Environ. Sci. Techno/. 1995, 29, 2467-2469
Observation of Hydroxycarbonyls from the OH Radical-Initiated Reaction of Isoprene E R I C S . C . KWOK, R O G E R A T K I N S O N , * , +A N D JANET AREY*ft Statewide Air Pollution Research Center, University of California, Riverside, California 92521
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Introduction Isoprene [CH2=CHC(CH3)=CH21 is emitted into the atmosphere from vegetation during daylight hours (1-3). In the troposphere, isoprene reacts with OH radicals, NO3 radicals, and O3 (41, with the daytime OH radical reaction dominating (5- 7 ) . Previous product studies of the reaction of isoprene with the OH radical in the presence of NO, identified and quantified methyl vinyl ketone, methacrolein, and HCHO as the major products, with methylvinyl ketone and methacrolein formation yields of -0.32 and -0.23, respectively (8-10). Approximately 40% of the carbon has not been identified or quantified in the OH radical-initiated reaction, although organic nitrates and hydroxycarbonyls have been postulated to account for this “missing”carbon (4, 8, 9). In this work, we have used a direct air sampling, atmosphericpressure ionization tandem mass spectrometer to investigate the products of the OH radical reaction with isoprene and isoprene-dB in the presence of NO.
Results and Discussion The API MS spectra of the initial CH30NO-NO-isoprene (or isoprene-de)-air mixtures showed weak ion peaks at 69 * Authors to whom correspondence should be addressed.
Also at the Department of Soil and Environmental Sciences.
0013-936X/95/0929-2467$09.00/0
1995 American Chemical Society
ISOPRENE-de
mlz FIGURE 1. API MS spectra of irradiated CHSONO-NO-isopreneair and CHaONO-NO-isogrene-4-air mixtures.
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Experimental Section Reactions were carried out in a 6500-L Teflon chamber, equipped with black lamps for irradiation, at 296 f2 K and 740 Torr total pressure of purified air at -5% relative humidity. Hydroxyl radicals were generated by the photolysis of methyl nitrite-NO mixtures in air at wavelengths ’300 nm. The initial CH30N0, NO, and isoprene (or isoprene-de) concentrations were 1.2 x 1014molecule cm-3 each. Irradiations of CH30NO-NO-isoprene (or isoprene&)-air mixtures were carried out for 2 min at 20% of the maximum light intensity, resulting in reaction of 25-32% of the initial isoprene or isoprene-de. The Teflon chamber was interfaced to a PE SCIEX API I11 MS/MS direct air sampling, atmospheric pressure ionization tandem mass spectrometervia a 25 mm diameter x 75 cm length Pyrex tube, with a sampling flow rate from the chamber of -20 L min-’. The operation of the PE SCIEX API mass spectrometer instrument in the MS (scanning) and MS/MS [with collision activated dissociation (CAD)] modes is described elsewhere (11,12).Use of the MSlMS mode with CAD allows the “daughter ion” spectrum of a given ion peak observed in the MS scanning mode to be obtained. Isoprene (99+%)and isoprene-% (98%De)were obtained from the Aldrich Chemical Co. and Cambridge Isotope Laboratories, Inc., respectively.
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77
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amu ([M HI+)and 98 amu ([M NO]+)for isoprene and corresponding peaks at 77 and 106 amu, respectively, for isoprene-& together with water vapor and nitrogen cluster ions (11)from the chamber diluent air. Figure 1 shows the API MS spectra of the reacted isoprene and isoprene-dB mixtures. The intense peak observed at 71 amu in the API MS spectrum of the irradiated CH3ONO-NO-isopreneair mixture is due to the [M + HI+ ions of methyl vinyl ketone and methacrolein, known products of the OH radical-initiated reaction of isoprene (8-10). The ion peak at 77 amu observed in the isoprene-& reaction is then the [M HI ions of methylvinyl ketone-& and methacrolein-
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de.
