A.
J. HAAGEN-SMIT
California Institute of Technology, Pasadena, Calif.
M. M. FOX Los Angeles County Air Pollution Control District, Los Angeles, Calif.
Ozone Formation in Photochemical Oxidation of Organic Substances Most active atmospheric ozone-forming organics-olefins, alcohols, and aldehydes. Reduction of nitrogen oxides as well as hydrocarbons is essential for control of Los Angeles smog
IT
IS well known that oxidation of organic material forms peroxides. Only in recent years: as a result of a n unfortunate set of circumstances. was it found that in this oxidation ozone is formed. The experiments leading to this discovery were carried out on a rather unusual scale in Los .4ngeles where. over a n area of several hundred square miles, thousands of tons of hydrocarbons and their derivatives plus about 400 to 500 tons of nitrogen oxides were released daily ( 8 ) . This fumigation caused a typical damage to plants. eye irritation, haze, excessive rubber cracking, and odors often resembling bleaching powder. Several years ago, each of these phenomena had been reproduced in laboratory fumigations where hydrocarbons or mixtures of hydrocarbons and their oxidation productse.g.: gasoline or automobile exhaustwere oxidized either Lsith ozone direct1)or photochemically in the presence of nitrogen oxides and sunlight ( 2 , 3? 5 ) . These experiments \vex carried out in concentrations closely resembling those experienced in the Los .4ngeles area. The characteristic odor pointed to the presence of peroxides. \vhile the intense rubber cracking indicated abnormally high ozone content of the air. The presence of these materials as ivell as nitrogen oxides, is manifested chemically by the high oxidizing property of the air which can be measured by oxidation of potassium iodide or phenolphthalin solutions. Routine measurements over a 2-year period have shown that the oxidant reaches a maximum during the
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day and is nearly absent at night. The optimum is not reached everywhere in the basin a t the same time, because of' location of the major emission sources. Also, the absolute value of the oxidant maximum is not constant because of weather conditions and the time necessary for the photochemical production of the oxidant. Human activities establish a definite pattern in which the oxidant values obtained on Sundays are significantly lower than those obtained on other days of the week. This is shown by the following data which gives by day of week the frequency with which total oxidant concentration was 0.50 p.p.m. or more (phenolphthalin method standardized against hydrogen peroxide).
These values were obtained a t California Institute of Technology, Pasadena,California. The value of 0.50 p.p.m. of total oxidant is one a t which all the observers on our panel agree that there is definite smog. Using the characteristic cracking of rubber as a test method, as well as chemical means, it has been established that a substantial part of the oxidant consists of
INDUSTRIAL AND ENGINEERING CHEMISTRY
ozone. The ozone concentration rise sharply during smog periods, and values as high as 0.5 p.p.m. (volume/volume) have been reported. which exceed by a factor of 20 thc ozone concentrations found in uninhabited areas (7, J ) .
