Environ. Sci. Technol. 1993, 27, 1032-1041

Gas-Phase Reactions of 2-Vinylpyridine and Styrene with OH and NO3 Radicals and 0 3

Ernest0 C. Tuaron,' Janet Arey,? Roger Atkinson,? and Sara M. Aschmann

Statewide Air Pollution Research Center, University of California, Riverside. California 9252 1

The products of the gas-phase reactions of styrene and 2-vinylpyridine with 03, OH radicals, and NO3 radicals have been studied at room temperature and atmospheric pressure of air. The reaction kinetics were also measured for those reactions not previously examined. The major products of the ozone reaction with styrene were form- aldehyde and benzaldehyde, each with yields of -40%. The OH radical reaction with styrene also formed benz- aldehyde and formaldehyde with yields of 63 f 6% and 72 f 7 % , respectively. The reaction of styrene with the N O 3 radical formed equal, minor yields (ca. 11 % ) of HCHO and CGH~CHO, with the remaining unidentified products consisting of transient and stable species which contain ONO2, OONO2, and C=O groups. The 2-vinylpyridine reaction with ozone produces 2-pyridinecarboxaldehyde and formaldehyde, with respective yields of 80 f 9 % and 34 f 5 % . 2-Pyridinecarboxaldehyde was also the major product of the OH radical reaction with 2-vinylpyridine, with a yield of 78 f 14%.

Introduction

Chemical compounds present in indoor environments arise from two sources, infiltration from ambient outdoor air (1-3) and direct emission in the indoor environment from combustion sources (including tobacco smoking) (2, 4-6), the materials of the building and its contents (7- 11), and the uses of chemicals in the indoor environment (8, 12). In addition to an incomplete knowledge of the identities and concentration levels of organic compounds present in indoor environments, the fates and transfor- mations of chemical compounds under indoor conditions are presently not well understood (see, for example, ref 13).

In this work, we have investigated the kinetics and products of the gas-phase reactions of the structurally related compounds styrene and 2-vinylpyridine with OH radicals, NO3 radicals, 03, and NO2, reactions which are expected to be of potential importance as loss processes in indoor and outdoor environments.

Styrene (CsH&H=CHz) has been identified as a hazardous air pollutant in the 1990 Clean Air Act and is often found in both indoor and outdoor environments (14, 15). Styrene is emitted into the atmosphere from solvents (16) and certain combustion emissions (1 7) as well as from interior sources such as adhesives (8, 9) and has been observed in the Los Angeles atmosphere in the low (0.5-3) ppb mixing ratio range (18). Styrene is currently being reviewed by the California Air Resources Board as a toxic air contaminant (19). 2-Vinylpyridine is a minor con- stituent of environmental tobacco smoke (ETS) (5 ) , and its concentration has been observed to increase in UV-

* Author to whom correspondence should be addressed. t Also at the Department of Soil and Environmental Sciences,

University of California, Riverside.

1832 Envlron. Sci. Technol., Vol. 27, No. 9, 1993

irradiated tobacco smoke (5) . 3-Vinylpyridine is more prevalent than 2-vinylpyridine in ETS, by a factor of -5- 10 (51, and has been suggested as a vapor-phase marker for ETS (20, 21). However, in contrast to its isomer, 2-vinylpyridine, 3-vinylpyridine is not commercially avail- able.

Experimental Section

The experimental methods used for the kinetic and product studies were generally similar to those described previously (22-24) and are briefly discussed below.

0 3 Reactions. Kinetics. Rate constants for the re- actions of 2-vinylpyridine and styrene were determined by monitoring the ozone decay rates in the presence of known excess concentrations of 2-vinylpyridine or styrene. Under conditions where the decay of ozone is governed by the reactions

(1)

(2) then for [organic] >> [03l in i t id , the 0 3 decay rate, -d ln- [Oal/dt, is given by

(1) and a plot of the ozone decay rate against the 2-vinylpy- ridine or styrene concentration should be a straight line of slope k2 and intercept of kl.

Experiments involving styrene were carried out in a 160-L Teflon reaction chamber, using the experimental methods described in detail previously (22).

Experiments involving 2-vinylpyridine were carried out in a 6400-L all-Teflon chamber fitted with a mixing fan rated at 300 L s-l. In these experiments, O3 in 0 2 diluent was flushed into the chamber in the presence of excess concentrations of 2-vinylpyridine, with rapid mixing being achieved by use of the mixing fan during the introduction of the reactants. For both reactions, O3 was monitored by a Monitor Labs 8410 chemiluminescence ozone analyzer, and styrene and 2-vinylpyridine concentrations were measured during the reactions by gas chromatography with flame ionization detection (GC-FID). Styrene was ana- lyzed using a 10 f t X 0.125 in. stainless steel column of 10% Carbowax E-600 on C-22 firebrick, operated at 75 "C. For the GC-FID analyses of 2-vinylpyridine, 100 cm3 gas samples were collected from the chamber onto Tenax solid adsorbent with subsequent thermal desorption at -250 "C onto a 15-m DB-5 megabore column held at 40 "C and then temperature programmed at 8 "C min-'. The initial reactant concentrations (in molecule ~ m - ~ units) were as follows: styrene (0-4.7) X 1014 or 2-vinylpyridine (0-2.2) X 1014 and O3 (1.0-2.8) X 1013.

Products. Product studies of the reactions of 0 3 with styrene and 2-vinylpyridine were carried out in a 5800-L evacuable, Teflon-coated chamber equipped with a mul- tiple reflection optical system interfaced to a Nicolet 7199 Fourier transform infrared (FTIR) absorption spectrom-

0013-936X/93/0927-1832$04.00/0 0 1993 American Chemical Society

0, + chamber walls - loss of O3 0, + organic - products

ozone decay rate = k, + k2[organicl

eter. The reactants and products were analyzed by FTIR absorption spectroscopy, with each spectrum being re- corded with 64 averaged interferograms (1.8 min mea- surement time) using a path length of 62.9 m and a full- width at half-maximum resolution of 0.7 cm-'. 03 in 0 2 diluent was flushed into the chamber by a flow of Nz, with rapid mixing (<2 min) by means of fans located inside the chamber. Gas samples were also collected on Tenax solid adsorbent from selected styrene and 2-vinylpyridine reactions for thermal desorption and analysis by GC-MS (Hewlett Packard (HP) 5890 GC interfaced to an H P 5970 mass selective detector) and/or by GC-FTIR (HP 5890 GC interfaced to an H P 5965B FTIR detector).

