Environ. Sci. Technol. 1987, 21, 52-57
Mechanism for the Formation of Gaseous and Particulate Products from Ozone-Cycloalkene Reactions in Air Shiro Hatakeyama, * Masafumi Ohno,+Jian-hua Weng,$ Hiroo Takagi, and Hajime Akimoto The National Institute for Environmental Studies, P.O. Tsukuba-gakuen, Ibaraki 305,Japan
Both gaseous and particulate products of the reactions of ozone with C5 and C7cycloalkenes in air were analyzed. Major gaseous products were CO, C02, formic acid, and aldehydes consisting of C, dialdehyde, C,l dialdehyde, and Cn-l monoaldehyde. The sums of the primary yields of aldehydes were 34 f 15% for cyclopentene and 64 f 15% for cycloheptene. In cyclopentene reactions, formaldehyde and ethylene were additional major products with yields of 13 f 3% and 12 f 2%, respectively. The main particulate products were C, and C,-l dialdehyde, w-oxo carboxylic acid, and dicarboxylic acid, and the absolute yields of each component were determined as a function of ellapsed time after the reaction. Transformation of aerosols was clearly demonstrated to be dialdehyde w-oxo carboxylic acid dicarboxylic acid. A generalized mechanism of the ozone-cycloalkene reactions was proposed, and the branching ratio of each pathway of the Criegee-type intermediates was evaluated. The lower and upper limits of aerosol yield were determined to be 1 f 1 and - 5 % , 3 f 1 and 13 f 3%, and 4 f 1 and -10% for C6, Cg, and c7cycloalkenes, respectively.
-
-
Introduction
The importance of cycloalkenes as precursors of organic aerosols in photochemical smog has been suggested in the early studies ( I , 2 ) on the photochemical oxidation and ozone reactions of these compounds. Studies have since been devoted to characterize the chemical identity of the particulate products of these reactions, and dicarboxylic acids, dialdehydes, oxo carboxylic acids, oxo alcohols, etc. have been reported to be the major constituents (3-6). Field studies have also revealed that these kinds of organic aerosols are the most abundant secondary aerosols in the urban air (7, 8). The oxidation reaction of cycloalkenes is also important as a prototype reaction of the atmospheric oxidation of monoterpenes, which have been assessed to be possibly the most important source of organic aerosols in the troposphere (9, 10). As,for the reaction mechanisms, however, studies on the oxidation of cycloalkenes are quite limited. Except for the discussion by Grosjean and Friedlander (3),none of the study had been specifically aimed at elucidation of the reaction mechanism of ozone-cycloalkene reactions before we recently reported (11)on the ozone-cyclohexene reaction on the basis of the quantitative analysis of both gaseous and particulate products. In this study, gas-phase reactions of ozone with cyclopentene and cycloheptene were studied as an extention of our previous study (11)for the purpose of establishing a general scheme for ozonecycloalkene reactions. As for the product analysis of the ozone-cycloalkene reaction, Niki et al. (12) has presented Fourier-transform infrared (FTIR) spectra of gaseous products for cyclopentene and cyclohexene. Qualitative identification of particulate prod-
* Author to whom correspondence should be addressed. ‘Present address: Environmental Pollution Control Center Co. Ltd., Higashikojiya, Tokyo 144,Japan. Present address: Chinese Research Academy of Environmental Sciences, Beijin, China.