As shown in Figure 1, additional prominent ion peaks are observed in the API MS spectra of reacted isoprene and isoprene-& including those at 101,87,83,and 55 amu for the isoprene reaction and those at 108, 92, 90, 89, and 46 amu for the isoprene-& reaction. The API CAD MSlMS daughter ion spectra of the 101 and 108 amu product ions from the isoprene and isoprene-dBreactions, respectively, are shown in Figure 2. It has been observed that fragmentation occurring in the API source region generally results in ions also observed in the CAD spectrum of the [M + H]+ ion (11, 12). Comparison of the CAD spectrum shown in Figure 2A with the API MS spectrum of the isoprene reaction shown in Figure 1 indicates that many of the ion peaks in the API MS spectrum may be attributed to a single product of molecular weight 100, namely, [M + H H20]+at 119 m u , [M + HI+ at 101 amu, [M + H-H201+ at 83 amu, and additional fragment ions at 73, 59, 55, and 43 amu. The remaining ion peaks from the reaction mixture
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A
SCHEME 1 OH
+ CH,=C(CH,)CH=CH,
- HOCH,C(CH,)CH=CH,
HOCH,C(CH,)=CHCH,
HOCH,C(CH,=CHCH,OO )
-
NO
NO,
isomer
HOCHC(CH,) =CHCH,OH 83
I?'
HO,
B
89
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HOCH,C(CH,)=CHCH,O
1
O2
HOCH,C(CH>)=CHCHO + H02
CH(O)C(CHJ=CHCH,OH
group of the alkoxy or P-hydroxyalkoxy radicals leads to dihydroxycarbonyls.] The ion peak at 101 a m u from the isoprene reaction is then attributed to the [M HI+ion of the hydroxycarbonyls HOCH2CH=C(CH&H0 and HOCH2C(CH3)=CHCH0.The ion peak at 108 a m u from the isoprene-ds reaction is attributed to the initial formation of DOCD2CD=C(CD3) CDO and DOCD2C(CD3)=CDCD0,followed by rapid D/H exchange of the OD group to an OH group in the presence of water vapor (11, 14). The CAD MSlMS spectra shown in Figure 2 can be rationalized by the expected fragmentation of the hydroxycarbonyls. The 83 amu peak in the isoprene reaction is the [M H-H20]+ion, and the 88,89, and 90 a m u peaks from the isoprene-& reaction are due to loss of D20, HDO, and H20, respectively, from the 108 amu ion. Loss of CO from these 83 amu (isoprenereaction) and 88, 89, and 90 amu (isoprene-& reaction) ions leads to the ion peak at 55 amu in the isoprene reaction and those at 60,61,and 62 a m u in the isoprene-d8reaction. The ion peaks at 43 (isoprene reaction) and 46 amu (isoprened8reaction) in the CAD spectra (Figure2) are due to [CH3COI- and [CD3CO]-,respectively. Standards of the hydroxycarbonyls HOCH$(CH3)= CHCHO and HOCH2CH=C(CH3)CH0are not available,and because the API MS sensitivity varies from one compound class to another (111,quantification of the hydroxycarbonyls was therefore not possible. Furthermore, because each of the two hydroxycarbonyls can be formed by isomerization andlor reaction with 0 2 (Scheme 1) of the two d-hydroxyalkoxy radicals formed after OH radical addition to the 1or 4-positions in isoprene, the relative importance of isomerization versus reaction with O2 for these hydroxyakoxyradicals cannot be determined from our present data. The CAD spectra of the weak ion peak at 87 amu from the isoprene reaction suggests that it is the [M HI' ion of a C4H602hydroxycarbonyl(s). The 92 amu ion from the isoprene-& reaction would be the deuterated analog, after OD to OH exchange. In conclusion, the spectra shown in Figures 1 and 2 indicate that one or both of the hydroxycarbonyls HOCH2CH=C(CH&HO and HOCH2C(CHs)=CHCHO (and their deuterated analogs) are formed from the isoprene and isoprene-& reactions. The previously postulated formation of these multifunctional products from the OH radical reaction of isoprene in the presence of NO (8, 9) is now confirmed.
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m/z FIGURE 2. API CAD MSMS spectra of (A) the 101 amu ion peak from the isoprene reaction and (B)the 108 amu ion peak from the isoprene4 reaction.