OH Radical Reactions. Kinetics. The rate constant for the gas-phase reaction of the OH radical with 2- vinylpyridine was determined using a relative rate method, in which the relative disappearance rates of 2-vinylpyridine and a reference organic, whose OH radical reaction rate constant is reliably known, were measured in the presence of OH radicals (24). Hydroxyl radicals were generated by the photolysis of methyl nitrite in air a t wavelengths >300 nm (R = H)

RCH20N0 + hv -+ RCH20 + NO RCH20 + 0, - RCHO + HO,

HO, + NO 4 OH + NO, and NO was added to the reactant mixtures to avoid the formation of 0 3 and hence of NO3 radicals. Isoprene (2- methyl-l,3-butadiene) was chosen as the reference organic, and 2,3-dimethyl-2-butene [which is much more reactive than isoprene toward NO3 radicals and 0 3 (2511 was also added to the reactant mixtures to check that any con- tributions of O3 and/or NO3 radical reactions to the 2-vinylpyridine and isoprene decays were of negligible significance (24). 2-Vinylpyridine was analyzed by GC- FID as described above, and isoprene and 2,3-dimethyl- 2-butene were analyzed by GC-FID using a 20 f t X 0.125 in. stainless steel column of 5% DC703/C20M on 100/120 mesh AW, DMCS Chromosorb G, operated at 60 "C.

Products. Product studies were carried out in a 6400-L all-Teflon chamber with black lamp irradiation (Zvin- ylpyridine) and the 5800-L evacuable, Teflon-coated chamber with irradiation being provided by a 24-kW xenon arc and with analyses by FTIR absorption spectroscopy (styrene). The formation of 2-vinyl pyridinium salts during the OH radical reactions with 2-vinylpyridine (see Results and Discussion section) precluded the use of the FTIR system for product identification. For experiments conducted in the 6400-L all-Teflon chamber, analyses were carried out by GC-FID with confirmation by GC-MS. OH radicals were produced by the photolysis of methyl nitrite (6400-L chamber) or ethyl nitrite (5800-L chamber), with the use of ethyl nitrite photolysis allowing the detection and quantification by FTIR spectroscopy of HCHO as a reaction product (26, 27).

NO3 Radical Reactions. The products of the gas-phase reactions of the NO3 radical with styrene were investigated in a similar manner to the OH radical reaction product study, using the thermal decomposition of N2O5 as a source of NO3 radicals (24, 28)

M N,O, - NO, + NO,

Chemicals. The chemicals used, and their stated purities, were as follows: 2,3-dimethyl-2-butene (99 % ),

0 2 4 6 ~ 1 0 ' ~

[STYRENE] molecule cmS3

Figure 1. Plot of the O3 decay rate against the styrene concentration.

Chem Samples; benzaldehyde (98%), isoprene (2-methyl- 1,3-butadiene) (99+%), pyridine (99+%), 2-pyridinecar- boxaldehyde (99%), styrene (99+ % 1, and 2-vinylpyridine (97%),AldrichChemicalCo.;andNO (199.0%),Matheson Gas Products. Methyl nitrite and ethyl nitrite were prepared as described by Taylor et al. (291, and N206 was prepared as described by Atkinson et al. (28). NO2 was prepared by reacting NO with an excess amount of 02 just prior to use, and O3 in 0 2 diluent was obtained as needed from a Welsbach T-408 ozone generator.

Results and Discussion In all of the chambers utilized, dark decays of styrene

and 2-vinylpyridine in air were negligible over the time scales of the experiments.

O3Reactions. Styrene. The ozone concentrations were monitored for 4-55 min after the reactions were initiated. In the presence of styrene, the 03 decays were exponential over 4-8 half-lives, and the 03 decay rates were 2 orders of magnitude higher than those in the absence of styrene. The O3 decay rates are plotted against the styrene concentration in Figure 1. A least-squares analysis leads to the rate constant

k,(styrene) = (1.71 f 0.18) X cm3 molecule-' s-'

at 296 f 2 K, where the indicated errors are 2 least-squares standard deviations combined with an estimated overall uncertainty of *lo% in the GC-FID calibration factor for styrene. This rate constant is in good agreement with our previous room temperature rate constant of (2.16 f 0.46) X 10-17 cm3 molecule-' s-l (22) and in reasonable agreement with the rate constant of 3.0 X 10-17 cm3 molecule-' s-1 a t 303 K reported by Bufalini and Altshuller (30).

Two experiments were carried out to study the products of the dark reaction of styrene with ozone: run EC-1332 where 1.2 X 1014 molecule ~ m - ~ of O3 was mixed with 2.4 X 1014 molecule cm3 of styrene in air and run EC-1333 where 1.2 X 1014 molecule cm-, of styrene was injected into a mixture of 2.4 X 1014 molecule cm-, of O3 in air. In both cases, 195% of the reactant with the lesser concen- tration was consumed after 15 min. For both reactions, the stoichiometry was A[031/A[styrenel = 0.93 f 0.10. Figure 2B shows the infrared spectrum from EC-1332 at a reaction time of 28 min, when all of the added O3 had reacted. The major products identified were HCHO and CsHsCHO, with respective molar yields of 37 f 5 % and

Environ. Scl. Technol., Vol. 27, No. 9, 1993 1833

%L *

* * *

650 650 1oso IZSO l l t 5 C 1650 1850 E050

Figure 2. FTIR spectra from the styrene f O3 reaction (run EC-1332). (A) Initial styrene. (B) At t = 28 min after addition of 1.2 X loi4 molecule ~ m - ~ O3 (no O3 remaining). (C) Residual spectrum from (8) after subtracting remaining styrene and HCHO, HCOOH, and CeH5CH0 absorptions; asterisks indicate band positions of HOCH20CH0, a possible product. Numbers in parentheses are concentrations in units of 10'3 molecule cm-3.

WflVENUMBER

.. - A [ S Y R E N E ] , ' ? ' I T O ecL le C T

Flgure 3. Least-squares plots of benzaldehyde and formaldehyde concentrations against the amounts of styrene consumed during the styrene 4- 03reactions(runs EC1332andEC-1333). The benzaldehyde data have been vertically offset by 1.0 X lo i3 molecule ~ m - ~ for clarity.

41 f 5 7% as determined from the slopes of the plots shown in Figure 3. CO (7 f 2 % ), COz (4 f 2 % 1, and HCOOH (1-2 % ) were also observed as minor products. The errors quoted above represent the total of random errors (the 2 least-squares standard deviations) and the systematic errors arising from -5% uncertainties in the FTIR calibration of each component.

