Envlron. Scl. Technol. 1982, 16, 844-846
Rates and Temperature Dependences of the Reaction of OH with Isoprene, Its Oxidation Products, and Selected Terpenes Tadeusz E. Kieindienst, Geoffrey W. Harris, and James N. Pitts, Jr.'
Statewide Air Pollution Research Center and Department of Chemistry, University of California, Riverside, California 92521 Absolute rate constants determined by using the flash photolysis-resonance fluorescence technique are reported for the reactions of hydroxyl radicals with isoprene, a-and &pinene, methyl vinyl ketone, and methacrolein in the temperature range 297-424 K, and with methylglyoxal at 297 K. These results contribute to a more quantitative understanding of the tropospheric fate of gas-phase biomass-related organics and serve as input to models of the chemistry of the natural troposphere. Introduction Recently there has been considerable discussion as to the importance of naturally emitted organics, especially hydrocarbons emitted by biomass, in three distinct but related areas of atmospheric chemistry: first, as an effective source of CO that may rival in magnitude CO production from the methane oxidation cycle (I);second, as a source of ozone in rural and background air of possibly comparable strength to the intrusion of stratospheric ozone (2);third, as a source of reactive hydrocarbons and thus of oxidant formation in urban and suburban environments (2) where anthropogenic hydrocarbon emissions have been thought to dominate. Detailed evaluation of the importance of the role of natural hydrocarbons in these and other aspects of atmospheric chemistry requires the development of experimentally validated chemical models of the atmospheric reactions of representative natural organics, as well as a wider data base on their emission rates and integrated source strengths (1,2). The products from the NO,-air photooxidation of isoprene, via irradiation either of HONO-NO-isoprene& (3)or of NO,-isoprene-air ( 4 , 5 ) mixtures, have recently been investigated. The a,j3-unsaturated carbonyls methyl vinyl ketone and methacrolein, along with formaldehyde, were observed to be the major initial products and are consistent with those postulated by Zimmerman et al. (I), who also proposed the formation of methylglyoxal from the photodegradation of methyl vinyl ketone. As part of a larger study involving laboratory and field experiments dealing with the role of natural hydrocarbons in the polluted and unpolluted troposphere, we have determined the absolute rate constants for the reactions of OH radicals with the representative natural hydrocarbons isoprene and a- and 8-pinene over the temperature range 297-423 K. Since the OH isoprene system has received particular attention, rate constants for the reaction of OH with methyl vinyl ketone, methacrolein, and methylglyoxal have also been measured.
+
Experimental Section The flash photolysis-resonance fluorescence apparatus has been described previously (6),and hence only a brief review will be given here. A Pyrex reaction cell was enclosed in a furnace that could be held to a constant temperature of f l K over the range 295-425 K. The gas temperature inside the vessel was measured by using a Chromel/Alumel thermocouple. Experiments were carried out under slow-flow conditions so that the gas mixture in the cell was replenished every 844
Environ. Sci. Technol., Vol. 16, No. 12, 1982
few flashes. Water vapor, argon buffer gas, and the reactant were metered through calibrated flow meters and premixed prior to entering the reaction vessel where hydroxyl radicals were produced by the flash photolysis of water vapor in the wavelength region above -115 nm (the MgF2 cutoff). The concentrations of OH radicals were monitored as a function of time after the flash by resonance fluorescence-photon counting as described previously (6). Isoprene was obtained as a 0.025% nominal mixture in argon (Liquid Carbonic Corp.) and its concentration determined by gas chromatography with flame ionization detection using a 20 f t X 1/8 in. stainless steel column of 5% DC703/C20M on 100/120 mesh AW, DMCS Chromosorb G, operated at 333 K. Methyl vinyl ketone, methacrolein, a-pinene and @-pinene(Aldrich Chemical Co., Research Grade) were admitted into the system by saturating a known fraction of the total flow of argon with reactant vapor at subambient temperatures obtained by using the appropriate slush baths. The partial pressure of these reactants in the argon flow was determined by ultraviolet absorption using a Cary 15 spectrophotometer, which was calibrated by using known pressures of the pure vapors as determined by an MKS Baratron capacitance manometer. Methylglyoxal,obtained