*
52
Environ. Sci. Technol., Vol. 21, No. 1, 1987
ucts has been reported by Grosjean and Friedlander (3) on the basis of mass spectrometry. Experimental Section
Experimental procedures and analytical techniques are similar to those used in the previous work (11). Briefly, experiments for quantitative analysis of gaseous products was carried out mainly in the evacuable and bakable smog chamber (-6 m3) by means of FT-IR spectroscopy (0.7-2 ppm of cycloalkenes and 0.7-2 ppm of ozone) and also in a quartz vessel (11L) for the runs with 1803. Gas chromatographic analysis of gaseous products was also performed by use of the 4-L bulbs as reactors (200 ppm of cycloalkenes and 270 ppm of ozone). Particulate products were analyzed quantitatively by means of gas chromatography (GC) after desolving the deposited aerosol mist in ether and treating with diazomethane for esterification of acids (initial concentrations of cycloalkene and ozone were 70-470 and 80-570 ppm, respectively, in a 4-L bulb). Cyclopentene and cycloheptene were commercially available from Wako Pure Chemical Industry, Ltd., and used after trap-to-trap distillation. Pimelic acid was obtained from Tokyo Kasei. Other compounds except for pimelaldehyde and 7-oxoheptanoic acid were prepared in the same manner as described before (11). Pimelaldehyde was prepared by analogy with the method to prepare adipaldehyde (13); oxidative cleavage of 1,2-cycloheptanediol (Aldrich) with lead tetraacetate was employed. Methyl 7-oxoheptanoate (methyl ester of 7-oxoheptanoic acid) was prepared directly by the method reported by was Schreiber et al. (14). Isotope-labeled ozone, 1803, (Nippon Sanso; atomic purity 99%) by prepared with 1802 silent discharge. Results and Discussion
Gaseous Aldehydes, Gaseous products from the cycloalkene (C,Hz,-J-ozone reactions measured by FT-IR are listed in Table I. The main products are aldehydes (C,.-l and C, dialdehydes and monoaldehydes),formic acid, CO, and COz. Ethylene and formaldehyde are also main products in the case of cyclopentene. As shown in Figure 1, the IR spectra of c4-6 monoaldehydes and C5-, dialdehydes are very similar, and it has been confirmed that IR absorption coefficients of dialdehydes for the band at 2715 cm-I are about twice as large as those of monoaldehydes (-0.28 Torr-l m-l) (11). In Table I, “formyls” is the estimated total yield of formyl (-CHO)groups (except for formaldehyde) based on the IR bands at 2715 cm-*, which can be equated to [ (AC,-, monoaldehyde) + 2(AC,1 dialdehydes) + 2(AC, dialdehydes)]/ (-Acycloalkene). The yields of monoaldehydes listed in Table I are based on the GC analysis in the high-concentration runs. No dependence of the yields of monoaldehydes on the initial concentration of reactants was observed. Since dialdehydes have much higher vapor pressure than oxo acids and dicarboxylic acids, they are expected to exist mostly in the gas phase in the smog chamber runs at the ppm concentration range. Therefore, the yield of dialdehydes can be calculated to be 23 f 14% and 46 f 12% for cyclopentene
0013-936X/87/0921-0052$01.50/0
0 1986 American Chemical Society
Table I. Yields (%) of Gaseous Products of Cycloalkene (C,Hz,-z)-Ozone Reactions reactant
formyls"
HCOOH
co
c-C~HB 57 f 27 11 f 3 35 f 16 80 f 20 12 f 1 18 f 2 C-Cl3H10 110 f 20 4 f 2 9fl c-C~HIZ Formyls = [2(ACHO(CH,),-,CHO) + B(ACHO(CH,),,CHO)
COZ
C2H4
HCHO
42 f 6 42 f 6 30 f 7
12 f 2 I* 0.3b
13 f 3 0 0
CH,(CH,),-,CHO
dialdehydes' 23 f 14 32 f 11
11 f l b
17 f b' 18 f 36
46 f 12
+ (ACH,(CH2),~,CH0)]/(-Acycloo1efin).bDetermined by GC. 'See text. I
I
I
I
I
0 8
e 03 V Formyls C2H4 c02 A HCOOH A HCHO
co
0
5a
. C
2 2 0.5
c
C
4-
m
:A
0,
VI
C
u0
n U
adipaldehyde
n
rc.