laboratory Studies on the Formation of Ozone
Determination of Total Ozone. In duplicating the ozone formation observed in polluted air, it is essential to adhere to a rather narrow concentration range of hydrocarbon and nitrogen dioxide. This was confirmed by a set of experiments in which the hydrocarbon, 3-methylheptane, and nitrogen dioxide concentrations were systematically varied. Artificial light from a bank of Westinghouse 40-watt blue fluorescent light bulbs was used for the irradiation to eliminate the variable intensity of sunlight. The bulbs were arranged in the form of a U, four bulbs to a side and spaced so that when a N i t e r flask was placed in the center. the bulbs were approximately one half inch from the sides and bottom of the flask. An oxygen atmosphere adjusted to 30% humidity, and a n irradiation time of 10 hours were selected as standards in the experimental procedure. The ozone formed was measured by the cracking observed on bent rubber strips of standard size (8 X 20 X 2 mm.) suspended in 5-liter flasks during irradiation. T o remove volatile impurities. the rubber strips were exhaustively extracted
A I R POLLUTION with carbon tetrachloride a t room temperature, and the last traces of solvent were removed in vacuum. It was found advantageous to use as ozone measure, the sun1 of the depth of all cracks as measured under the microscope (1 00 X ) using an ocular micrometer. The measurement of crack depth $vas made on freshly cut surfaces exposed by making longitudinal cuts through the test strip 1 mm. from the edges. I n the experiments, the rubber was bent and the ends tied together \vith glass thread used to suspend the test strip in the flask. The calibrations against known ozone concentrations \vex made in flasks of the same size as those used in the irradiation experiments. I n the concentration range usrd. the total crack depth is proportional to the ozone concentration. 1 .0 mm. corresponding to 3 p.p.m. of ozone. \Vhen 2-butene in concentrations of of 3 p.p.m.. and nitrogen dioxide in concentrations varying. from 0 to 20 p.p.rn. are irradiated. it is notic-d that cracks begin to appear at a concrntration approximating 0.2 to 0.4 p.p.m. of nitrogen dioxide. The cracking increases with increased nitrogen dioxide concentration until a t about 2 to 3 p.p.m. a maximum is reached. After passing this maximum, the rubber cracking diminishes and at about 20 p.p.m. of nitrogen dioxide. only a fe\v cracks appear during the 10-hour irradiation. The results of thes: experiments at concentrations of 0 . i . 1.0. 2.0, and 3.0 p.p.m. of 2-butene and var!-ing concentrations of nitrogen dioxide (Figure 1 ) . are similar to those obtained with 3methylheptane. describrd earlier ( 6 ) . The general shape of the curves obtained in these experiments can be attributed to a t least t\vo simultaneous reactions: the formation of ozone; and the removal of ozone b!- both nitrogen dioxide and the Oxidation products of the hydrocarbon. That nitrogen dioxide does react with ozone is \vel1 known: and especially a t higher concentrations this removal of ozone uill play an important role. In our experiments a t the upper limits of nitrogen dioxide concentrations, when no cracking appears after 10 hours. longer irradiation does produce cracking. I t is most likely that during this induction period, the nitroyen dioxide concentration has decreased until it comes within the range of ozone formation. This shift in the composition of the mixture is also brought about b!. the reaction of nitrogen dioxide \.r-ith the hydrocarbon and its oxidation products. IVhen ozone formation stops. nitroyen dioxide is practically absent. The rubber strip suspended in the flask during irradiation continually removes the ozone formed, and is in competition with products that \vi11 react Ivith ozone.
s
3
5-3fPM.
2-BUTENE
I
I C
0' I
I
04
08
16
3
6
12
24
NITROGEN DIOXIDE PARTS PER MILLION Figure 1. Rubber cracking with 2-butene and nitrogen dioxide with artificial light in oxygen
of irradiation time in the absence of' rubber, will represent the excess of ozone caused by different rates of these reactions. The reaction by ivhich ozone is formed is only slightly faster than those which destroy it. IVhen UT attempt to isolate any ozone, \ y e isolate only the slight excess resulting from the difference in the rate of these reactions. In the analysis of air samples it is this Cxcess of ozone Lvhich is measuryd. In duplicating these phenomena in the laboratory, hydrocarbons and their oxidation products Ivere irradiated in the presence of nitrogen dioxide and the ozone concentration, measured by two methods, was determined after exposure for different lengths of time. One method was by rubber cracking, in which the rubber \vas introduced after and not during the irradiation as previousl>-described. I n the other method, advantage was taken of the difference in vapor pressures of ozone and nitrogen dioxide. By passing the irradiated mix-
Although the reaction with the rubber strip takes from 20 to 30 minutes to remove the ozone nearly quantitatively from a 5-liter flask, the rate a t which the rubber reacts with the ozone is sufficientl). greater than the degradation reaction, to show considerable ozone formation. \Ye find that the average rate of ozone formation during the first 10 hours irradiation of 3-methylheptane ( 3 p.p.m.) and nitrogen dioxide (1 p.p.m.) is about 0.8 p.p.m. per hour. After 100 hours slight cracking is still observed, and the total crack depth corresponds to 20 p.p.m. of ozone? which clearly indicates a chain reaction of considerable extent. The high values obtained in these experiments are possible only because the rubber strip continuously removes the ozone formed. If, ho\vever, the ozone is not removed, it !vi11 react u i t h the hydrocarbon and its oxidation products and the nitrogen oxides. Consequently, the ozone, measured after a certain length
TIME
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Figure 2. Photochemical rubber cracking with 3 p.p.m. of hydrocarbon and NOz 2 p.p.m. of nitrogen dioxide VOL. 48, NO. 9
SEPTEMBER 1956
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__
Q---
___
r: I z 10
........... n - PARAFFINS -- OLEFINS FINS
t
-.-.
