Environ. Sci. Technol. 1985, 19, 151-155
Hydroxyl Radical Oxidation of Isoprene Chee-llang Gu, Carolyn M. Rynard, Dale G. Hendry,+and Theodore Mlll"
Physical Organic Chemistry Department, SRI International, Menlo Park, California 94025 We have used the low total pressure (10 torr) flowing gas system and the high total pressure (760 torr) CH30N0-NO photolysis system to study the reaction of OH with isoprene. Both systems give methyl vinyl ketone, methacrolein, and 3-methylfuran as the major products; 1,2addition is favored over 1,4-additionby a fador of 7. From the proposed mechanism and reaction pathways, we conclude that more than 2 mol of NO will be oxidized to NO2 for each mole of isoprene oxidized by OH. Direct reaction of isoprene with NOz was also measured.
Introduction Two general types of natural hydrocarbon emissions have been identified (1). The first type is low molecular weight hydrocarbons including ethylene, produced in minor amounts by decaying organic matter. The second and more important type is largely isoprene and a variety of terpenes emitted from living plants (2, 3). The significance of natural hydrocarbons as a source of ozone in the atmosphere is a subject of recent controversy (3-9). Arnts and Gay (5) have reported smog chamber results using isoprene and a number of terpenes as the reactive hydrocarbons. They find that these hydrocarbons are very reactive, with most of them having half-lives of about 1-2 h; however, little ozone was generated in these experiments. This is believed to be due in part to the rapid reaction of these compounds with ozone, as long as the hydrocarbons are present in significant amounts. Killus and Whitten (10) recently have established that outdoor smog chamber experiments of isoprene can be computer modeled with available kinetic data and known principles of structure-reactivity for hydrocarbons. Unlike Arnts and Gay (5),they believe that isoprene alone can yield as much as 50% of the peroxy radicals formed in ambient rural air even though isoprene might be only 6% of the ambient hydrocarbon level. With sufficient NO, all peroxy radicals will produce ozone; however, in typical rural environments NO, is low, and the efficiency of ozone production by this route is unknown. Our own analyses (6)of smog chamber experiments leads us to suggest that one must be extremely careful in extrapolating the smog chamber experiments to the environment primarily for three reasons. First, smog chambers favor reaction of organics with ozone because chamber experiments are typically run with higher than normal atmospheric concentrations of reactants and initially generate ozone faster than would be observed in the atmosphere. Consequently, smog chambers tend to overemphasize ozone chemistry at the expense of the free radical chemistry initiated by OH. Second, the higher initial concentrations of reactants and higher proportion of O3 to OH reactions in a smog chamber generate higher concentrations of aerosol precursors and ultimately a larger fraction of aerosol. The limited amount of data on the reaction of terpenes and ozone indicates that aerosols are formed in most cases (11)and that aerosol formation is concentration dependent (12). In smog chamber experiments, aerosol formation removes organic Deceased. 0013-936X/85/0919-0151$01.50/0
carbon from further reaction and thereby limits ozone formation. Thus, the hydrocarbons may appear to have less ability to form ozone in the smog chambers than in the environment. Third, the chamber experiments are short in duration and do not provide an adequate test of the ability of initially formed, less reactive, produds to generate ozone over a long period. In the environment, where aerosol loading is smaller and the time scale is longer, organics that form the aerosol particulate may revolatilize to provide hydrocarbons for the free radical reactions that lead to ozone formation. Thus, information from smog chambers regarding the chemistry of isoprene and terpene is limited, and conclusions may be misleading. We believe that the most effective way to understand the chemistry of these compounds is to study their reactions with OH and O3 individually rather than in smog chambers where the proportion of OH and O3 chemistries is uncontrolled. This paper reports the product analysis of OH reaction with isoprene. We have used two types of experimental systems to study the reaction: (1)a low total pressure (10 torr) flowing gas H-atom-NO2 system and (2) a high total pressure (760 torr) CH,ONO-NO photolysis system. We discuss the mechanism of product formation as well as the implication of the significance of the reaction to atmospheric chemistry.
