Fourier transform infrared ( F T I R )studies are being conducted to answer questions concerning the chemical mechanism of
Atmospheric ozone-olefin reactions Hiromi Niki Paul D. Maker Carleton M. Savage Larry P. Breitenhach Ford Motor Company Research Staff Dearborn, Mich. 48121 Among various classes of organic compounds present in the troposphere, the olefins are unique in exhibiting significant reactivity toward ozone (0,) as well as toward the hydroxyl (HO) radical. Numerous potentially important roles of the 0,-olefin reactions have been recognized for some time. In brief, these reactions can provide mutual sinks for both 0, and the olefins and concomitantly serve as sources for partially oxidized compounds, e.g., CO, aldehydes, ketones, and organic acids. Some of the intermediates formed in the O3-olefin reactions can also lead to HO-radical chain reactions, regenerate O3by oxidizing NO to NO*, and convert SO2 to sulfate aerosols. Existing theoretical and experimental bases for these reaction mechanisms have been reviewed recently by Herron and his co-workers (3). A general scheme for the 0,olefin reactions, which emerged over the years, can be represented by reaction 1 followed by reactions 2a-2d. 312A
Environ. Sci. Technol..
VoI. 17, No. 7, 1983
L
General scheme for 03-olefin reactions
In memoriam: Bernard Weinstock This article is based largely on material presented at the Atmospheric Chemistry Symposium, held at the 1981 Fall National American Chemical Society Meeting in New Yo& and organized by the late Bemard Weinstock. (It also contains additional updated information.)For that meeting, Weinstock had suggested that Hiromi Niki address unanswered questions concerning the mechanism of gas-phase reactions that are important in the troposphere. Comprehensive assessments of chemical kinetics data needs for modeling purposes are made from time to time by various groups ( 1. 2). This article is therefore intended primarily to illustrate the current status of knowledge and to point out as-yet unanswered questions raised by ongoing Fourier transform infrared (FTIR) studies of the reactions between ozone (Os) and several differenttypes of olefins,such as simple mono-, di-. chloro-, and cyclic olefins.
+
>c=c
0,I ,(-
W0\OI / )
/c--C, --t
=,=,
b 0 0+
/ Criegee
(1)
intermediate \c--00
--
decomposition ( 2 a )
/ (forexample, CO, CO,. RH,
RO,,
RCO; R = H or alkyl)
isomerization
+ RCHO
(2b)
products,
suchas \ I
0-
,cy
+ (H,O,SO,. NO)
-+
!/
(Zc)
\
products (2d)
The primary ozonide initially formed in reaction 1 decomposes rapidly to the Criegee intermediate and the accompanying carbonyl product. The Criegee intermediate may isomerize to other more stable configurations prior to unimolecular and bimolecular processes, as shown in reactions 2a-2d. Thus, numerous competing reactions involving the Criegee intermediate and the ensuing secon-
0013-936X/83/09150312A501.50/0
@ 1983 American Chemical Society
FIGURE 1
Fourier transform infrared ( m R ) spectrometer system
-Multiple optical pass long-path (180 m) photoreactor
-Pyrex
-UV
tube reactor/ absorption cell
fluorescent tubes
Elliptical cross section (gold-coated, tapered. metal-walled cell) Multiple optical pass intermediate path (22 in) heated reactor Single optical pass photoreactors
Optical path of IR light Flat mirrors
reference laser
Eiect&al signal lines
Maanetic Multiple interactive storage-display terminals
tap;, archives
Auxiliary
PDP 11/60
Network link to mainframe comwter
Software. computer printouts
-Envirm. %I. Technol., Vol. 17. No. 7. 1983
313A
dary free radicals can give rise to the formation of a large variety of products. For the majority of the olefinic compounds these products are still poorly characterized. Clearly, togain further insight into these complex reactions, it is imperative to identify and quantify the relevant products. The current knowledge of atmospheric reactions is, in general, limited largely by the lack of analytical methods suitable for monitoring the reactants and products under simulated atmospheric conditions. However, in recent years, the long-path Fourier transform infrared (FTIR) method has emerged as a powerful tool for this purpose ( 4 , 5 ) . Several new, previously unknown aspects of the O3-olefin reactions are revealed with this method. Long-path FTIR facility Infrared (IR) absorption spectroscopy offers many advantages as an analytical technique for the laboratory study of atmospheric chemistry. It provides a means for the in situ, nondestructive measurement of reacting