i
2-POLEFINS ALDEHYDES
- n-
I ~
i8 n
I I
i
Y
-.-._.
0
2
Figure 3.
i
i
Photochemical rubber cracking with organic compound 3 p.p.m. and
NO2 2 p.p.m. 10 hours' exposure ture through several traps held a t -183" C., only ozone passes through, while nitrogen dioxide with its higher boiling point (24' C.) is trapped. The ozone passing through the traps was determined by both the phenolphthalin and the potassium iodide method. In irradiation experiments using 3 p.p.m. of 3-methylheptane and 1 p.p.m. of nitrogen dioxide, we found that a level of 0.9 p.p.m. of ozone was reached after four to five hours, which then remained nearly constant for twenty hours (7). When duplicate flasks were kept in the
dark for one hour before measuring the ozone content, the ozone concentration was reduced to one-half the value obtained directly after irradiation. In order to keep the ozone concentration at 1 p . p m . for 20 hours, approximately 14 p.p.m. of ozone must have been formed during that time. The experiments in which the rubber strips were suspended in the flasks during irradiation indicatrd the formation of 12 p.p.m. of ozone during the period between 5 and 25 hours. These experiments, although of a qualitative nature, support the conclusion
I.
reached earlier, that a considerably greater amount of ozone is produced than could be expected on the basis of a stoichiometric reaction of 3 p.p.m. of hydrocarbon and 1 p.p.m. of nitrogen dioxide. Similar observations have been made using 2-butene in the photochemical oxidation with nitrogen dioxide. At concentrations of 3 p.p.m. of 2-butene and 2 p.p.m. of nitrogen dioxide, a level of 0.5 p p . m . of ozone is reached after only 15 minutes, as compared to several hours required in the case of 3-methylheptane. The maximum ozone level obtained with 2-butene is lower than that found with 3-methylheptane, This is attributed to side reactions which may occur between butene and ozone, forming ozonides, and thereby removing butene from the reaction mixture. These reacticns are greatly reduced in experiments where rubber strips are suspended in the flask during irradiation, for then the ozone is continuously removed and prevented from reacting with the olefin. Under these conditions butene forms more ozone a t a faster rate than does 3-methylheptane. This method is probably a better measure of the nuisance value of the ozone-forming reactions because it indicates the total ozone-forming capacity of a particular compound. The ozone formed may react with many acceptors present in polluted air, which function as the rubber pieces in the laboratory experiments.
