Environ. Sci. Technol. 1993,2 7 , 278-283
Product Formation from the Gas-Phase Reactions of OH Radicals and with ,8-Phellandrene
O3
Hannele Hakola,t Baslma Shorees, Janet Arey, * J and Roger Atkinsod
Statewide Air Pollution Research Center, University of California, Riverside, California 9252 1
rn 4-Isopropyl-2-cyclohexen-l-onewas identified as a product of the gas-phase reactions of P-phellandrene with OH radicals (in the presence of NO,.) and with ozone, and formation yields of 0.29 f 0.07 and 0.29 f 0.06, respectively, were determined. Sampling several 500-L volumes from the reaction chamber onto polyurethane foam solid adsorbent, followed by solvent extraction and HPLC cleanup, gave sufficient product for identification by lH NMR. Introduction Isoprene, monoterpenes (Cl,)H16), sesquiterpenes (CISH2J, and oxygenated compounds are released into the atmosphere by vegetation (1-15). The magnitude of these biogenic non-methane organic emissions is comparable to the emissions of non-methane hydrocarbons (NMHC) from anthropogenic sources in the United States (16) and is estimated to be a factor of 10 higher than anthropogenic NMHC on a worldwide basis (17, 18). Although large uncertainties in the overall biogenic non-methane organic emissions inventory remain, the monoterpene hydrocarbons are known to be a significant fraction of the overall biogenic emissions (16, 18, 19). The monoterpenes are highly reactive toward OH radicals, NO3 radicals, and 0, (20-25), and the reactions of these compounds and isoprene appear to dominate the chemistry of the planetary boundary layer (19, 26-29). Furthermore, the emissions of isoprene and monoterpenes from vegetation have been implicated in the formation of enhanced ozone levels in urban (30,31)and rural (31,32) areas. Because of their high reactivity, the impact of isoprene and the monoterpenes on the chemistry of the free troposphere appears to be at most minor (19,26,27). While the rate constants for the gas-phase reactions of the monoterpenes with OH and NO, radicals and O3have been measured at room temperature (20-25,33),there are less data available concerning the products of these reactions and the reaction mechanisms under atmospheric conditions (34-46). To date, the only quantitative product yield data arise from the studies of Hatakeyama et al. (36, 41), concerning the reactions of OH radicals and 0, with a- and &pinene, and of Arey et al. (38) for the reactions of the OH radical with a-and P-pinene, 3-carene, limonene, myrcene, sabinene, and terpinolene. Carbonyl compounds were the major products observed (36,38,41),although the reported formation yields from the OH radical reactions show significant discrepancies. For many of the monoterpenes, such studies are rendered difficult due to the lack of commercially available standards for the expected product species (38). In this work, we have extended our previous study (38) to investigate the products of the gas-phase reactions of 0-phellandrene with OH radicals and 0,. 0-Phellandrene (1-methylene-4-isopropylcyclohex-2-ene) is a monoterpene
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+Visiting Scientist. Present address: Air Quality Department, Finnish Meteorological Institute, Helsinki, Finland. Also Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521.
*
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Environ. Sci. Technol., Vol. 27, No. 2, 1993
emitted from a variety of coniferous trees (4-7,9,14,47, 48) and Liquidambar styraciflua (American sweet gum) (14)and is the major emission from commercial tomatoes (12). Because of the lack of available standards for the expected product(s), an experimental technique was developed and tested to allow sufficient amounts of the products formed in these reactions to be collected and purified for spectroscopic identification. Experimental Section The reactions of P-phellandrene with OH radicals and 0, were carried out in a 6400-L all-Teflon chamber equipped with two parallel banks of blacklamps for irradiation, in a manner similar to our previous kinetic study of the reactions of OH and NO, radicals and 0, with Pphellandrene (25). All reactions were carried out at 297 f 2 K and at 740 Torr total pressure of pure air. Product Quantification from Small-Volume Samples. As described previously (25), OH radicals were generated by the photolysis of methyl nitrite (CH,ONO) in air at wavelengths of >300 nm
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CH30N0 + hv CH30 + O2
HO2 + NO
+ NO HCHO + H02 OH + NO2 CH,O
+