Experimental Procedures Materials. Isoprene (Aldrich, gold label, 99+%) was distilled through a 75-cm Heli-Pak column before use. Bromoethane (Baker) was distilled through a 12-in. Vigreux column. Nitric oxide was purified by vacuum distillation before use. Nitrogen dioxide was supplied by Liquid Carbonic as a 0.698% custom mixture in helium, and the concentration was determined to be 0.58% by ultraviolet absorption. Oxygen, synthetic air, argon, and hydrogen were obtained from Liquid Carbonic. Synthesis of Methyl Nitrite (CH30NO). CH30N0 was prepared by following the procedure of Hortung and Eressley (13)and Taylor et al. (14) using NaN02 and 60% H2S04in MeOH. The trapped and dried CH30N0 was vacuum distilled and stored in the dark. The UV absorption cross section at 339 nm is 3.3 x cm2mol-'. Low-Pressure Flowing Gas System. The flowing gas system used in these studies was described elsewhere (15) and is shown in Figure 1. Total pressures in the system were 10 torr, and the temperature was 22 f 2 "C. Typically, a Datametrics Barocell Pressure Sensor was used to measure the pressure. Hydrogen was added to the system to 0.6 torr, and mixed with argon to total pressures of 2-3 torr. A scintillonics Model HV15A microwave generator produced a plasma and dissociated molecular hydrogen. The resulting hydrogen atoms reacted with 5-6 torr of 0.5% NO2 in the He to produce hydroxyl radicals and nitric oxide. The N02/He inlet consisted of a linearly adjustable diffuser that allowed variation of the position of hydroxyl radical generation relative to the inlet positions for other gases. Isoprene vapor was admitted from a gas sample bulb to 0.05 torr and mixed with 1.5 torr of oxygen. After a reaction was complete, the system was cleaned by rinsing
0 1985 American Chemical Society
Environ. Sci. Technoi., Vol. 19, No. 2, 1985
151
Table I. Products of the Reaction of Isoprene with OH at 10.0 torr and 15-20% Conversion (Averaged Results of 20 Reactions)a % of total reaction productsb
compound
+ Ar
Figure 1. Flowing gas system used in the experiments.
first with acetone, then with alcoholic KOH, and finally with distilled water. From the position of discharge to the point of addition of NOz, the walls of the system were coated with phosphoric acid (Baker, 85%) to minimize recombination of H. At the position of generation of OH radicals, the walls were coated with Halocarbon Series 6-00 wax. Reaction mixtures from the flow system were collected by drawing the gases through absorbent cartridges with a vacuum pump. Cartridges were prepared with 10 cm long by 0.4 cm i.d. Pyrex tubes and packed with glass wool and Poropak Q (50-80 or 100-200 mesh, Waters Associates) or Tenax GC (60-80 mesh, Applied Science Laboratories). The cartridges were spliced into the injector system of the gas chromatograph using a vacuum-tight gas sampling valve. Products were desorbed by rapidly heating the columns, which were flushed with carrier gas. Products were analyzed with a Varian 3700 gas chromatograph using a flame ionization detector, and the data were collected by a Varian CDS 111integrator. Reaction products and authentic samples were separated on a 15 f t by 1/8 in. 0.d. Poropak QS (80-100 mesh) column and on a 18 f t by l / , in. 0.d. 10% OV-17 column. Gas chromatography/mass spectrometry (GC/MS) was performed with similar columns on a LKB 9000 GC/MS using 70-V ionization. Atmospheric Pressure CH30NO-NO-Isoprene Photolysis Experiments. Experiments at 760 torr were performed by irradiating mixtures of NO, CH,ONO, and isoprene (16) in 760 torr of synthetic air in 5-L Pyrex reaction bulbs. Irradiations were performed with an Oriel Model 6153 2500-W mercury lamp adjusted to give light intensity equivalent to a rate of photolysis of NO2 of 0.32 min-' in N2 at the bulb center. Reaction mixtures were analyzed by gas chromatography using bromoethane as internal standard under the same conditions as used for low-pressure experiments. Before photolysis, the reaction bulb was rinsed first with HF solution and then with distilled water, and finally baked at 80 "C under 0.01 torr. A Bendix ozone monitor, 8002, and a Bendix NO-NOz-NO, analyzer, 8101-B, were used to minitor NO, and O3during the reaction. Dark Reaction between Isoprene and NOz. Known amounts of purified NOz and isoprene were injected into a 10-cm-pathUV cell at an initial concentration of (1.5-2.6) x 10l6 and (0.4-1.2) X 10le molecule/cm3 of NOz and isoprene, respectively. Synthetic air was used to pressurize the cell to 1atm, and loss of NO2 was followed spectrally at 400 nm. Results and Discussion In most experiments, the amounts of isoprene collected from the flow system were a few percent less than the amounts expected from the concentration of the starting material, even in the absence of reaction. This may be a consequence of unequal rates of evaporation of isoprene relative to bromoethane from the starting mixture or of reaction of isoprene with NOz. 152 Environ. Sci. Technol., Vol. 19, No. 2, 1985
methyl vinyl ketone (MVK) methacrolein (MA) 3-methylfuran (MF) formaldehyde (FA)
48.9
* 5.1
41.7 f 5.0
7.6 f 6.3 1.8 f 1.8
See Experimental Procedures for detailed flow system conditions; [isoprene] = 0.05 torr, [H2] = 0.03-0.3 torr, [NOz/HO] = 4.5-6.2 torr, and [O,]= 4.50 torr. All reactions were conducted at ambient temperature, 22 f 2 "C. bPercent based on moles of total uroducts. Table 11. Rate Constants for Relevant OH Reactions
compound bromoethane isoprene MVK MA MF
SAR kOH X 10'2,0 environmental lifetime,b cm3 mol-' s-' ti/z(OH), h-' 0.39 96' 17.gd 31.4d 130
258 1.1 3.7
1.5 0.77
"SAR (structure activity relationship) (17). [OH] = 1.9 X lo6 mol cm-3 in the atmosphere (18). 'Measured value (16). dMeasured value (19).