Photochemical Ozone Formation with Organic Compounds (3 p.p.m.) and Nitrogen Dioxide (2 p.p.m.) in Oxygen (Irradiation, 10 hours; Humidity, 30% ; t, 30-33' C.) Total Total Total Total Total so, Crack Crack Crack ('rack Crack Carbon Saturated Death, Cnsaturated Depth. Droth, Aldshudrs Death, .lIiscellaneous Devth. Hydrocarbons .T~%L. Hydrocarbons .Ilm .Ilc9hols .lIm and Ketones Jim Compounds Jjm. Atoms __ ______ Methanol 5 1 0 Formaldehyde 4 Formic acid Methane 0 Ethanol 5 0 Ethylene 2 Acetaldehyde 4 Acetic acid 2 Ethane 0 Acetylene 0.5 Ethyl bromide 0.1 Ethyl nitrate 0.5 Nitroethane 2 Ethyl mercaptan 5 0 1-Propene 5 n-Propanol 6 Propionaldehyde 4 3 Propane Isopropanol 6 Acetone 0 Propionic acid 4 n-Butanol 6 n-Butyraldehyde 4 0 . 1 1-n-Butene 5 4 %-Butane tert-Butanol 0 cis-2-n-Butene 8 Biacetyl 7 Butyric acid 6 Butyl nitrite trans-2 -n-Bu8 (without NO*) 12 tene Butadiene 12 5 Isobutene 5 Pyridine 2 5 n-Pentane 0 . 2 1-n-Pentene Amyl alcohol 6 cis-2-n-Pentene 8 trans-2-n-Pentene 8 4 Cyclohexanol 4 %-Hexane 1 1-n-Hexene 6 2-n-Hexene 7 Benzene 0 7 1 Methylben0.6 Phenylethyl alcohol 1 n-Heptane 3-Ethylpentane 1 zene 2,2,3-Trimethyl0 butane 8 n-Octane 2 1,Z-Dimethylbenzene 6 3-Methylheptane 3 1,3-Dimethylbenzene 7 9 n-Nonane 3 1,3,5-Trimethylbenzene 7
Table
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INDUSTRIAL AND ENGINEERING CHEMISTRY
A I R POLLUTION Relative Ozone Forming Potential of Organic Compounds The phenomenon of ozone formation being limited to definite relative proportions of hydrocarbon and nitrogen dioxide, observed with 3-methylheptane and 2-butene, is apparently equally true for other hydrocarbons and their osidation products. At a concentration of 3 p.p.m. and varying concentrations of nitrogen dioxide, the hydrocarbons: nbutane, n-pentane, n-hexane, n-heptane, n-octane? n-nonane, 1-n-hexene, and diisobutylene gave curves similar to the one obtained with 3-methylheptane: Xvith optimum ozone formation a t from 1 to 3 p.p.m. of nitrogen dioxide. Ozone formation during photochemical oxidation is not limited to hydrocarbons; it is also shown by their oxidation productsacids, aldehydes, ketones, and alcohols. Figure 2 shows the relative ozone forming capacity, determined a t different time levels, of triptane, 3-methylheptane, 1-butene, and butadiene a t concentrations of 3 p.p.m, of hydrocarbon and 2 p.p.m. of nitrogen dioxide. A comparison of a number of organic compounds tested for a 10-hour irradiation is shown in Figure 3 and Table I . AS the length of the carbon chain in the straight chain paraffins increases from 4 to 3 atoms, the ozone forming capacity becomes greater. Methane, ethane, and propane were found to be inactive. The highly branched hydrocarbon. 2,2,3trimethylbutane (triptane), does not cause measurable rubber cracking during the 10-hour irradiation period, which parallels its well known resistance to oxidation. Introduction of a double bond increases ozone formation which is more pronounced when the bond is in the 2 position. Cis- and transconfiguration did not affect the activity, as shown by cis- and trans- 2-butene and 2pentene. The optimum ozone formation is shown with olefins of four to six carbon atoms. The introduction of a second double bond, as in butadiene, doubled the activity when compared to that of the corresponding 1-butene. Pronounced activity was established for the alcohols and aldehydes with one to five carbon atoms. Tertiary butyl alcohol did not form ozone during the 10-hour irradiation period. The effect of substituting the hydrogen by different functional groups has been studied for ethane. The ozone-forming capacity of the resulting compounds is indicated by Functional
Group -ON0 -SH -CHO -NHq -NO? -0NOz
Total Crack Depth, Mm. IO 5 4 4 2
0 5
Functional Group -COOH -CjH, -CsHa
Total
Crack Depth, LMm. 0 5 0 2 0 1
_ _
-Rr-.
n i
--CHI -H
0 0
TIME Figure 4.