The residual spectrum shown in Figure 2C, obtained by subtracting the absorptions of the remaining styrene and selected products, exhibits absorption bands in the - 1750 cm-I and 1000-1300 cm-' regions which could indicate the presence of esters and, possibly, ozonides. GC-FTIR and GC-MS analyses of samples collected on Tenax solid adsorbent from a separate run in excess 03 (with initial reactant concentrations similar to those of run EC-1332) also indicated benzaldehyde as a product and showed the presence of small amounts of benzoic acid and a product with a molecular weight of 122 and major infrared absorption bands at 1766,1195, and 1104 cm-l, tentatively identified as phenyl formate. From the residual spectra such as that shown in Figure 2C, the yields of benzoic acid

in runs EC-1332 and EC-1333 were estimated to be I1 % . The absorption bands of phenyl formate (an authentic sample was not available), although most likely present., were also weak and were not distinct in the residual spectra at any stage during the reactions.

The available kinetic and product data suggest that the reaction proceeds by

CGHSCHO + [CHzOb]* [CSH&HO~] ' +- HCHO

where [ 1 * denotes an initially energy-rich biradical species. The subsequent reactions of the [CHzOOI * radical formed from the 03 plus ethene reaction have been discussed previously (25, 31). At atmospheric pressure of air and room temperature (25)

(37%)

I- decomposition products (63%)

with the thermalized CHzOO biradical being expected to react with water vapor to form HCOOH under atmospheric conditions (25). The reactions of the [66H&H00] * radical are not presently known. The product data obtained are reasonably consistent with the above reaction scheme, providing that the [C6H&HOO] * biradical does not produce C$&CHO in high yield.

The products which have been identified and quantified do not account for the decomposition products of the biradicals [(?,HzOOl* and [C6H&HOO] *. By analogy with the mechanisms of reactions of simple alkenes with ozone (see, for example, refs 31-33), under laboratory conditions the thermalized biradicals can react with aldehydes to form secondary ozonides and hydroxyl-substituted esters:

OH 0 R i \ P-9 ,Rz Ri, I I I - H/C-o-c-R2

R,CHO~ + R~CHO - H,c.o.c, H

Hence, the possible ester products from the styrene-03 system are HOCH2QCH0, ~6H&H(OH)OCHO, HOCH2- OC(O)CF&, and c6H&H(OH)OC(O)C6H5. The major infrared bands of HOCHzOCHO at 1760 (trans C-0 stretch), 1737 (cis C=O stretch), 1167,1047, and 922 cm-l reported by Niki et al. (33) coincide or nearly coincide in both position and shape with discernible maxima in the spectrum of Figure 2C. While weak absorption bands in the 3400-3600 cm-l region (not shown in Figure 2) may indicate the presence of OH groups, the poor response of the IR detector in this frequency range and the possible merging of absorptions from more than one alcohol species make this spectral region less useful. There are no published infrared spectra of the three other hydroxy esters, and authentic samples of these compounds are not available. However, the major vapor-phase infrared absorptions of the analogous compounds CH30C(O)CsHb and C&I5CH@C(0)6& (34) are nearly coincident with those of Figure 2B: for CH30C(O)CgH5,1749,1445,1278, 1110, and 702 Cm-l; for C6H5CH~OC(O)C6H~, 1743,1267,

1834 Environ. Sci. Technol., Vol. 27, No. 9, 1893

2-VINYLPYRIDINE I (19.41 /

- 1 w

I I 0 I 2 3 ~ 1 0 ' ~

[ 2-ViNYLPYRiDiNE ] molecule cm-3

Flgure 4. Plot of the O3 decay rate against the 2-vinyipyridine concentration.

1103, and 701 cm-l. The spectrum of liquid CsH&Hz- OCHO (35) likewise shows major absorption bands at 1735, 1163, 760, 741, and 698 cm-'. I t is expected that the corresponding hydroxy esters will have very similar spectra, with the generally medium to strong C-0 stretch absorp- tion of the >C-OH moiety additionally contributing to the absorptions in the 1000-1100 cm-' region. Thus, the proposed hydroxy ester products are consistent with the observed residual spectra and may explain to a large extent the decomposition products of the energy-rich biradical species. I t should be noted that, if present as an observable product, the secondary ozonide would also likely absorb most strongly in the 1070-1130 cm-l region (33, 36, 37).

2-Vinylpyridine. A plot of the ozone decay rates, -d ln[03l/dt, against the 2-vinylpyridine concentration is shown in Figure 4. A good straight line is observed with the O3 decay rates in the absence of 2-vinylpyridine being over 2 orders of magnitude less than those in the presence of 2-vinylpyridine. A least-squares analysis of these data yields the rate constant of

K2(2-vinylpyridine) = (1.46 f 0.17) X

10-l~ cm3 molecule-' s-l a t 298 f 2 K, where the indicated error is 2 least-squares standard deviations combined with an estimated overall uncertainty of *lo% in the GC-FID response factor for 2-vinylpyridine.

The reaction of 1.94 X 1014 molecule ~ r n - ~ of 2-vinylpy- ridine with 1.26 X 1814 molecule ~ m - ~ of O3 (run EC-1384) is depicted in the spectra presented in Figures 5 and 6. The initial 2-vinylpyridine (Figure 5A) showed a 42 % loss after 5 min and a 58% loss after 15 min (Figure 5B) of reaction, with essentially equal consumption of O3 for each period. Figure 6A shows a spectrum of the products a t t = 15 min after subtracting the absorptions of the remaining reactants and those of the H20 formed from Figure 5B. Further successive spectral subtraction, using calibrated

j -V INYLPYRIDINE (8.31

OZONE

x1

650 650 1050 1250 1450 1650 1850 2050 WFiVENUMRER

Flgure 5. FTIR spectra from the 2-vinyipyridine 4- O3 reaction (run EC-1384). (A) Initial 2-vinylpyridine. (B) At t = 15 rnin after addition of 1.26 X 1014 molecule ~ m - ~ 03. Numbers in parentheses are concentrations in unlts of 10'3 molecule cm-3.

