J . Phys. Chem. 1989, 93, 1091-1096
1091
Studies of NO3 Radical Reactions with Some Atmospheric Organic Compounds at Low Pressures Edward J. Dlugokencky and Carleton J. Howard* Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303, and Cooperative Institute for Research in Environmental Sciences and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado (Received: March 11, 1988)
Rate constants for the reactions of NO, with trans-2-butene (l), isoprene (2), a-pinene (3), and acetaldehyde (4) have been measured as a function of temperature at low pressures in a fast flow system with LIF detection of the NO, reactant and NO2 product. Rate constants for reaction 1 were found to be independent of pressure from 0.44 to 4.5 Torr, and the Arrhenius plot was curved. The data were fit by the four-parameter equation, k l ( T = 204-378 K) = (1.78 f 0.36) X exp[-(530 10O)/Tl + (1.28 I 0 . 2 6 ) X exp[(570 1lO)/Tl (where all the error limits are the 95% confidence levels including a factor for systematic error, and the units are cm3 molecule-Ls-l). The data for isoprene were fit by a normal Arrhenius equation, kz(T = 251-381 K) = (3.03 0.45) X exp[-(450 70)/Tl. The Arrhenius plots for a-pinene and acetaldehyde were linear, and the fits gave k3(T = 261-384 K) = (1.19 0.31) X exp[(490 7 0 ) / q and k4(T = 264-374 K) = (1.44 f 0.18) X exp[(-1860 f 300)/Tl. The efficiencies for the conversion of NO, to NO2were determined for reactions of trans-2-butene and isoprene at low pressures and room temperature in He, N2,and O2 carrier gases, and at 360 K in 1 Torr of He. The measured yields of NO2 decreased with increasing size of the organic reactant, with increasing pressure, with decreasing temperature, and with increasing deactivation efficiency of the carrier gas. These observations and the temperature dependencies of the rate constants indicate that reactions 1-3 proceed by way of an addition mechanism. From the analysis used to determine the NO2 yields, NO, fluorescence quenching rate constants were determined for isoprene, k = (3.5 f 1.1) X lO-'O, and trans-2-butene, k = (3.0 0.9) X 10-lo.
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Introduction The oxidation of natural and anthropogenic organic compounds has been recognized as an important atmospheric source of CO and other reactive intermediates such as H 0 2 and R0Z1 which ultimately produce ozone. Many kinetic and mechanistic studies of the oxidation of organics by OH and 0, have been but less is known about reactions with nitrate radical (NO3) as the oxidizing agent. The reactions of NO3 with atmospheric organic compounds can lead to formation of "0,: peroxyacyl nitrate (PAN),5 R02,536 and toxic compounds such as dinitrates6 Since NO3 is rapidly photolyzed by sunlight, it is present in appreciable quantities only at night. Reactions of organic compounds with NO3 can then provide a significant loss process for the organic species at night. Therefore, kinetics and product information about these reactions are important for atmospheric models. Several groups have made room temperature studies of the kinetics and mechanisms of oxidation by NO3,'-l6 but little is known about the primary products for reactions of NO3 with organic compounds. Most of the kinetic measurements are from indirect studies at room temperature and atmospheric pressure, and no studies of the temperature and pressure dependencies of the rate constants or the products have been reported. Using the fast flow system with LIF detection, we are able to detect NO2 and NO3 with excellent sensitivity and to study reactions as functions of temperaure and pressure in the low-pressure region. We report here the results of studies of NO, reactions with the four species: trans-2-butene (reaction l), isoprene (2), a-pinene (3), and C H 3 C H 0 (4). A comparison of our results with the earlier studies done at higher pressures will serve as a test of the applicability of our work to pressures as high as 1 atm. Experimental Section
The flow tube reactor with laser-induced fluorescence (LIF) detection of NO3 and NO2 has been described in detail," so only a brief description is given here. Temperature control of a 2.54 or a 4.13 cm i d . Pyrex flow tube was achieved by circulating either silicone oil ( T > 298 K) or ethanol ( T < 298 K) from a thermoregulated reservoir through