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Photochemical rubber cracking with n-butylnitrite 3 p.p.m.
the total crack depth in millimeters obtained after 10 hours of irradiation. Previously it has been reported (6) that biacetyl. bibutyryl, pyruvic acid, and butyl nitrite produce ozone upon irradiation without the addition of nitrogen dioxide. I n the absence of nitrogen dioxide? ozone formation can be established by irradiation of biacetyl a t a concentration of 40 p.p,m.; in its presence the rate of formation is still quite pronounced in the 1 p,p.m. range. and equals that of butene-2. Butyl nitrite without the addition of nitrogen dioxide is even more active than butene-2, as shown in Figure 4. After 94 hours, the total crack depth of the rubber pieces corresponded to the production of 80 p.p.m. of ozone, demonstrating again a chain mechanism in its formation. In laboratory experiments. photochemical ozone formation with hydrocarbons and their derivatives in the presence of nitrogen dioxide has been observed in concentrations as low as 0.1 p.p.m. Analytical results obtained from atmospheric samples have shown the presence of 1 to 2 p.p.m. of hydrocarbon and aldehyde and 0.4 to 0.8 p,p,m. of nitrogen dioxide. The mass spectrographic investigations of Shepherd and coworkers (9) on polluted Los Angeles air, established the presence of a variety of hydrocarbons known to form ozone in the presence of nitrogen dioxide. The emission sources of hydrocarbons and their derivatives are well known, for they originate mainly from incomplete combustion processes and from the evaporation of gasoline. The oxides of nitrogen are formed in the combustion of fuels-gas. oil and gasoline-mainly by fixation of nitrogen from the air a t high temperatures. We have demonstrated that gasoline, as well as automobile exhaust after proper dilution, is able to form ozone upon irradiation in the presence of nitrogen dioxide ( 6 ) .
In photochemical experiments conducted in sunlight and in concentrations of 3-methylheptane and nitrogen oxides approximating those known to occur during smog periods, it was found that ozone formation is proportional to the product of the hydrocarbon and nitrogen oxide concentrations (3, 7). Similar conclusions can bedrawnfrom the experiments with 2-butene (Figure 1). The present control measures are directed mainly toward reduction of hydrocarbon, while nitrogen oxides from high temperature combustion sources continue to increase. This increase in nitrogen oxides demands greater and greater efficiencv in hydrocarbon recovery. Since the reduction of hvdrocarbon emission will only be partial. it is essential to study the other component of the smog-forming system, nitrogen oxides, and to institute engineering research necessary for the drastic reduction of nitrogen oxides released by combustion processes. Literature Cited (1) Bradley, C. E., Haagen-Smit, A . J , Rubber Chem. and Techrml. 24, 750 (1951) ( 2 ) Haagenkmit, A. J., Eng. and Sci. 18. 11 ( 1... 9 5 4 ) .. mit, 4. J., IND.Eric. CHEM.
Plant Physiol. 27, 18 (1952). 16) Haagen-Smit, A . J., Fox., M. M.,Air
Repazr 4, 105 (1954). 17) Haagen-Smit,A. J.,Fox, M. M.,S.A.E.
Trans 63, 575 (1955). ( 8 ) Los Angeles Co. Air Pollution Control Dist., Los Angeles, Calif., 2nd Tech. and Admin. Reut., 1950-51 May, 1952. ( 9 ) Shepherd, M., Rock, S. M., Howard, R., Stormes, J., Anal. Chem. 23, 1431 L
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11951). . , RECEIVED for review December 12, 1955 ACCEPTED April 23, 1956 VOL. 48, NO. 9
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