I

2-PYRIDINECARBOXALDEHYDE (8.7)

C

I- , 8 -

650 850 1050 1250 lit50 1650 1850 E050 dAVENUMRER

Flgure 6. Product spectra at t = 15 rnin from the 2-vinylpyridine + 0 3 reaction (run EC-1384). (A) From Figure 5B, after subtraction of absorptions by the remaining reactants and H20. (B) Reference spectrum of 2-pyridinecarboxaldehyde. (C) Residual spectrum from (A). Numbers in parentheses are concentrations in units of l O I 3 molecule cm-3.

reference spectra, was employed to analyze the products obtained. The major products observed were 2-pyridine- carboxaldehyde and formaldehyde. The detailed time- concentration data of the above experiment and those of another run in excess 0 3 , Le., 2.6 X loi4 molecule ~ r n - ~ of O3 and 1.0 X 1014 molecule ~ m - ~ of 2-vinylpyridine (run EC-1387), lead to molar yields of 80 f 9% for 2-pyridine- carboxaldehyde and 34 f 5% for formaldehyde (Figure 7). The other products and their yields were pyridine (6 f 2%), COz (11 f 2%), CO (10 f 2 % ) , HCOOH (12%) , and performic acid [HC(O)OOHl with an undetermined but minor yield. Absorption features due to still un- identified products are evident in the residual spectrum (Figure 6C). The ratio of O3 consumed to that of 2-vinylpyridine reacted as determined from both exper- iments was 1.00 f 0.08. The errors quoted for the above ratio and yields are the 2 least-squares standard deviations plus the estimated systematic errors arising from the uncertainties in the calibrations.

GC-MS analysis of samples collected on Tenax solid adsorbent from a separate run with similar conditions to EC-1384 (2-vinylpyridine in excess) confirmed the for-

Environ. Sci. Technol., Vol. 27, No. 9, 1993 1835

I

00 2L1 40 6 0 8 0 1 0 0 1 2 c

4[2- \ l i u~LPYF?l3 lYE] , 13" m3lecuie crn-'

Flgure 7. Least-squares plots of 2-pyrldlnecarboxaldehyde and formaldehyde concentrations against the amounts of 2-vinylpyridine consumed during the 2-vlnylpyridine -t O3 reactions (runs EC-1384 and

mation of 2-pyridinecarboxaldehyde and a small amount of pyridine.

The observed products of the 2-vinylpyridine-ozone reaction are consistent with the reaction process

EC-1387).

L i

The initially formed primary ozonide decomposes to yield 2-pyridinecarboxaldehyde in path a and formaldehyde in path b as stable products. The initially energy-rich biradical coproducts may rearrange to a hot acid and then decompose or be stabilized, or react with other species (26, 33).

HC(0)OH

H20 + C O -E H2 + COP

[ d ~ , 0 6 ] ~ +' [HC(Q)OH]*

CHz06 -t H20 HCjOjOOH + Hz

r oc(O'uH D O H + co

As noted above, among the possible decomposition products of the biradical containing the pyridine moiety, only pyridine has so far been identified. Since, however,

C2H50N0 I r- (21.8)

650 850 ldS0 1250 1450 I650 -5C WRVENUMBER

Figure 8. FTIR spectra from the styrene-C2H50NO-NO-air irradiation (run EC-1336). (A) Initial mixture. (B) Reaction mixture at irradiation time t = 28 min. Numbers In parentheses are concentratlons in units of 10'3 molecule cm-3.

the sum of the yields of formaldehyde and 2-pyridine- carboxaldehyde exceeds unity, then one or both of these aldehydes must also be produced through stabilization or decomposition of the respective Criegee biradical. Since a significant decomposition of the biradical [ ~ H z O ~ ] * to HCHO has not been observed (see, for example, ref 31), it is likely that 2-pyridinecarboxaldehyde is also formed from the reaction@)

Reactions of the biradicals with formaldehyde and 2-py- ridinecarboxaldehyde to form secondary ozonides and hydroxy esters, as proposed above for styrene, may account for the products which exhibit the stronger absorption bands in the infrared regions -1750 and 1000-1200 cm-' (Figure 6C).

Our observations that the rate constants for the 03 reactions with styrene and 2-vinylpyridine are similar (to within 20 7Z ) and that the major products are the aldehydes arising from oxidative cleavage of the CH=CHz bond shows that the 0 3 reactions occur a t the CH=CHz substituent group and that the aromatic C6H5 or C5HdN rings have little effect on the kinetics or mechanisms of these reactions, although there are differences in the detailed product yields.

OH Radical Reactions. Styrene. Since the rate constant for the OH radical reaction with styrene has been measured previously under atmospheric conditions (24, 38, 39), no kinetic studies were carried out in this work. Two product experiments (EC-1336 and EC-1337) were carried out which consisted of irradiations of mixtures of styrene (2.4 X 1014 molecule ~ m - ~ ) , ethyl nitrite (2.2 X 1014 molecule ~ m - ~ ) , and NO (1.8 X 1014 molecule cm-9 in air.

Figures 8 and 9 show spectra from run EC-1336 after 28 min of irradiation. The major products expected and observed from styrene were HCHO and CsHtjCHO. The infrared absorptions of the products arising from CZH5- O N 0 itself (namely, CH3CH0, Cd"bjONoz, and CHsC-

1836 Environ. Scl. Ischnol.. Vol. 27, No. 9, 1993

( 2 X ) C '

I L t = IOmin

650 850 1050 1250 1450 1650 1650 2650 WAVENUMBER

Flgure 9, Product spectra from the styrene-C2H50NO-NO-air Irradiation (run EC-1336). (A) From Flgure 8B ( t = 28 min), after subtractlon of absorptions by the reactants and products from C2H5- ON0 (see text). (B) Residual spectrum from (A) after subtraction of CeHsCHO and HCHO absorptions. (C) Residual spectrum at t = 10 min. Numbers in parentheses are concentrations in units of loi3 molecuie ~ m - ~ .

10.8, , , , I I I I

0 7

U G O.8.0 3.0 6.0 9.0 12.0 15.0

-A[STYRENE], 1 01' molecule crn-'

Flgure 10. Least-squares plots of benzaldehyde and formaldehyde concentrations against the amounts of styrene consumed during the photolyses of styrene-C2HsONO-NO-alr mixtures (EC-1336 and EC- 1337). The concentrations of CeHsCHO and HCHO plotted have been corrected to take into account reactions with the OH radical. The HCHO data have been vertically offset by 1.0 X 1013 molecule cm3 for clarity.