and NO was added to the reactant mixtures to avoid the formation of O3 and hence of NO, radicals. The initial reactant concentrations were as follows (in molecule cm-,): CH,ONO, (1.0-2.5) X 1014;NO, -2.4 X 1014;and P-phellandrene, 2.2 X lo1,. Irradiations were carried out at 20% of the maximum light intensity for 1-4 min. A limited amount of a purified sample of P-phellandrene (90.2% P-phellandrene/9.8% limonene) was available, and this sample was used for the product quantification studies. As noted below, the product was isolated from a technical formulation of P-phellandrene and limonene. The 0, reactions were carried out in the presence of sufficient cyclohexane to scavenge the OH radicals formed in the 0, reaction with P-phellandrene (49). The initial reactant concentrations were as follows (in molecule cm-,): cyclohexane, 1.2 X 10l6. P-phellandrene (1.5-1.8) X Five additions of O3 in O2 diluent were made to the chamber in each experiment (each addition corresponding to initial concentrations of O3in the chamber of -5 X 10l2 molecule cm-,). The purified (90.2%) P-phellandrene sample was used in these 0, reactions. 0-Phellandrene and the reaction products were quantified by gas chromatography with flame ionization detection (GC-FID). As expected (50), isomeric monoterpenes have equal FID responses (12). Similarly, isomeric ketones should give FID responses equivalent to one another (50). Therefore, a monoterpene calibration factor was used to quantify P-phellandrene and a calibration factor developed from standards of 6,6-dimethylbicyclo[3.l.l]heptan-2-one was used to quantify the reaction product. p-Phellandrene was analyzed using two methods: in one, 100-cm3gas samples were collected from the chamber in
0013-936X/93/0927-0278$04.00/0
62 1993 American Chemlcal Society
all-glass, gas-tight, syringes and introduced through a stainless steel (SS) loop and gas sampling valve onto a 10 f t X 0.125 in. SS column of 10% Carbowax E-600 on C-22 firebrick, operated at 100 "C; and in the second, 100-cm3 gas samples were collected from the chamber onto Tenax solid adsorbent for subsequent thermal desorption at 225 "C onto a 15-m DB-5 megabore column held at 0 "C and then temperature programmed to 200 "C at 8 "C mi&. The reaction products were analyzed using the thermal desorption/DB-5 megabore column system. As noted previously (25), the presence of NO2 in the OH radical reaction system led to a marked loss of 8-phellandrene during the adsorption/ thermal desorption analysis procedure, presumably due to a reaction of the 0-phellandrene with NO2 on the Tenax solid adsorbent during the sampling, and this precluded the use of the thermal desorption/DB-5 column system for analysis of 8-phellandrene in the OH radical reactions. Thus, the sampling loop/ Carbowax E-600 column system was used for the quantification of 0-phellandrene in the OH radical reactions, with the products being monitored by the thermal desorption/DB-5 column system. For the O3reactions, both the 0-phellandrene and reaction products were analyzed by thermal desorption. In addition, 50-100-cm3 gas samples were collected from the chamber onto Tenax solid adsorbent for analysis by combined gas chromatography/mass spectrometry (GC/ MS), using thermal desorption onto a 50-m HP-5 fusedsilica capillary column in a Hewlett-Packard (HP) 5890 GC interfaced to a HP 5970 mass selective detector (MSD) operated in the scanning mode (38). As noted above, the 90.2 % O-phellandrene/9.8% limonene sample was used, and the 0-phellandrene and limonene were not resolved with either the Carbowax E600 or DB-5 column, but could be resolved by GC/MS using the mass fragment ion chromatograms. In calculating the product yields, the presence of reactive limonene in the p-phellandrene sample was taken into account, as described previously by Shorees et al. (25). Product Identification from Large-Volume Samples. (A) Production and Collection. Experiments were carried out to generate high concentrations of the products in the chamber with the products then being collected from the majority of the chamber volume for subsequent product separation and identification. On the basis of the results of our kinetic (25) and product studies of P-phelthe O3reaction landrene (see below) and limonene (38,511, was most suitable. Thus, while the product yields from the OH radical and O3 reactions with 0-phellandrene are similar, the dark reaction of NO2with 8-phellandrene (25) makes the OH radical reaction more complex and high product concentrations difficult to achieve. Furthermore, using the sample collection and analysis procedures employed here, the O3reaction with limonene leads to no significant interfering products (51). Reactions of O3with 0-phellandrene were carried out as described above, but in the absence of added cyclohexane. Because further samples of the 90.2% P-phellandrene/9.8% limonene mixture were unavailable, a 40% P-phellandrene/GO% limonene mixture was employed in these experiments. Analyses for reactants and products were carried out during these reactions by GC-FID using the DB-5 column. For the experiment which resulted in the spectroscopic identification data presented here, the initial reactant concentrations were -1.1 x 1014molecule li8-phellandrene and -2.0 X 1014molecule monene with -3.5 X 1014molecule cm-3 O3added to the chamber in seven equal additions. In this experiment, after
