 0.1) × 10^(-12) × (T/300))^(-(1.9±0.5)) cm^3 molecule^(-1) s^(-1), using the expressions for rate constants adopted by the NASA data evaluation panel (F_c = 0.6). The reaction rate was significantly enhanced in the presence of methanol due to a chaperone effect involving an HO_2·CH_3OH complex. Enhancement parameters for this process were quantified as a function of temperature. During the course of our studies, we observed an unexpected time-dependent UV absorption unaccounted for in previous examinations of reaction 1 that employed UV spectroscopy to monitor HO_2. We show that this absorption, which may have led to errors in those prior studies, is due to the process NO_2 + NO_2 ⇄ N_2O_4 (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 NO_2. This represents the first measurement of k_(-3) at T < 250 K.

Additional Information

© 2004 American Chemical Society. Received: July 2, 2003; In Final Form: October 21, 2003. Publication Date (Web): December 5, 2003. This research was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This work was supported by the NASA Upper Atmosphere Research and Tropospheric Chemistry Programs and the NASA Graduate Student Researcher Program (GSRP). This research has also been supported in part by a grant from the U.S. Environmental Protection Agency National Center for Environmental Research's Science to Achieve Results (STAR) program, through Grant R826236-01-0. It has not been subjected to any EPA review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. We acknowledge the scientific and technical support of Barna László, Dave Natzic, Jürgen Linke, Siamak Forouhar, Dave Dougherty, and Sam Keo of the Jet Propulsion Laboratory.

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