-
U L 3000
R
glutaratdehyde
2 000
Wavenumberlcm-' Flgure 1. IR spectra of C .& monoaklehydes and C,-C,
1000
dialdehydes.
and cycloheptene, respectively, by subtracting the yield of monoaldehyde from that of "formyls" and dividing the difference by 2. Total yields of gaseous aldehydes other than formaldehyde are estimated to be 34 f 15% and 64 f 15% for cyclopentene and cycloheptene, respectively. Table I clearly shows that the yields of C, and C,l gaseous aldehydes increase as the carbon number increases. Typical time profiles of the reactants and products in ozone-cyclopentene and ozone-cycloheptene reactions for the smog chamber runs are shown in Figures 2 and 3, respectively. The decay of "formyls" after reaching a maximum was observed in contrast to the constancy of other gases due possibly to the gas-to-particle conversion and deposition on the wall of dialdehydes. Particulate Products. Particulate products identified conclusively in this study are succinic acid [HOOC(CH2),COOH], glutaraldehyde [CHO(CH,),CHO], &OXOpentanoic acid [OPA, CHO(CH,),COOH], and glutaric acid [HOOC(CH,),COOH] from cyclopentene and glutaric acid, adipaldehyde [CHO(CH,),CHO], 6-oxohexanoic acid [OHA, CHO(CH2)4COOH],adipic acid [HOOC(CH2)4COOH], pimelaldehyde [CHO(CH2)&HO], 7-oxoheptanoic acid [OHpA, CHO(CH2)&OOH], and pimelic acid [HOOC(CH2)&OOH] from cycloheptene. Among these particulate products, glutaraldehyde, 5-oxopentanoic acid, and glutaric acid from cyclopentene have been analyzed qualitatively by Grosjean and Friedlander (3). Although succinaldehyde [CHO(CH,),CHO] and 4-oxobutyric acid [CHO(CH2)2COOH]would also be produced from the cyclopentene reaction as assessed (3) from the mass fragmentation pattern, conclusive identification could not be made in this study due to the lack of authentic samples. Two additional unidentified products, one of which is acidic and the other of which is nonacidic as judged by whether it eluted without esterification with diazomethane, exist for the ozone-cycloheptene reaction. Table I1 shows the yield of each particulate product as a function of elapsed time after the reactants were mixed.
e
x i
0
I
20 Timelmi n
"10
I
I
30
Flgure 2. Typical time profile of reactants and products of cyclopentene (1.0 ppm)-ozone (0.72 ppm) reaction in the smog chamber. I
I
I
I
1
Tirnelrnin
Flgure 3. Typical time profile of reactants and products of cycloheptene (0.98 ppm)-ozone (0.92 ppm) reactlon in the smog chamber.
As in the case of the ozone-cyclohexene reaction ( I I ) , all dialdehydes decreased monotonically, oxo acids increased first and then decreased, and dicarboxylic acids increased monotonically. These results clearly indicate that the sequential oxidation process of dialdehyde oxo acid dicarboxylic acid takes place in the reaction systems possibly on the wall surface. Such a sequential oxidation of the difunctional compounds is also expected to occur on the aerosol surface in the ambient atmosphere and would explain the presence of dicarboxylic acids in the sampled aerosol (3-6). Figure 4 for cyclopentene and Figure 5 for cycloheptene depict the dependence of the aerosol organic carbon (AOC) yields as measured at 1 h of reaction time on the initial concentration of reactants. Here, the total AOC yield (%)
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-
Envlron. Sci. Technol., Vol. 21, No. 1, 1987 53
Table 11. Yields (%) of Deposited Difunctional Compounds against Consumed Cycloalkenes from the Cyclopentene- and Cycloheptene-Ozone Reactiona as Functions of Time
cyclopentene,
cycloheptene
products
5 min
dialdehyde oxo carboxylic acid dicarboxylic acid
2.2
f 0.8 1.3 f 0.5 0.4 f 0.1
C7 compounds
C6compounds
C5 compounds
l h
24 h
5 min
l h
24 h
5 min
l h
24 h
1.5 f 0.9 1.8 f 0.3 0.6 0.2
0.3 f 0.2 1.4 0.2 1.7 f 0.2
0.7 f 0.1 2.5 f 0.2 0.3 f 0.1
0.3 0.1 3.2 f 0.2
0.2 f 0.1
1.2 f 0.2 4.9 f 0.3 2.5 f 0.3
1.0 f 0.1 6.1 f 0.5 4.5 f 0.5
0.7 f 0.1
*
*
2.3 f 0.2
0.9 f 9.2 4.2 f 0.2 sum 3.9 3.9 3.4 3.5 4.4 6.7 a Cyclopentene, 270 ppm, and ozone, 340 ppm; cycloheptene, 200 ppm, and ozone, 270 ppm.