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94% of the initially present P-phellandrene + limonene had reacted, a -5000-L-volume gas sample was collected onto eight polyurethane foam (PUF) plugs, with the collection time for each PUF plug being limited to 0.5 min (corresponding to -600 L per PUF plug) to minimize breakthrough of the relatively volatile product species. (B) Product Isolation. Development and testing of the extraction and high-performance liquid chromatography (HPLC) separation procedures for the isolation of the product were carried out using nopinone (6,6-dimethylbicyclo[3.l.l]heptan-2-one) as a surrogate species for the 8-phellandrene product(s). To estimate the efficiencies of collection on the PUF plugs, solvent extraction, and the subsequent separation procedure, 4.0 X 1013molecule cmT3 nopinone was introduced into the 6400-L all-Teflon chamber and a 2000-L-volumegas sample collected onto six PUF plugs, collecting on each plug for 0.5 min. After collection, the PUF plugs were each extracted with 200 mL of dichloromethane for 4 h, and the extracts were concentrated with a rotary evaporator to 2 mL in preparation for purification by HPLC. The concentrations of the nopinone in the various extracts were determined by GCFID using a H P 5890 GC with a 30-m Supelco PTE-5 column. The recovery of nopinone from the PUF plug collection and extraction procedures for the six PUF plug samples varied from 44% to 75%, with an average efficiency of 58%. At least some of this variation in collection and extraction efficiency was probably due to differences in the gas volumes sampled by the different PUF plugs. While a constant interval was utilized, the high-volume sampler motor was not flow-controlled and differences in the resistance of the PUF plugs would lead to variations in the sampling flow rate. Losses of nopinone during the rotary evaporator concentration of the dichloromethane extracts to 2-mL volumes were small, and the average recovery during this stage of the procedure was 84%. HPLC fractionations of the concentrated extracts were performed using a HP 1050 HPLC system interfaced to a H P 1040M diode-array detector. An isocratic solvent program with a 60% methanol/40% H 2 0 mixture was used, since this program allowed separation of the 0phellandrene product of interest from the reactants and other reaction products. The HPLC fractionation was followed by solvent exchange into CDCl,, using solid-phase extraction (SPE) on octadecyl columns (500 mg, B & J Inert SPE System, Burdick & Jackson). The recovery of nopinone from HPLC followed by solid-phase extraction was 110% when the HPLC eluent was placed directly on the SPE column. Therefore the majority of the methanol was removed by rotary evaporation prior to placing the eluent on the SPE column. A small amount of methanol (-1 mL) was left in the sample to avoid volatilitization of the nopinone from the solution (passing additional water through the SPE column prior to eluting the organic product with solvent reduced the methanol in the final deuterated solvent to an acceptable level). After addition of the sample to the extraction column, water was removed by the application of reduced pressure. The column was then dried with a slow flow of N2 gas for 5 min and the product eluted with 1mL of CDC13. Recoveries of 50430% were achievable from the combined HPLC fractionation and SPE steps. Therefore, the overall recovery of nopinone from the chamber was -25-30%, thus allowing sufficient product to be collected from a single chamber experiment to allow full spectroscopic characterization. The above procedures were then applied to isolate the product from the P-phellandrene reaction with 03. Nopinone was used as an internal standard to quantify the
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Environ. Scl. Technol., Vol. 27, No. 2, 1993 279
1%
A
43
,
;19 ,
,;;3 120
,
,'I;
120
00
140
MASSXHARGE 1708
B
product has a molecular weight of 138, compared to that of P-phellandrene of 136. In order to identify this product, large-volume samples were collected on PUF plugs from the chamber after reacting Q3 with 40% p-phellandrene/60% limonene mixtures, as described above. '3' ince limonene has a rate constant for reaction with O3which is a factor of 4.4 higher than that for P-phellandrene (22, 25), then at the end of the reaction, after 94% of the initially present @-phellandrene+ limonene had reacted, the /3-phellandrene/limonene concentration ratio is calculated to have been >200 and the product concentration (as noted above, no liwas -2.4 X 1013 molecule monene products were observed). The GC-FID trace from the PUF plug extract again showed one dominant peak. HPLC was used to isolate this product from other minor product peaks, residual parent compound, and interferences from the PUF plug material. The amounts of the p-phellandrene product obtained from the PUF plug collection and solvent extraction steps ranged from 0.78 to 2.6 mg (average 1.9 mg) for the various PUF plugs, a recovery of -50%. The recovery of the HPLC fractionation and solvent exchange with SPE was 60%, thus giving 1 mg of the 8-phellandrene product from a single PUF plug sample. The GC/FTIR absorption spectrum of the product is shown in Figure 1B. The )C=O absorption at 1708 cm-', together with the olefinic C-H absorption at 3039 cm-', suggests (52)that this product is an a,@-unsaturatedcarbonyl. Consistent with a molecular weight of 138 and the infrared evidence for a )C=C-C=O unit, the product of the OH radical and O3 reactions with p-phellandrene is (I) on the basis identified as 4-isopropyl-2-cyclohexen-l-one of its lH NMR.