(0)OONOz) and those of the other expected products (HCOOH, CO, CO2, "03, HONO, and NO21 are all present in the product spectrum shown in Figure 8B. Subtraction of the absorption bands of these products from Figure 8B with the use of reference spectra resulted in the clearer presentation in Figure 9A of the products from styrene. The detailed time-concentration data of HCHO and C,&CHO from such analyses showed mod- erate declines in their yields during the -30-min irradi- ations, due to the reaction of these products with OH radicals (39). Plots of the HCHO and C&I&HO con- centrations, corrected for their reactions with OH radicals (401, against the amount of styrene reacted are presented in Figure 10, and the slopes of these plots lead to molar yields of 72 f 7 % for HCHO and 63 f 6 % for C6H&HO (the errors quoted are the total of systematic and random errors). The benzaldehyde formation yield of 0.63 f 0.06 observed in this work is significantly lower than that of 1.03 f 0.15 obtained by Bignozzi et al. (38) from the time- concentration profiles of styrene and benzaldehyde in an

irradiated NO,-styrene-air mixture, for as yet unknown reasons.

The decrease in the measured yield of C6H5CHO during the photolysis was accounted for in part by the formation of peroxybenzoyl nitrate (C6H&(O)OON02) [PBzNl, as illustrated in Figure 9B. The PBzN yield increased from 1.5% after 10 min of irradiation to 6% after 28 min of irradiation. Among the stable products observed was a compound (or compounds) containing a nitrate (ONO2) group, as indicated by the infrared absorption bands at 1667,1280, and 848 cm-l. The concentrations of the nitrate compounds were estimated from the integrated band areas of the NO2 asymmetric stretch in the range 1646-1694 cm-l, based on an average integrated absorption coefficient of (2.5 f 0.2) X 1O-l' cm molecule-l determined from the corresponding bands of CHsON02, C2H50N02, and CH3C- (0)00N02. The yield of the organic nitrate(s) was estimated to be -12% and could account for approxi- mately one-third of the amount of styrene consumed which was not accounted for by the amounts of C6H&HO and PBzN formed.

The magnitude of the OH radical reaction rate constant (39) indicates that the reaction of the OH radical with styrene proceeds by initial OH radical addition to the substituent CH=CH2 group (39)

M OH + C,H,CH=CH, -

C,H,CHCH,OH and C,H,CHOHCH,

and no evidence for products arising from initial OH radical addition to the aromatic ring was obtained. In the presence of NO, the reactions subsequent to this initial OH radical addition to the CH=CH2 group are expected to be (25)

M C6H5&lCH@l + 0 2 ---c C6H&H(O6)CH2OH

M C~HC,CH(ONO~)CH~OH

CeH5CH(06)CHzOH + NO 5 CeH&H(b)CH20H + NO2

C,H,CH@)CH,OH - C,H,CHO + CH,OH

CH20H + 0, - HCHO + HO,

and

M C,H,CH(OH)CH, + 0, - C,H,CH(OH)CH,OO'

C~HSCH(OH)CH~ONO, -t C6H,CH(OH)CH,O' + NO, C6H5CH(OH)CH2OO' + NO

C6H&H(OH)CH2O' ---) C6H5kHOH + HCHO

The observed high, and nearly equal (within the estimated uncertainties), yields of HCHO and CeH5CHO are in general agreement with this reaction scheme. The organic nitrates observed are most likely those that are formed from the reaction of the hydroxyalkylperoxy radicals with NO, and the presence of a medium intensity band at 1060 cm-l in Figure 9B,C is consistent with the C-0 stretch frequency of a C-OH group.

The OH-initiated reaction of C&CHO leads to the formation of PBzN (41)

Environ. Sci. Technol., Vol. 27, No. 9, 1993 1837

Rate constants for reactions c, d, and -d have been reported by Kirchner et al. (41).

2- Vinylpyridine. Initial experiments were carried out to check that 2-vinylpyridine did not undergo photolysis under normal room fluorescent lights, black lamp irra- diation, or react with NOz. Under "cool-white" fluorescent lighting at an intensity a factor of - 11 higher than that in an office room, <5% decay of 2-vinylpyridine occurred over a 5.2-h period (equivalent to -7 days of normal office lighting). Similarly, irradiation with black lamps at the maximum light intensity (which corresponds to an NO2 photolysis rate of -0.45 min-l) for 15 min led to no observable decay of 2-vinylpyridine within the measure- ment uncertainties.

2-Vinylpyridine and cyclohexane (present as a nonre- active tracer) were introduced into the 6400-L, all-Teflon chamber at initial concentrations of -2.4 X 1013 molecule ~ m - ~ . After initial GC-FID measurements of the cyclo- hexane and 2-vinylpyridine, 1.2 X 1014 molecule ~ m - ~ of NO2 was introduced into the chamber, and GC-FID measurements were continued throughout the course of the reaction. After a rapid initial decrease in the 2-vinyl- pyridine concentration (7% within 3 min of the intro- duction of NO2 into the chamber), the 2-vinylpyridine concentration remained constant to within *2% over a period of 145 min. As expected, the concentration of the cyclohexane tracer remained constant to within <1% throughout the experiment. The rapid (and small) initial decrease in the 2-vinylpyridine concentration was almost certainly caused by the reaction of 2-vinylpyridine with nitric acid present in the Nos. Thus, previous FTIR measurements in this laboratory have shown that NO2 prepared from reacting NO with an excess of 0 2 contains typically 1-2 % nitric acid, consistent with the observed ratio (-A[2-vinylpyridinel/ [NO21 1 = 0.014.

A least-squares analysis of the 2-vinylpyridine data after this initial period leads to an upper limit to the rate constant at 298 f 2 K for the gas-phase reaction of 2-vinylpyridine with NO2 of

k(N0, + 2-vinylpyridine) <7 X lo-'' cm3 molecule-' s-'

Initial kinetic experiments to determine the OH radical reaction rate constant were carried out in which CH3- ONO-NO-2-vinylpyridine-isoprene-2,3-dimethyl-2- bu- tene-air mixtures were irradiated for 1-8 min at 20% of the maximum light intensity. The initial reactant con- centrations (in units of 10'3 molecule cm-3) were as follows: CH30N0, 2.5-25; NO, 2.5-25; 2-vinylpyridine, isoprene, and 2,3-dimethyl-2-butene, -2.4 each, with the CHBONO and NO concentrations being maintained ap- proximately equal. 2-Vinylpyridine, isoprene, and 2,3- dimethyl-2-butene all reacted with approximately similar disappearance rates, and 2-pyridinecarboxaldehyde was observed as EL reaction product by GC-MS. However, plots

I I I 0 0.5 I .o I .5

In([: ISOPRENE I t o / [ ISOPRENE 1,)