8.6
11.5
.
2.It
I
100
I
200 LO10 / pprn
I
I
300
400
1J
Figure 4. Total aerosol organic carbon (AOC) yield for cyclopentene-ozone reactions as a function of initial concentratlonof cyclopentene at l-h reaction time. The circles wlth error bars are the averages of more than three data and errors are 1 SD. 20k
‘
I
I
I
I
I
I
A I
100
200 300 [O~o/ppm
I
I
A
400
Figure 5. Total aerosol organic carbon (AOC) yield for cycloheptene-ozone reactions as a function of initial concentrationof cycloheptene at l-h reaction time. The circles with error bars are the averages of more than three data and errors are 1 SD.
is defined by 100 X C[(Aparticulate product/mol) X (carbon number of product/carbon number of reactant cycloalkene)]/ (-Acycloalkene/mol). In these runs, the ratio of the initial concentration of cycloalkene to ozone was kept constant at -0.8, and the concentration of cycloalkene was changed from -70 to -470 ppm. As shown in Figures 4 and 5, the total yield of particulate products increased as the concentration increased. For cyclopentene, the plots gave a straight line within the concentration range studied. A least-squares calculation gave the yield of total AOC at the low concentration limit to be 3.1%. Since succinaldehyde and 4-oxobutyricacid are not counted, the total yield should be a little higher than this and is estimated to be -5%. For cycloheptene, the plots gave a curved line, as is shown in Figure 5. If we take the unidentified two products and a large uncertainty of the measured yields a t low concentration into consideration, the total AOC yield at the low concentration can be es54
Environ. Sci. Technol., Vol. 21, No. 1, 1987
4.0 f 0.4 8.8 f 0.5 13.5
timated to be -10 % These values can be compared to the AOC yield, 13 f 3%, for cyclohexene (11). The aerosol yield of cyclopentene is thus much lower than that of cyclohexene, and that of cycloheptene is about the same as that of cyclohexene within experimental error. Since the ambient concentrations (3)of cycloalkenes are much lower (by a factor of ~ 1 0than ~ )the initial concentrations employed in this experiment, the above values obtained in this study should be taken as upper limits for estimating the AOC yield in the atmosphere. It would be worthwhile to compare the values obtained in our study for cyclopentene ( - 5 % ) and cyclohexene (13 f 3%) (11) with the AOC yield in the photooxidation of these compounds (0.5-2.0 ppm) in the presence of NO, (0.15-0.30 ppm) studied by Grosjean and Friedlander (3),since they reported that the aerosol formation is correlated well with the ozone reaction rather than with the OH reaction under their experimental conditions. The AOC yields reported in the photooxidation were 33-39% and 5-17% for cyclopentene and cyclohexene, respectively. Thus, although the initial concentration range of their study is 2 orders of magnitude lower than ours, their value for cyclohexene agreed well with ours. On the contrary, their AOC yield for cyclopentene is unreasonably much higher than the value obtained in this study. Since our value for cyclopentene, which is smaller than that for cyclohexene, seems to be consistent with the higher yield of gaseous products, their high value may be subjected to some artifact during sampling as they noted (3). FragmentationProducts. Formation of formaldehyde and a high yield of ethylene is characteristic of the ozone-cyclopentene reactions (12), although a small amount of ethylene is produced also in the ozone-cyclohexene and ozone-cycloheptene reactions (see Table I). The 1a03-cyclopentene reaction in unlabeled oxygen/ nitrogen revealed that the product formic acid was completely labeled with I8O (HC1s0180H), whereas formaldehyde was not labeled at all. Thus, it can be concluded that formic acid is the primary product of ozone-cycloolefin reactions (not a oxidation product of formaldehyde), while formaldehyde is a secondary product of fragmentation reactions. Almost all C02 and CO were fully