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a condenser-like jacket surrounding the reaction zone. The inside surfaces of the reactor were coated with a halocarbon wax to minimize wall effects. The loss of NO, on the halocarbon wax coated reactor walls was small, k, 2 1 s-l, at all temperatures. We further tested for wall effects in the reaction of NO, with a-pinene at room temperature by changing the surface area to volume ratio, Le., the diameter of the flow reactor by a factor of 1.6. Curved decay plots were observed for the NO3 reactions with isoprene and a-pinene at temperatures 1 2 5 0 K. This was taken to be evidence of wall effects; therefore data for these two reactants taken below 250 K were rejected. Nitrate radicals were produced in a source reactor by thermal decomposition of NzOs at ~ 4 0 K0 and -3 Torr pressure N205
+ He
-
NO3 + NOz + H e
(5)
where k5 = 50 s-l.18 (1) Zimmerman, P. R.; Chatfield, R. B.; Fishman, J.; Crutzen, P. J.; Hanst, P. L. Geophys Res. Lett. 1978, 5, 679. (2) Atkinson, R. Chem. Rev. 1986, 86, 69. (3) Atkinson, R.; Lloyd, A. C. J . Phys. Chem. Ref. Data 1984, 13, 315. (4) Atkinson, R.; Plum, C. N.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N. Jr. J . Phys. Chem. 1984, 88, 2361. ( 5 ) Cantrell, C. A.; Davidson, J. A.; Busarow, K. L.; Calvert, J. G. J. Geophys. Res. 1986, 91, 5347. (6) Bandow, H.; Okuda, M.; Akimoto, H. J . Phys. Chem. 1980.84, 3604. (7) Atkinson, R.; Aschmann, S. A.; Winer, A. M.; Carter, W. P. L. Environ. Sci. Technol. 1985, 19, 87. (8) Atkinson, R.; Plum, C. N.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N. Jr. J. Phvs. Chem. 1984. 88. 1210. (9) AtkiGon, R.; Aschrnann, S . A.; Winer, A. M.; Pitts, J. N. Jr. Environ. Sci. Technol. 1984, 18, 370. (IO) Atkinson, R.; Carter, W. P. L.; Plum, C. N.; Winer, A. M.; Pitts, J. N. Jr. Int. J. Chem. Kinet. 1984, 16, 887. (11) Atkinson, R.; Plum, C. N.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N. Jr. J . Phys. Chem. 1984,88, 2361. (12) Tuazon, E. C.; Sanhueza, E.; Atkinson, R.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N. Jr. J. Phys. Chem. 1984, 88, 3095. (13) Japar, S. M.; N i b , H. J . Phys. Chem. 1975, 79, 1629. (14) Morris, E. D.; Niki, H. J . Phys. Chem. 1974, 78, 1337. (15) Hoshino, M.; Ogata, T.; Akimoto, H.; Inoue, G.; Sakamaki, F.; Okuda, M. Chem. Lett. 1978, 1367. (16) Ravishankara, A. R.; Mauldin, R. L. 111 J . Phys. Chem. 1985, 89, 7144 ..
*Author to whom correspondence should be addressed at NOAA R/E/ AI-2, 325 Broadway, Boulder, CO 80303.
0022-3654/89/2093-1091$01.50/0
(17) Hammer, P. D.; Dlugokencky, E. J.; Howard, C. J. J. Phys. Chem. 1986, 90, 249 1.
0 1989 American Chemical Society
Dlugokencky and Howard
1092 The Journal of Physical Chemistry, Vol. 93, No. 3, 1989
The NO, fluorescence was excited with an Ar' laser pumped W) dye laser (-0.4 W) which was tuned to the peak of the strong NO, absorption feature at 662 nm and then detected by a cooled photomultiplier tube. The NO2 fluorescence was excited by using the Ar+ laser (A = 514 and 488 nm), and its signal was calibrated directly by adding measured flow rates of NO2. The NO, fluorescence signal was calibrated relative to NO2 by converting NO, to NO2 by the reaction NO NO, 2N02 (6) (-6
+
k6 = 3
X
-
10-l' cm3 molecule-'
s-l
(ref 17)
in excess NO, ([NO] N 1 X lo1, molecules cm-,). The detection limit was -1 X lo8 molecules cm-, for NO, and -8 X lo7 molecules cm-, for NO, with 15-s signal averaging. All kinetic measurements were made under pseudo-first-order conditions ([organic reactant]/[N03] > 100) which produced linear plots of In Zf(N03)vs reaction distance, z , Le., the distance between the tip of the moveable inlet and the detection region. The reaction distance is equivalent to reaction time, t = z/v,where u is the average flow velocity. In the kinetic studies, the [NO,], was in the range (0.6-3) X loio molecules cm-,, The use of such low radical concentrations precludes the possibility of any significant interference from secondary reactions. The concentrations of organic reactants were calculated from their flow rates, which were determined by measuring the rate of change in pressure in a known volume. trans-2-Butene, isoprene, and a-pinene were added to the flow tube through a moveable inlet. An inert carrier gas was added to the flow to prevent reactant condensation in the inlet. For slow reactions, the necessary addition of large reactant flows through the inlet produced a discontinuity in the flow velocity at the addition point. This problem was circumvented for the reaction of NO, with acetaldehyde by modifying the experimental arrangement so that NO, was added through the moveable inlet, and the acetaldehyde was added to the carrier gas through a fixed port. The NO, product yields were determined by reacting NO, to completion (>99.9%) with excess organic reactant and then measuring the product [NO,]. The large excess organic reactant concentration assured that there was no significant production of NO, by secondary reactions (