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are due to the [M HI+ of methacrolein and methyl vinyl ketone at 71 amu, water adducts of these, [M + H + HZO]', at 89 amu, an NO adduct of isoprene at 98 amu,and a weak ion peak at 87 amu, suggesting an additional product of molecular weight 86. Similarly, the CAD spectrum shown in Figure 2B shows that, apart from deuterated methyl vinyl ketone and methacrolein, the ion peaks observed in the isoprene-& reaction (Figure 1)are the 108amu ion peak and its fragment ions plus a weak ion peak at 92 amu. These data indicate that the major products of the isoprene and isoprene-d8 reactions observed with our API MSlMS are methyl vinyl ketone and methacrolein and their deuterated analogs and products giving rise to ion peaks at 101 and 108 amu, respectively. The OH radical reactions with isoprene and isoprene-d8 proceed almost entirely by initial addition of the OH radical to the > C=C bonds (4,131, In the presence of NO, initial OH radical addition at the 1-, 2-, 3-,or 4-positions with subsequent addition of 02 at the 2-, 1-, 4-, or 3-positions, respectively, leads to the formation of P-hydroxyakoxy radicals (4). The dominant tropospheric reaction of p-hydroxyalkoxy radicals is decomposition (41, leading to the formation of methyl vinyl ketone plus HCHO or methacrolein plus HCHO from isoprene ( 4 , 8, 9). However, as shown in Scheme 1, initial OH radical addition at the 1-position can also lead to the formation of the hydroxycarbonyls HOCH2CH=C(CH3)CH0and HOCH2C(CH3)= CHCHO by isomerization of the alkoxy radical HOCH2C(CH3)=CHCH20or by its reaction with 0 2 . Initial OH radical addition at the 4-position gives rise to the identical two hydroxycarbonyls by an analogous reaction scheme. [Note that isomerization via H-atom abstraction from the CH3 2468
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Acknowledgments The authors gratefully thank the U.S. Environmental Protection Agency for support of this research through Cooperative Agreement CR821787-01-0(ProjectOfficer,Dr. Marcia C. Dodge) and thank the National Science Foundation (Grant ATM-9015361) and the University of California, Riverside, for funds for the purchase of the PE SCIEX API
MSlMS instrument. While the research described in this article has been funded by this Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
( 7 ) Yokouchi, Y. Atmos. Environ. 1994,28,2651-2658. (8)Tuazon, E. C.;Atkinson, R. Int. J. Chem. Kinet. 1990,22, 12211236. .__.
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Literature Cited (1) Rasmussen, R. A. J. Air. Pollut. ControlAssoc. 1972,22,537-543. (2) Tingey, D.T.; Manning, M.; Grothaus, L. C.; Burns, W. F. Physiol. Plant 1979,47, 112-118. (3) Guenther, A. B.; Monson, R. K.; Fall, R. J. Geophys. Res. 1991,96, 10799- 10808. (4)Atkinson, R. 1.Phys. Chem. Ref: Data 1994,Monograph 2,1-216. (5) Martin, R. S.; Westberg, H.; Allwine, E.; Ashman, L.; Farmer, J. C.; Lamb, B. J. Atmos. Chem. 1991,13, 1-32. (6)Montzka, S. A.; Trainer, M.; Goldan, P. D.; Kuster, W. C.; Fehsenfeld, F. C.1. Geophys. Res. 1993,98,1101-1111.
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(9)Paulson, S. E.; Flagan, R. C.; Seinfeld, J. H. Int. J . Chem. Kinet. 1992,24,79-101. (10) Miyoshi, A.; Hatakeyama, S.; Washida, N. J. Geophys. Res. 1994, 99,18779-18787. (11)Atkinson, R.; Kwok, E. S. C.; Arey, J.; Aschmann, S. M. Faraday Discuss. Chem. SOC., in press. (12)Atkinson, R.; Tuazon, E. C.; Kwok, E. S. C.; Arey, J.; Aschmann, S . M.; Bridier, I. J. Chem. SOC. Faraday Trans., in press. (13)Atkinson, R. J. Phys. Chem. Ref: Data 1989,Monograph 1,1-246. (14)Dunlop, J. R.; Tully, F. P. J. Phys. Chem. 1993,97,6457-6464.
Received for review April 18, 1995. Revised manuscript received June 15, 1995. Accepted June 16, 1995. ES950266J
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