labeled, so that these compounds are also primary products. AS shown in Table I, such a fragmentation reaction is minor for cycloheptene since the yields of formic acid, CO, and ethylene are very low in comparison with the cases of cyclopentene and cyclohexene. Table I demonstrates a clear tendency that the yield of fragmentation products (C, and C2products) decreases while the yield of “formyls”, i.e., gaseous aldehydes except formaldehyde, increases as the ring size of cycloalkene increases. The enthalpy of the reaction of C-C,H~,-~+ O3 HOOC(CHz),-&HO was calculated to give (1) C-CbHs + O3 CHO(CHz),COOH AH = -180.02 kcal/mol
-
-
+
c-C6HI0 O3
-
-
CHO(CH2)&OOH
(2)
Scheme I
AH = -175.52 kcal/mol c-C,HI2
+ O3
CHO(CH2)6COOH
(3)
AH = -179.52 kcal/mol by use of AHo;s of cycloalkenes and HOOC(CH2),zCHO estimated from the group additivity rule (15). Since the AH values do not differ very much, the decrease of the fragmentation pathway as the ring size increases would be explained by the effect of the number of vibrational degrees of freedom. Generalized Reaction Scheme and Branching Ratio, While the mechanisms of ozone-olefin reactions have been studied extensively (16),ozone-cycloalkenereactions have been scarcely investigated. The product analysis for cyclopentene and cycloheptene in this study has revealed the similarity of the product type to that for cyclohexene observed in our previous study (11). From these results, we propose generalized reaction schemes for ozone=,5, 6, 7) reactions as Schemes cycloalkene ( C - C ~ H ~ n, - ~ I and 11. In Scheme I, steps 1and 2 are envisioned as molecular and radical mechanisms of COz elimination, which corresponds to the reaction of the simplest Criegee intermediate, CH200,giving C02 H2and COz + 2H, respectively (17). Thus, the formation of the C,-l monoaldehydes, which were detected for the first time in these reaction systems, can be explained by the molecular mechanism of the C02 elimination. The C,l dialdehydes are assessed to be produced through the CHO(CHz),3CH20 radicals, which would be formed by the self-reaction of CHO(CHz),-3CH200 radicals formed via step 2. Disproportionation reaction of CHO(CH2),-3CH200could also give CHO(CH2),3CH0 as well as CHO(CH2),-3CH20H, although the latter compound could not be identified in this study. Subsequent oxidation of C,-l dialdehyde, possibly via heterogeneous processes, gives C,l oxo carboxylic acids and C,-l dicarboxylic acids as noted earlier. As shown in Scheme I, the primary C, oxo carboxylic acids are thought to be produced via collisional stabilization of the hot species formed by isomerization of the Criegee-type intermediates. The puzzling thing is the relatively high yield of C, dialdehyde in the ozone-cycloalkene reactions. As shown in Figures 2 and 3, nonstoichiometric excess consumption of cycloalkenes as referenced to ozone is also observed both for cyclopentene and for cycloheptene. These results might be due to the side-chain reactions involving OH radicals as proposed in our previous study (11). The C, dicarboxylic acids are all secondary oxidation products as expected and confirmed before. Formation of ethylene, primary production of formic acid, and secondary production of formaldehyde can be explained by the reaction mechanism depicted in Scheme 11. Production of approximately equal amounts of formic acid, formaldehyde, and ethylene in the case of cyclopentene is consistent with the proposed mechanism while some unspecified products other than ethylene would compensate the yield of formic acid for cyclohexene and cycloheptene. According to the proposed reaction scheme, the branching ratio of each reaction pathway for the ozonecycloalkene reaction could be evaluated on the basis of the product distributions given in Table I. First, the ratio of the molecular mechanism of C02 elimination from the [HCO(CHZ),&OOH]~ can be equated to the yield of monoaldehydes. Then, the remaining yield of C02is at-
Scheme I1
U
n=5
+
n-7
5.-