systems by the simple expedient of using the chemical reactor itself as the IR absorption cell. The spectra of the relatively simple gaseous molecules encountered under simulated atmospheric conditions are extremely detailed and provide definitive identification. Long absorption paths provide sensitivities down to a few parts per billion (ppb). However, to exploit this detail and sensitivity fully and also to enable use of Beer’s law for direct quantitative concentration measurements, high instrumental resolution [>O.l cm-’ (spectra)], which is timeconsuming and cumbersome, is needed. Extended data acquisition times are required in order to obtain adequate records of signal-to-noise ratio (SIN). These otherwise prohibitive constraints were lifted in the mid-1970s with the advent of the FTIR interferometer spectrometer (6). It provides digitized, computer-based spectra of extremely accurate wavelength calibration and excellent S/N at rates 1000 times faster than had been possible with earlier instruments. This remarkable feat is achieved primarily by multiplexing the IR signals. Each IR wavelength element is modulated by a scanning Michelson interferometer at a characteristic frequency and recorded during theentire measurement period, whereas earlier dispersive machines discarded all hut one wavelength element, measured it briefly, then advanced to the next. 3141
Environ. Sci. Technol.. Vol. 17, No. 7, 1983
Since theS/N grows as thesquare root of the measurement interval, -50 000 resolution elements in a spectrum imply a direct improvement factor of -700. However, the raw data emerging from the interferometer spectrometer have to be demultiplexed. This is accomplished by applying a Fourier transform, the mathematical operation from which this method derives its name. Figure 1 shows the apparatus employed in the studies to be described. At its center is the interferometer (currently an Idac model 1000). with its integral reference laser, and the IR detector, a germanium:copper element, cooled to 12 K by a closed-cycle
liquid-helium refrigerator. Through a simple mirror rearrangement, any one of four different absorption cellchemical reactors can he selected. Three of these are surrounded by ultraviolet (UV) lamps for photochemical studies; the fourth can he heated. This choice enables studies at reactant partial pressures ranging from tens of parts per billion to tens of torr. Simple construction The reactors are simply constructed and demountable for cleaning. This facilitates exploratory studies, wherein the interplay between reactants, concentrations, and diluents can be surveyed quickly. To acquire, Fourier-
frequency regjona*aC (a) i = 0 min
C~HI(9.46) +-I 0 3
1
(10.10)
A
* CzH4
C2H4
I
‘(b) t = 15 min
coz
HCHO
+ l
t = 40 min
l/A,-cm-’
transform, process, and compare spectra, a sizable minicomputer is required. Currently, a DEC 11/60 CBU with 256-Kbyte memory, a floating point processor, three 13-Mbyte disk drives, magnetic tape storage, an electrostatic printer/plotter, and several direct-view storage display terminals make up the facility. To optimize the process of information retrieval, extensive software, mostly in assembly language, is available. In common practice, an experiment involves some tens of minutes for sample preparation, 1.5 min for data acquisition (16 scans), 30 s for Fourier transformation, and several minutes for background correction and raFIGURE 3
tioing, and results in an absorbance spectrum of Ill6 cm-I resolution, with an RMS (root mean square) noise level of fO.OO1 absorbance (base 10) units. With a storage display terminal, these data can be examined in full detail, as compared with any of several hundred spectral records stored on-line (the archives contain >IO OOO!), and processed in any number of ways with or without operator intervention. Experimental results and discussion Ethylene. Reactions occurring in mixtures containing O3 and ethylene (CzH4) in the ppm range of concen-
