Temperature Dependent Rate Coefficient for the Reaction O( 3 P) + NO 2 f NO + O 2 Tomasz Gierczak, † James B. Burkholder, and A. R. Ravishankara* ,‡ National Oceanic and Atmospheric Administration, Aeronomy Laboratory, 325 Broadway, Boulder, Colorado 80303, and CooperatiVe Institute for Research in EnVironmental Sciences, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: October 5, 1998; In Final Form: December 8, 1998 The rate coefficient (k 1 ) for the reaction O( 3 P) + NO 2 f O 2 + NO was measured under pseudo-first-order conditions in O( 3 P) atom concentration over the temperature range 220-412 K. Measurements were made using pulsed laser photolysis of NO 2 to produce oxygen atoms and time-resolved vacuum UV resonance fluorescence detection of O atoms. The NO 2 concentration was measured using three techniques: flow rate, UV absorption, and chemical titration (NO + O 3 f NO 2 + O 2 ). The NO 2 UV absorption cross section at 413.4 nm was determined as a function of temperature using the chemical titration and flow methods. Including the low-temperature data of Harder et al. 1 , the temperature-dependent NO 2 cross section is given by σ 413.4 (T) ) (9.49 - 0.00549 T) × 10 -19 cm 2 molecule -1 . The measured rate coefficients for reaction 1 can be expressed as k 1 (T) ) (5.26 ( 0.60) × 10 -12 exp[(209 ( 35)/T] cm 3 molecule -1 s -1 , where the quoted uncertainties are 2σ and include estimated systematic errors. This result is compared with previously reported measurements of k 1 . Introduction Nitrogen oxides, NO and NO 2 (collectively called NO x ), play a crucial role in atmospheric ozone chemistry: they lead to photochemical ozone production in the troposphere and catalytic ozone destruction in the stratosphere. In the stratosphere, NO x chemistry affects both the ozone abundance and its vertical profile. Of the many possible catalytic ozone destruction cycles involving NOx, the following is the most important: net: Atmospheric model calculations of ozone abundances and vertical profiles rely on the temperature-dependent rate coef- ficients for reactions 1 and 2. Reaction 1 is the rate-limiting step in this catalytic cycle and has been studied many times over the past few decades. However, a careful examination of the available data shows that there are significant discrepancies and that a more accurate rate coefficient would be beneficial. Current recommendations 2,3 for reaction 1 give k 1 (T) ) 6.5 × 10 -12 exp(120/T) cm 3 molecule -1 s -1 and are based on the studies of Davis et al., 4 Slanger et al., 5 Bemand et al., 6 Ongstad and Birks, 7 and Geers-Muller and Stuhl. 8 Other earlier studies, which yielded lower values of k 1 (298 K) and positive activation energies, have not been included in deriving the recommenda- tions. The values of k 1 (298 K) reported in the above five studies agree within 10%. However, the temperature dependence of k 1 from these studies disagree significantly; the activation energies reported from various groups fall in the range 0 to ∼-400 cal mol -1 . The recommended value of the activation energy, 240 ( 240 cal mol -1 , has been derived from the studies of Davis et al., Ongstad and Birks, and Geers-Muller and Stuhl. The current recommendations suggest an uncertainty of ∼60% in the value of k 1 (200 K); this large range for k 1 (200 K) is mostly due to the uncertainty in the activation energy. This level of uncertainty has significant implications in the interpretation of atmospheric measurements of trace species and model calculated abundances and trends of ozone. The rate coefficient has been identified as a major source of uncertainty in stratospheric models (see, for example, ref 9) Here, we report the temperature dependence of k 1 measured using the technique of pulsed laser photolysis with resonance fluorescence detection of O( 3 P) atoms (PP-RF). During these experiments, special emphasis was placed on the determination of NO 2 concentration and measurements of k 1 at stratospheric temperatures. Our results are compared with previous measure- ments and a new value for stratospheric modeling is suggested. Experimental Section The accuracy of the value of k 1 , determined in a system where the temporal profile of O( 3 P) atoms are measured under pseudo- first-order conditions, depends on how well the concentration of NO 2 is known. Even though NO 2 is a stable gas, there are a few difficulties associated with its handling and knowing its concentration accurately. First, NO 2 can react on the walls of the reactor, thermally decompose (e.g., in electronic flow meters), and be photolyzed by room light. Second, it can undergo self-association † Permanent address: Department of Chemistry, Warsaw University, ul. Zwirki i Wigury 101, 02-089 Warszawa, Poland. * Address correspondence to this author at NOAA/ERL, R/E/AL2, 325 Broadway, Boulder, CO 80303. E-mail: ravi@al.noaa.gov. ‡ Also affiliated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309. O( 3 P) + NO 2 f O 2 + NO (1) O 3 + NO f O 2 + NO 2 (2) O( 3 P) + O 3 f 2O 2 (3) 2NO 2 a N 2 O 4 (4) 877 J. Phys. Chem. A 1999, 103, 877-883 10.1021/jp983962p CCC: $18.00 © 1999 American Chemical Society Published on Web 02/03/1999