The flow and static systems used in this study provide two different ways to study HO-isoprene oxidation. The flow system produces HO by the sequence
microwave
Hz 2H* (1) H. + NO2 -* HO. + NO (2) If properly designed, the system can provide a clean source of OH and NO with few secondary reactions, but only at relatively low pressure and under slow-flow conditions where product analyses are difficult. The static system uses continuous photolysis of CH30N0 in 1atm. of air to form HO, but secondary reactions and aerosol formation complicate interpretation of the results and typically make product mass balances difficult to attain. We believe that use of both systems described here adds considerable confidence to our analyses and conclusions. A number of flow experiments were performed under the following conditions: [isoprene] = 0.05 torr, [H,] = 0.04 torr, [N02/He] = 4.5 torr, and discharge power 30 W; [Ar] was varied to keep the total pressure constant at 10.0 torr. Oxygen was varied from 0.5 to 2.5 torr. Over this small range, there was no observable change in the product yields or branching ratios. Products of the reaction of hydroxyl radical, OH, with isoprene at 10.0 torr are listed in Table I. Products were identified by GC/MS and by comparing the observed spectra with published mass spectra. The addition of extra NO (to 0.03 torr) did not change the products or their relative amounts appreciably. Although OH and NO are initially produced in equal amounts, reaction 2, OH is lost in wall and disproportionation reactions; thus, NO is already in excess, and added NO is not expected to have an effect. Table I1 summarizes estimated rate constants for reaction of OH with bromoethane, isoprene, and the observed oxidation products, on the basis of the Hendry structure-activity relationship (SAR) method (17). Bromoethane is only l/m as reactive as isoprene toward OH radical and thus should be unreactive and a suitable standard for use in these flow experiments.
T a b l e T I . Products of the Reaction of Isoprene with OH in Air at 30-40% Conversion and 760 torrn [isoprene], PPm 4.3 4.0 100
Scheme I. 1,a-Addition of HO to Isoprene
// t H02*
% productsb
gas Air 02 Air
MVK 32 34 43
MA
MF
% yieldC
54 50 46
14 16 11
44 f 12 40 52 10
See Experimental Procedures for detailed photolysis system; the initial concentration ratio for NO:CH30NO:isoprene:bromoethane was kept at 1:2.5:1:0.2.All reactions were conducted at ambient temperature, 22 & 3 "C. bPercent based on moles of total products. Yield based on moles of product per mole of isoprene consumed.
The material balance for the reaction of OH with isoprene was determined by measuring the amount of unreacted isoprene and collected products relative to the internal standard, bromoethane. Material balances obtained for isoprene oxidations in molar units indicate that methyl vinyl ketone (MVK), methacrolein (MA), and 3-methylfuran (MF) account for 72 f 10% (molar) of the total isoprene reacted. The mass balance is not as high as reported for oxidation of toluene or xylene (93%) (15). However, the primary products from isoprene are both very reactive toward OH (see Table 11) and are subject to wall interactions and aerosol formation. HO Oxidation of Isoprene at 760 torr. The experimental technique of Ch,ONO-N&isoprene photolysis was based on the modified procedure of Atkinson et al. (16). OH radical was generated by irradiating CH30N0 at a290 nm in the presence of air. CH30N0 2CH@ + NO (3) CH30 + Oz HCHO + HOz(4) -OH NO2 HOz* NO (5) NO was added to the system to minimize the concentration of ozone and its reaction with isoprene.
+
--
-
+
NO+NO+O (6) 0 + Oz O3 (7) O3 + NO Oz + NOz (8) A series of experiments was performed at 23 f 3 "C in which isoprene concentrations were varied from 4.3 to 100 ppm. Irradiation times were carefully controlled so that O3 formation was below that detectable by an O3monitor (
