Chemical aspects of the photooxidation of the ... - ACS Publications

dant, peroxyacyl nitrates, and formaldehyde were measured when irradiations were carried out with only dilution air in the chamber. A large number of ...
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CURRENT RESEARCH

Chemical Aspects of the Photooxidation of the Propylene-Nitrogen Oxide System A. P. Altshuller, S. L. Kopczynski, W. A. Lonneman, T. L. Becker, and R. Slater Bureau of Disease Prevention and Environmental Control, National Center for Air Pollution Control, U. S. Department of Health, Education, and Welfare, Public Health Service, Cincinnati, Ohio 45226

The photooxidation of part-per-million concentrations of propylene in the presence of similar concentrations of nitric oxide has been investigated. The product yields were measured over a wide range of reactant concentrations and reactant ratios. Series of experiments were carried out with varying light intensities, with both static and dynamic flow conditions. The products measured by colorimetric, gas chromatographic, and various monitoring instruments included nitrogen dioxide, oxidant (ozone and other oxidizing agents), formaldehyde, acetaldehyde, carbon monoxide, peroxyacetyl nitrate, and methyl nitrate. The mechanism of the reactions involved is considered, including stoichiometry and the role of free radical intermediates. Carbon and nitrogen balances are computed, and difficulties in obtaining balances are considered.

T

he photooxidation of various olefin-nitrogen oxide systems has been studied (Altshuller and Bufalini, 1965; Leighton, 1961 ; Wayne, 1962). Much of this previous work is limited, however, by use of reactant concentrations well above atmospheric levels and by measurements of only a limited number of products over a narrow range of reactant cancentrations. Furthermore, measurements of eye irritation and plant damage often have not been obtained along with the chemical or physical results. Although it is not possible to represent fully all of the diverse effects associated with photochemical air pollution by studies of a single hydrocarbon, propylene was chosen as a representative reactive hydrocarbon. The propylene-nitrogen oxide system when irradiated reacts readily t o produce oxidant, formaldehyde, acetsldehyde, carbon monoxide, peroxyacetyl nitrate (PAN), and methyl nitrate and causes 070ne and PAN-type

plant damage and eye irritation. Thus, major “smog” manifestations can be reproduced, although not necessarily at the intensities experienced in the ambient atmosphere. This paper reports the chemical and physical measurements of the photooxidation of propylene-nitrogen oxide over a range of reactant concentrations, a t several light intensity evels, and under static or dynamic flow conditions. Biological indicator measurements will be reported in another paper. Experimental

The propylene concentrations used included 0.25, 0.5, 1, 2, and 3 p.p.m. by volume, while the nitric oxide concentrations included0.125,0.20,0.25,0.50,0.6,0.7,0.75,1.0, 1.25, 1.50,2, 3, and 4 p.p.m. by volume. Irradiations also were conducted with 1, 2, or 3 p.p.m. of propylene in the chamber. but with only the background nitric oxide concentrations of 0.02 to 0.04 p.p.m normally present in the dilution air. The molar ratio3 of propylene to nitric oxide were varied from about 100 to 1 to 8. I n most experiments, however, the reactant ratios varied from 16 to 1 to 1 to 2. Many experiments were carried out with only dilution air to determine the background levels of reaction products and of effects of biological indicators. Fewer experiments over a narrower range of reactant concentrations were made with irradiations under static conditions than under dynamic conditions. The concentrations of nitric oxide, nitrogen dioxide, and oxidant were monitored instrumentally. Laboratory analyses were made at frequent intervals before and during the irradiations for the following reactant or products : propylene, nitric oxide, nitrogen dioxide, ozone (oxidant corrected for nitrogen dioxide and peroxyacetyl nitrate), formaldehyde, acetaldehyde, carbon monoxide, peroxyacetyl nitrate, and methyl nitrate. The nitrogen dioxide was analyzed both manually and instrumentally by the colorimetric procedure suggested by Volume 1, Number 11, November 1967 899