-
0 4000
3500
3000
2500
2000
1500
1000
FREQUENCY (cm-1) Flgure 1. (A) Mass spectrum kom GC/MS analysis of the product. (B) I R spectrum from GC/FTIR analysis of the product.
recovery of the 0-phellandrene product by GC-FID analysis, as described above. (C) Spectroscopic Identification. The HPLC separation with diode-array detection allowed measurement of the product's UV absorption spectrum from 190 to 600 nm. The mass spectrum (MS), infrared absorption spectrum, and 'H NMR spectrum were obtained for the product species using the HP 5890 GC-HP 5970 MSD system described above for the mass spectrum, a H P 5965 FTIR detector interfaced to a H P 5890 GC with a 25-m HP-5 thick phase (0.52 pm) fused-silica capillary column for the IR spectrum, and a GE 300 MHz NMR spectrometer for the 'H NMR spectrum. Chemicals. The chemicals used, and their stated purity levels, were cyclohexane (high-purity solvent grade), American Burdick and Jackson; nopinone (6,6-dimethylbicyclo[3.l.l]heptan-2-one;98%) and 2-cyclohexen-l-one (97%), Aldrich Chemical Co.; and NO (199.0%), Matheson Gas Products. Samples of 90.8% p-phellandrene/9.8% limonene and 40% P-phellandrene/GO% limonene were generously donated by Union Camp Corp. Methyl nitrite was prepared and stored as described previously (23),and O3was generated as needed using a Welsbach T-408 ozone generator. Results Samples (loO-cm3volume),collected on Tenax adsorbent and analyzed by GC-FID and GC/MS, from the OH radical and O3 reactions with @-phellandreneshowed the presence of a single, identical, major product from both reactions, and the mass spectrum of this product is shown in Figure 1A. The mass spectrum indicates that this 280
Environ. Scl. Technoi., Vol. 27, No. 2, 1993
1
The chemical Lifts (ppm), multiplicities (L, doL et; t, triplet), coupling constants (Hz), and the number of protons for the product in CDC13were as follows: 6.91 (dt, J = 10.3,2.0, 1H), identified as H,;6.02 (dd, J = 10.3, 2.9, 1 H), identified as Ha; 2.58-2.48 (dt, 1 H); 2.41-2.28 (m, 2 H); 2.07-1.98 (m, 1H); 1.9-1.7 (m, 2 H); 0.98 (d, J = 4.48, 3 H), identified as CH, group hydrogens; 0.96 (d, J = 4.46, 3 H), identified as second CH, group hydrogens. The NMR spectrum is in agreement with that reported by Hawley and Schreiber (53). The chemical shifts of the olefinic hydrogens Ha and Hb are similar to those measured for the corresponding hydrogens in 2-cyclohexen-l-one of Ha = 6.02 ppm and Hb = 7.02 ppm. Furthermore, the ultraviolet/visible absorption maximum of the 8-phellandrene product at 232 nm was similar to that for 2cyclohexen-l-one (Amm = 235 nm). The concentrations of 4-isopropyl-2-cyclohexen-l.-one were measured in the OH radical and O3 reactions with @-phellandrene,assuming, as noted above, that the GC-FID response for 4-isopropyl-2-cyclohexen-l-one was identical to that for nopinone (50). As discussed previously (25), p-phellandrene reacts with NOz in the dark and in the presence of NO (as in the CH,ONO/NO/NO,/air mixtures used in the OH radical reaction system) OH radicals are generated, leading to further dark loss of the 0-phel-
landrene. Because of this dark decay of the 0-phellandrene in the CH,ONO/NO,/P-phellandrene/air mixtures, only one irradiation was carried out per reaction mixture, and the 0-phellandrene concentrations measured before and after the irradiation were corrected (assuming first-order kinetic decays) to the midpoint of the irradiation period (25). These corrections to the 0-phellandrene concentrations measured immediately before and after the irradiation period were