Flgure 11. Plot of In([2-vlnylpyrldlne] b/[2-vlnylpyrldlne] t) against In- ([isoprene] b/[lsoprene]d, wlth the least-squares analysis line used to calculate the OH radical rate constant ratio shown.

of In{ [2-vinylpyridinel tJ [2-vinylpyridinelt} against ln- {[isoprene] td [isopreneltJ were slightly curved, more so than expected from the uncertainties in replicate analyses. Therefore, as anticipated from our previous observations that pyridine reacts in the gas phase with nitric acid to form the pyridinium nitrate salt (23) and consistent with our observations concerning the reaction of NO2 with 2-vinylpyridine (see above), we suspected that part of the 2-vinylpyridine removal was caused by reaction with nitric acid generated through the reaction

M OH + NO, - HNO,

Accordingly, we then conducted irradiations in which pyridine was added to scavenge gaseous nitric acid. The initial reactant concentrations were (in units of 1013 molecule cm-9 as follows: CH30NO,24; NO, 24; 2-vinyl- pyridine, isoprene, and 2,3-dimethyl-2-butene, 2.4 each; and pyridine, 12-24. Relative to isoprene, the 2-vinylpy- ridine disappearance rate was now significantly slower and plots of In( [ 2-vinylpyridinel td [ 2-vinylpyridinel t ) against ln([isoprene] td [isopreneItj were good straight lines, as shown in Figure 11. The slopes of such plots did not depend on the pyridine concentration over the range employed. A least-squares analysis of the data obtained using the higher concentrations of pyridine (Figure 11) leads to a rate constant ratio of

k(OH + 2-vinylpyridine)/k(OH + isoprene) = 0.561 f 0.036

where the indicated error limit is 2 least-squares standard deviations. Using a rate constant for the reaction of the OH radical with isoprene of 1.01 X cm3 molecule-' s-' at 298 K (39), this leads to

k(OH + 2-vinylpyridine) = (5.67 * 0.37) X

IO-'' cm3 molecule-' s-'

at 298 f 2 K, where the indicated error does not take into account the f25% estimated overall uncertainty in the

1838 Environ. Scl. Technol., Vol. 27, NO. 9, 1993

/

0 0 I I I 0 0.5 I .o 1.5 1013

-A [2-VINY LPYRlDl NE 1 molecule cm-3

Figure 12. Plot of the amount of 2-pyridine carboxaldehyde formed (corrected to take Into account reaction with the OH radical) against the amount of 2-vinyipyridine reacted.

rate constant for OH radical reaction with isoprene (39). This rate constant is very similar to that for the analogous reaction of the OH radical with styrene (24, 38, 391, consistent with the expectation (39) that the reaction occurs by OH radical addition to the CH=CH2 substituent group.

As noted above, 2-pyridinecarboxaldehyde was observed as the major product of the reaction of the OH radical with 2-vinylpyridine. 2-Pyridinecarboxaldehyde was con- firmed as the product of the OH radical reaction through GC-MS analyses of Tenax samples taken from the chamber, comparing the spectrum and GC retention time of the product peak observed with those of an authentic standard. In order to determine the formation yield of 2-pyridinecarboxaldehyde from the OH radical reaction with 2-vinylpyridine, it was necessary to take into account the losses of the 2-pyridinecarboxaldehyde through its reaction with the OH radical. This was carried out as described in detail by Atkinson et al. (40), using a rate constant for the reaction of the OH radical with 2-py- ridinecarboxaldehyde of 1.3 X 10-l1 cm3 molecule-l s-l, estimated by assuming that the 2-pyridinecarboxaldehyde rate constant is identical to that for benzaldehyde (39). A plot of the amount of 2-pyridinecarboxaldehyde formed (corrected to take into account reaction with the OH radical) against the amount of 2-vinylpyridine reacted (in the presence of pyridine) is shown in Figure 12. A reasonable straight line plot is obtained, and a least-squares analysis leads to a molar formation yield of 2-pyridine- carboxaldehyde from 2-vinylpyridine of 78 f 14%, where the indicated error limits are the 2 least-squares standard deviations combined with estimated f10 % uncertainties in the calibration factors for 2-vinylpyridine and 2-py- ridinecarboxaldehyde.

The formation of 2-pyridinecarboxaldehyde from 2- vinylpyridine is analogous to the formation of benzalde- hyde in high yield from the reaction of the OH radical with styrene (see above) and confirms that the reaction also proceeds by initial OH radical addition to the CH=CH2 group.

Styrene + Nos. The dark reaction of N2O5 (9.6 X 1013 molecule ~ m - ~ ) with styrene (2.4 X 1014 molecule ~ m - ~ ) in

650 850 I050 1250 1450 1650 I850 zbso WRVENUMBER

Figure 13. Residual spectra corresponding to different reaction tlmes during the styrene + N205 reaction (run EC-1335). (A) At t = 1 mln. (B) At t = 5 min. (C) At t = 33 mln. Absorption bands which indicate the presence of three distinct products are marked.

air (EC-1334) resulted in the disappearance of 194% of the N2O5 after 7 min. A similar experiment which employed 1.2 X 1014 molecule ~ m - ~ of N205 and 2.4 X 1014 molecule cm-3 of styrene (EC-1335), and which generated a higher amount of NO2 (i.e., 1.2 X 1014 molecule ~ m - ~ vs 8.9 X lO13molecule cm-3), resulted in a slower rate of N205 disappearance (198% after 17 rnin). In both runs, HCHO and CsH&HO were formed in equal yields but in amounts which were only 10-12% of the styrene consumed.

Figure 13 shows residual spectra from run EC-1335, obtained by subtracting the absorptions of HCHO, C6H5- CHO, and the other obvious products NO2 and “03. A comparison of the residual spectrum obtained after 1 min of reaction (Figure 13A) with that after 5 min (Figure 13B) reveals that a product that was formed early (designated here as product U1) and associated with the peak at 1726 cm-l also decayed rapidly. A second unknown (product U2) associated with a band at 1636 cm-l was more stable than product U1, but a steady decrease in its concentration was observed after attaining a maximum after approximately 9 min of reaction. The concentration of the most stable product (product U3) remained constant as product U2 decayed. The rise in the slopes of the baselines a t the higher frequencies in Figure 13 is indicative of light scattering by aerosols. The addition of NO (1.8 X 1014 molecule ~ m - ~ ) to the mixture after all the N2O5 had been consumed appeared to accelerate the disap- pearance of U2 but did not affect U3. The residual spectrum shown in Figure 14A, obtained -20 rnin after the addition of NO, represents the spectrum of U3. The differences in the curves of growth enabled the spectra of U2 and U1 to be derived by intersubtraction from the residual spectra, and they are presented in Figure 14B,C with their intensities (relative to that of U3) corresponding to their maximum observed yields.