from Aldrich as a 40% aqueous solution, was dried prior to use as follows: approximately 50 cms of the 40% solution was pumped on for -24 h. The dry but polymerized methylglyoxal was then gently pyrolyzed in the presence of P205,and the gaseous monomer that evolved was passed through an additional P205 trap and collected at dry ice-acetone temperature. Immediately following collection, a known pressure of methylglyoxal was expanded into an evacuated, darkened 12-L bulb, which was then filled to 760 torr with Ar (299.998% purity). The known methylglyoxal-argon mixture was admitted into the reaction vessel through a calibrated capillary flow meter. Results The reactions of OH radicals with isoprene, methyl vinyl ketone, methacrolein, a-pinene, and @-pinenewere studied over the temperature range 297-424 K, while the reaction of OH radicals with methylgyoxal was studied solely at 297 K. Most experiments were conducted at a total pressure of 50 torr of Ar, but for each compound an OH radical decay rate was also measured at a total pressure of 200 torr to assure that the high-pressure limit had been reached. In all cases, no effect of varying the total pressure between these limits was observed. The reactions were carried out under conditions of pseudo-first-order kinetics. In such cases, the integrated rate law for OH reduces to the form [OHI,/[OHl, = &/So = exp -[(k,
+ kl[reactant])(t - to)] (I)
where [OH], and [OH],, S , and So are the OH radical concentrations and resonance fluorescence signals at times t and to, respectively, Itl is the rate constant for the disappearance of OH due to reaction with the substrate and
0013-936X/82/0916-0844$01.25/0
0 1982 American Chemical Society
4001
2 9 9 y
f
349K
::I 300
[CH3COHCO] molecules ~ r n - ~
Figure 2. Pseudo-firstorder hydroxyl decay rates due to the reaction with methylglyoxal at 300 f 2 K. The filled points were obtained at 200-torr total pressure.
I
2
3
4
5
6x10"
[C5H8] molecules cm-3
Flgure 1. Pseudo-firstorder hydroxyl decay rates due to reaction wlth isoprene at three temperatures. The filled point at 299 K was obtained at 200-torr total pressure.
Table I. Rate Constants for the Reaction of OH Radicals with Isoprene, with Its Oxidation Products. and with the Pinenes 10l1h1,cm3 temp, K molecule-' s-' reactant 9.26 f 1.5 isoprene 299 methyl vinyl ketone methy lglyoxal methacrolein Q!
-pinene
p-pinene
349 422 298 350 4 24 297 300 350 423 298 349 422 297 350 423
1.64 f 6.21 f 1.79 1.35 f 1.14 f 0.71 f 3.14 f 2.99 f 2.65 f 6.01 f 5.10 f 3.88 5 7.76 6.78 f 5.42 f
*
*
1.2 0.82 0.28 0.24 0.21 0.16 0.49 0.48 0.39 0.82 0.69 0.57 1.1 1.1 1.0
ko is the rate of disappearance of OH radicals by all processes other than reaction with the compound of interest.
ko was typically much smaller than IzJreactant] and was observed to be independent of the nature of the reactant. Hydroxyl radical decays were measured over 2-3 halflives, and except in certain experiments involving methylglyoxal (see below), the OH radical concentration decayed exponentially. From each decay the OH radical decay rate R, given by R = (t - to)-l In (So/S,) was determined for the measured concentration of reactant. At each temperature, a plot of R vs. reactant concentration proved to be linear over the whole concentration range. Figure 1 shows the OH decay rates as a function of isoprene concentration at three temperatures. The scatter in the data points shown in Figure 1 is representative of that for the other compounds (except methylglyoxal). Table I gives the measured rate constants for all reactants at each temperature studied. The error limits were calculated by combining the estimated uncertainties in the reaction concentrations (10%) and twice the standard deviation of the linear least-squares fit to data such as
5x10-12
24
28
32
IOOO/T(K)
Flgure 3. Arrhenius plots for the reactions of OH with isoprene, @-pinene, a-pinene, methacrolein, and methyl vlnyl ketone.
those shown in Figure 1. The uncertainty in the methylglyoxal concentration was estimated to be higher (20%) than the other reactants. For all the compounds studied, the flash energy was varied by at least a factor of 2 (e.g., from 60 to 120 J/flash) in order to investigate any effects of reactant photolysis and secondary reactions. No effect of varying the flash energy was observed for any compound except methylglyoxal. For the methylglyoxal system, the OH radical decay rates appeared to the nonexponential after -1 half-life when a flash energy of 60 J/flash was used and high concentrations of reactant were present. Presumably photolysis of the methylglyoxal during the flash resulted in complicating secondary reactions. At much lower flash energies (