trations in air have been examined recently with the long-path FTIR method by Calvert and co-workers (7) and in this laboratory (8,9).A major product, previously unidentified (compound X), was detected, and the kinetic and spectroscopic characterization of this compound was attempted. The results represent the latest knowledge of the O3-olefin reactions in general. The representative spectral data and the results of the computer-aided data analysis procedures ( 4 ) are illustrated in Figures 2-4. The IR spectrum of 0 3 can be uniquely identified by its fine rotational structures even when pressure broadened at 700 torr of air. The fully '80-labeled 03 was also used in some experiments to trace the fate of 0 3 in terms of I80-containing products. Figures 2a-c illustrate typical time-resolved spectra recorded for 03-CzH4-air mixtures. In addition to the sharp bands belonging to CO, COz, and C H z 0 (formaldehyde), broad bands indicated by a and bare seen in Figures 2b and c. Band A increases in height between 15- and 40-min reaction time, while band B remains virtually unchanged. This temporal behavior is readily discernible in the scale-expanded display of the difference spectrum, Figure 3a, derived by subtracting Figure 2b from Figure 2c. The residual spectrum, Figure 3b, was obtained from this difference spectrum by removing contributions from known products. This spectrum was identified as that of formic anhydride(CH0)zO. The spectrum shown in Figure 4a was derived from Figure 2b by removing the spectra of the reactants and all the known products including (CH0)zO. This species (compound X) is neither of the two suspected products, ethylene ozonide or glycolaldehyde, shown in Figures 4b-c. Glycolaldehyde can be formed in the secondary reaction involving CzH4 and the HO radical (10).Note in Figure 4a that the compound X exhibits two overlapping bands at 1737 and 1760 cm-I in the C = O stretch region and three bands a t approximately 1044, 1116, and 1170 cm-I in the neighborhood of the C-0 stretch region. Also, in the high-frequency region (not shown), two 0-H stretch bands were observed at 3405 and 3583 cm-' (8). Calvert and co-workers (7) tentatively X as identified compound CH2(0H)-O-CHO. This being the case, the observed spectrum suggests the possible presence of two isomeric forms as schematically shown by the following configurations: Envirm. Sci. Technol., VoI. 17, No. 7. 1983
9151
trans form
cis form
Compound X formation The following simplified reaction scheme represents the formation of compound X via the reaction of CHzO with the thermally stabilized entity of the Criegee intermediate CHzOO
03
CHzOO* CH200*
Y
-
+ C Z H ~ CH200*
+
Effect of added SO2 The effect of added SOz on the product distribution in the O3-CzH4 reaction has been reported recently by Calvert’s group (7). The present study generally confirms these findings.* Namely, the formation of compoun
CH,CHO
(3)
dissociation products (-60%) (4)
+M
+
-
CHz00
+M (54
Y
+ M(-40%)
(5b)
+ CHzO
-
CH,OO
+ CH2O
However, it was noted in the above experiment (Figure 5 ) , that the formation of compound X and anhydrides was not completely suppressed by the addition of CH3CHO. Furthermore, the formation of formic and acetic anhydrides cannot be explained adequately by the heterogeneous mechanism (reaction 7) and suggests the involvement of more than one isomeric form of the C H 2 0 0 entity.
of the product ensuing from the reaction of CHzOO with the added CH3CH0 and the competing reactions of CHzOO with the product CHzO. The extent of the reactant consumption and product formation is indicated by the difference spectrum in Figure 5. The most notable effect of the added CH3CH0 is the formation of propylene ozonide as the major product. Thus, the CH200 reacts with CHzCHO mainly via reaction 8.
CHz(OH)-0-CHO (compound X)
-
+
(“
CHz(OH)-0-CHO wall (CH0)zO Hz (7)
+
where the asterisks indicate the chemically activated species, and M represents nitrogen (Nz) and oxygen (02). The identity of species Y formed in reaction 5b remains uncertain. However, as discussed below and in Reference 3a, the existing evidence suggests that it is an isomer of the Criegee intermediate. The heterogeneous decay lifetime of compound X is highly sensitive to the surface conditions of reactor walls and ranges from 10 min to 1 h (7.8). Chemistry of CHzOO species Additional experiments revealed several new features of the chemistry of CH2OO species. One of the important questions is the possible effect of diluent pressures on the partitioning of reactions 4 and 5 . It was observed that the compound X was formed in significant amounts even at air pressures as low as -10 torr. Thus, a large fraction of the C H 2 0 0 appears to be produced initially “cold” without sufficient internal excitation to induce unimolecular dissociation. Interesting observations were also made concerning the reactions of the CHzOO with CH3CHO and with SOz. Figure 5 shows the results obtained by adding acetaldehyde (CH3CHO) to the 03-CzH4 system. A relatively short reaction time ( - 5 min) and low reactant conversion (