The infrared absorption bands associated with product U1 indicated that it contained both ON02 (1670, 1281, 846 cm-l) and 00N02 (1726, 1297, 790 cm-l) groups. Product U2 contained ON02 (1636,1283,852 cm-l) and C=O (1701 cm-9 groups. Product U3 also had absorption bands attributable to ON02 (1680, 1285, 843 cm-l) and C=O (1729 cm-l) groups. The observed disappearance of product U2 may possibly be more related to aerosol formation than to loss by reaction with added NO (the

Envlron. Scl. Technol., Vol. 27, No. 9, 1993 1839

li "3

b5C: &SO 1050 1250 1'150 1650 lES0 2bSO WRVENUMBER

Figure 14. Infrared spectra of the three unidentified products U1, U2, and U3 (see text) observed during the styrene 4- N205 reaction.

latter reaction loss being normally associated with com- pounds containing the peroxynitrate (OONO2) group).

Formaldehyde, benzaldehyde, U2, and U3 were the products of styrene present a t the time when N2O5 was totally consumed. Of the balance of nitrogen not ac- counted for by NO2 and HN03 formation, 80% in run EC-1334 and 86% in run EC-1335 could be found in products U2 and U3, if it is assumed that the combined amounts of these products were equal to the styrene consumed minus benzaldehyde formed.

The NO3 radical reaction with styrene is expected to proceed by initial NO3 radical addition to the CH=CH2 substituent group:

NO, + C,H,CH=CH, - C,H,CH(ONO,)CH, and

followed by the reactions (taking the C ~ H ~ C H C H ~ O N O Z radical as an example)

C,H,CHCH,ONO,

M C~HSCHCH~ONO~ + 0, -). C,H&H(06)CH20N02

C ,~HSCH(O~)CH~ONO~ + NO2 e C~HSCH(OONO~)CH~ONO~

(possibly product U1)

(NO C6H&H(06)CH20N02 + - C6H5CH(6)CH20N02 +

(where the RO2' radical can include the C&CH(Od)- CHzONOz or C6H5CH(ON02)CH20O9 radicals)

C,HSCH(~)CH~ONO~ -t 0 2 - CcH&(O)CH20N02 + HOP

(possibly produd U2 or U3)

C ~ H ~ C H ( ~ ) C H ~ O N O ~ - C ~ H ~ C H O + C H ~ O N O ~

1 fast t

HCHO + NO2

Analogous intermediates or first-generation products

are expected to be formed from the CsH&H(ON02)CH2 radical.

Lifetime5 of 2-Vinylpyridine and Styrene in Indoor and Outdoor Environments. Our experimental data indicate that photolysis of 2-vinylpyridine will not be an important loss process in indoor or outdoor environments. The expected loss processes are then reactions with OH radicals, NO3 radicals, and O3 for both styrene and 2-vinylpyridine, with 2-vinylpyridine additionally reacting

[C6H&H(ONO2)CH2OONO2 and C&,CH(ON02)CHO]

with gaseous nitric acid to form salts in the aerosol phase. Since all of the observed reactions of 2-vinylpyridine occur a t the CH=CH2 group, with rate constants similar to those of styrene, then it is expected that 3-vinylpyridine will react in a manner analogous to 2-vinylpyridine, with similar rate constants.

For ambient atmospheric conditions, using ambient concentrations of the following: OH radicals, a 12-h average of 1.6 X 10, molecule ern" (42 ) ; NO3 radicals, a 12-h average of 5 X lo8 molecule cm-3 (43 ) ; and 03, a 24-h average of 7 X 10" molecule cm3 (30 ppb) (44) , then the calculated lifetimes of styrene and 2-vinylpyridine (and presumably also 3-vinylpyridine) due to reaction with OH radicals, NO3 radicals, and Os are -3 h, -4 h, and -1 day, respectively [assuming that 2- and 3-vinylpyridine react with NO3 radicals with a rate constant similar to that of styrene (24, 43 ) l . It is possible that the removal of 2- and 3-vinylpyridine with gaseous nitric acid is competitive with these reactions as an atmospheric loss process.

Indoor ozone concentrations have been observed to track outdoor concentrations and to be from 20 to 80% of those outdoors (ref 1 and references cited therein). Because of lower 0 3 levels indoors, NO3 radical concentrations might also be expected to be significantly lower indoors. How- ever, it has recently been suggested that the lack of NO3 photolysis may more than counter the lower formation rates indoors ( 2 3 ) . The production of OH radicals from the dark reactions of monoterpenes and other alkenes with 0 3 (45, 46) suggests a potential indoor source of OH radicals, particularly considering the relatively high levels of monoterpenes observed in indoor environments from the use of cleaning products. Clearly our assumptions that the majority of atmospheric chemistry will only occur outdoors under the influence of sunlight deserves close examination.

Our current knowledge suggests that the lifetimes of styrene and 2- and 3-vinylpyridine will be significantly longer indoors than outdoors, but certainly both outdoor and indoor lifetimes will vary, for example, with ambient O3 levels. The short anticipated lifetimes of these com- pounds means that any riskassessment from their ambient concentrations should also consider the potential health effects of their gas-phase aldehyde reaction products.

Acknowledgments We gratefully acknowledge the financial support re-

ceived from the Center for Indoor Air Research (Award No. 90-005; Dr. Lynn Kosak-Channing, project monitor) and the California Air Resources Board (Contract A732- 107; RalphPropper, project monitor). The work described in the manuscript does not necessarily reflect the views of the funding agencies, and no official endorsement should be inferred.

Literature Cited (1) Weschler, C. J.; Shields, H. C.; Naik, D. V. J. Air Pollut.

( 2 ) Chuang, J . C.; Mack, G. A,; Kuhlman, M. R.; Wilson, N. K.

(3) Hisham, M. W. M.; Grosjean, D. Environ. Sci. Technol.

(4) Eatough, D. J.; Benner, C. L.; Bayona, J. M.; Richards, G.; Lamb, J. D.; Lee, M. L.; Lewis,E. A.; Hansen, L. D. Environ. Sci. Technol. 1989, 23, 679-687.

(5) Benner, C. L.; Bayona, J. M.; Caka, F. M.; Tang, H.; Lewis, L.; Crawford, J.; Lamb, J. D.; Lee, M. L.; Lewis, E. A.;

Control Assoc. 1989,39, 1562-1568.

Atmos. Environ. 1991,25B, 369-380.

1991,25, 857-862.

1840 Environ. Sci. Technol., Vol. 27, No. 9, 1993

Hansen, L. D.; Eatough, D. J. Environ. Sci. Technol. 1989,

(6) Brunnemann, K. D.; Kagan, M. R.; Cox, J. E.; Hoffmann,

(7) Brown, V. M.; Crump, D. R.; Gardiner, D. Environ. Int.

(8) Chan, C. C.; Vainer, L.; Martin, J. W.; Williams, D. T. J.

(9) Rothweiler, H.; Wgger, P. A,; Schlatter, C. Atmos. Environ.

(10) Rothweiler, H.; Wager, P. A,; Schlatter, C. Environ. Toxicol.

23,688-699.

D. Carcinogenesis 1990,11,1863-1868.

1990,16, 283-289.

Air Waste Manage. Assoc. 1990, 40, 62-67.

1992,26A, 2219-2225.

1992,13,891-896. (11) Weschler, C. J.; Hodgson, A. T.; Wooley, J. D. Environ. Sci.

Technol. 1992.26. 2371-2377. (12) Sack, T. M.; Steele, D. H.; Hammerstrom, K.; Remmers, J.

(13) Weschler, C. J.; Brauer, M.; Koutrakis, P. Environ. Sci.

(14) Harwell, T. D.; Perritt, R. L.; Pellizzari, E. D.; Michael, L.

(15) Namiesnik, J.; Gorecki, T.; Kozdron-Zabiegala, B.; Lukasiak,

(16) Nelson, P. F.; Quigley, S. M.; Smith, M. Y. Atmos. Environ.

(17) Spicer, C. W.; Holdren, M. W.; Smith, D. L.; Hughes, D. P.; Smith, M. D. J. Eng. Gas Turbines Power 1992,114,111- 117.

(18) Grosjean, D.; Fung, K. J. Air Pollut. Control Assoc. 1984,

Atmos. Environ. 1992,26A, 1063-1070.

Technol. 1992,26, 179-184.

C. Atmos. Environ. 1992,26A, 1519-1527.

J. Build. Environ. 1992, 27, 339-356.

1983, 17, 439-449.

34, 537-543. (19) ARB, Feb 19, 1993, Proposed identification of federal

hazardous air pollutants as the toxic air contaminants, State of California Air Resources Board, Stationary Source Division, Sacramento, CA.

(20) Heavner, D. L.; Ogden, M. W.; Nelson, P. R. Environ. Sci. Technol. 1992,26, 1737-1746.

(21) Nelson, P. R.; Heavner, D. L.; Collie, B. B.; Maiolo, K. C.; Ogden, M. W. Environ. Sci. Technol. 1992,26,1909-1915.

(22) Atkinson, R.; Aschmann, S. M.; Fitz, D. R.; Winer, A. M.; Pitts, J. N., Jr. Int. J. Chem. Kinet. 1982, 14, 13-18.

(23) Atkinson, R.; Tuazon, E. C.; Wallington, T. J.; Aschmann, S. M.; Arey, J.; Winer, A. M.; Pitts, J. N., Jr. Environ. Sci. Technol. 1987, 21, 64-72.

(24) Atkinson, R.; Aschmann, S. M. Int. J. Chem. Kinet. 1988,

(25) Atkinson, R. Atmos. Environ. 1990,24A, 1-41. 20, 513-539.

(26) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P.

(27) Tuazon, E. C.; Atkinson, R. Int. J. Chem. Kinet. 1990,22,

(28) Atkinson, R.; Plum, C. N.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J. Phys. Chem. 1984, 88, 1210-1215.

(29) Taylor, W. D.; Allston, T. D.; Moscato, M. J.; Fazekas, G. B.; Kozlowski, R.; Takacs, G. A. Int. J. Chem. Kinet. 1980,

(30) Bufalini, J. J.; Altshuller, A. P. Can. J. Chem. 1966, 43,

(31) Horie,O.; Moortgat, G. K. Atmos. Environ. 1991,254,1881-

(32) Su, F.; Calvert, J. G.; Shaw, J. H. J. Phys. Chem. 1980,84,

(33) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P.

(34) EPA/IR Library, Copyright 1990, Hewiett Packard Co. (35) Pouchert, C. J. The Aldrich Library of Infrared Spectra;

Aldrich Chemical Co.: Milwaukee, WI, 1975; p 892A. (36) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P.

Chem. Phys. Lett . 1977,46, 327-330. (37) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P.;

Hurley, M. D. J. Phys. Chem. 1987,91,941-946. (38) Bignozzi, C. A.; Maldotti, A,; Chiorboli, C.; Bartocci, C.;

Carassiti, V. Int. J. Chem. Kinet. 1981, 13, 1235-1242. (39) Atkinson, R. J. Phys. Chem. Ref. Data 1989, Monograph

(40) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J. Phys. Chem. 1982, 86, 4563-4569.

(41) Kirchner, F.; Zabel, F.; Becker, K. H. Chem. Phys. Lett .

(42) Prinn, R.; Cunnold, D.; Simmonds, P.; Alyea, F.; Boldi, R.; Crawford, A.; Fraser, P.; Gutzler, D.; Hartley, D.; Rosen, R.; Rasmussen, R. J. Geophys. Res. 1992, 97, 2445-2461.

(43) Atkinson, R. J. Phys. Chem. Ref. Data 1991,20,459-507. (44) Logan, J. A. J. Geophys. Res. 1985,90, 10463-10482. (45) Atkinson, R.; Aschmann, S. M.; Arey, J.; Shorees, B. J.

(46) Atkinson, R.; Aschmann, S. M. Environ. Sci. Technol. 1993,

Chem. Phys. Lett . 1981,80,499-503.

1221-1236.

12, 231-240.

2243-2250.

1896.

239-246.

J. Phys. Chem. 1981,85, 1024-1027.

1, 1-246.

1992,191, 169-174.

Geophys. Res. 1992,97,6065-6073.

27, 1357-1363.

Received for review Octcber 23, 1992. Revised manuscript received March 9, 1993. Accepted May 12, 1993.

Envlron. Sci. Technol., Vol. 27, No. 9, 1993 1841