Saltzman (1954). Nitric oxide was analyzed as nitrogen dioxide after oxidation with NazCr20,-impregnated paper. Oxidant was determined manually by the 1 % neutral potassium iodide method (Byers and Saltzman, 1959) and the Mast Oxidant instrument (Wartburg, Brewer, er al., 1964). A 40% response of peroxyacetyl nitrate relative t o ozone and a 10 % response of nitrogen dioxide relative to ozone were measured in the manual procedure. Formaldehyde was analyzed by means of the chromotropic acid method as applied to atmospheric systems (Altshuller, Miller, et al., 1961) but with collection in bisulfite solution. Propylene was separated by means of a 15-foot X l/a-inch stainless steel column packed with 10 1,2,3-tris(2-cyanoethoxy)propane on alumina (Alcoa) a t 42" C. and analyzed with a flame ionization detector. Acetaldehyde was separated by means of a 12-foot by l/s-inch stainless steel column packed with 10 % 1,2,3-tris(2-cyanoethoxy)propane on 60- t o 80-mesh Gas Chrom Z at 42" C. and analyzed with a flame ionization detector. Peroxyacetyl nitrate and methyl nitrate were separated on 8 feet of lis-inch borosilicate glass packed with 10 polyethylene glycol 400 on 60- to 80-mesh Gas Chrom Z at 28 O C. and analyzed by means of an electron-capture detector. Carbon monoxide was separated on a molecular sieve column, hydrogenated to methane with 5 to 10 % Raney nickel on a 40to 60-mesh '2-22 Celite catalyst and analyzed as methane with a flame ionization detector. The two irradiation chambers were identical and were fabricated of aluminum with plastic film windows of poly(vinyl fluoride). The air purification system was modified t o provide cooling of the air before passing over the charcoal bed within this system. The chamber temperature was held constant by controlling the dilution air temperature, the surrounding room temperature, and the use of infrared lamps before irradiation. The chamber temperature was maintained within 2 " C.in each individual run, whereas, over the series, the temperature was maintained at 31.5" + 2" C. The relative humidity was held near 50 %. Irradiation was done by two large banks of fluorescent lamps external to the chambers and parallel to the plane surface of the chambers containing the poly(viny1 fluoride) film windows. Each bank was divided into three sections that could be independently controlled. Each section contained a mixture of blue-light, black-light, and sunlight fluorescent lamps to approximate the ultraviolet portion of the solar spectrum. Use of nitrogen dioxide in nitrogen photolysis procedure for determining total light intensity gave a Kd of 0.40 minute-l a t full light intensity. In both dynamic and static experiments, charging was done dynamically to obtain the desired levels of reactants. For most of the static experiments, the chamber was charged by injecting the reactants into the chamber with syringes. Subsequently, the flow was stopped just before lights were put on. I n the dynamic experiments, the flow rate of 2.8 cubic feet per minute corresponded to an average irradiation time of 120 minutes. The chambers were flushed out at high flow rates of dilution air with the fluorescent lights on for a number of hours after each run. In addition, the chambers were treated with ozone frequently to destroy organic contaminants on the chamber walls. Even after such treatment, low concentrations of oxidant, peroxyacyl nitrates, and formaldehyde were measured when irradiations were carried out with only dilution air in the chamber, A large number of such irradiations of dilution air 900 Environmental Science and Technology

were carried out under dynamic flow conditions. The average concentrations were as follows : initial nitrogen oxides, 0.05 p.p.m. ; oxidant (as determined colorimetrically), 0.065 p.p.m. ; oxidant (Mast instrument), 0.05 p.p.m. ; peroxyacetyl nitrate, 0.006 p.p.m. ; formaldehyde, 0.06 p.p.m. ; acetaldehyde, 0.03 p.p.m.

Results and Discussion One objective of this study was to obtain measurements under both static and dynamic conditions. Comparisons of these results will be made in some detail. Aside from previous results from the authors' laboratory, almost all irradiation chamber experimental results have been obtained under static operating conditions. In static experiments, the chamber is changed to a predetermined concentration level of reactants in air, the ultraviolet lamps are turned on, and the reactions are followed for several hours without further additions of reactants and with removal limited to the small amounts needed for analyses. In dynamic experiments, the chamber is changed to a predetermined concentration level of reactants, the ultraviolet lamps are turned on, but the reactants are continually charged to the chamber and removed at the same rate. The dynamic experiments more nearly represent the continuous charging from emissions occurring in the atmosphere. Neither type of system duplicates the varying meteorological conditions in the atmosphere even with low wind speeds and limited vertical mixing. Consumption of Propylene. In the irradiation experiments under dynamic conditions, the fraction of propylene consumed near dynamic equilibrium reached 0.6 to 0.8 for the 2-to-1 to 4-to-1 molar ratios of propylene to nitrogen oxide. The fraction of propylene consumed was dependent on the ratio of reactants, but was almost independent of the absolute concentration of reactants (Table I). Therefore, the absolute

Table I. Fraction of Propylene Consumed as a Function of Ratio of Reactants under Dynamic Flow Conditions Fraction of Propylene Ratio of Reactants Propylene Concn. Consumed 1 to 2 1 0.30 1 to 1

2 1 0.5

0.34 0.29 0.38

2 to 1

3 2 1 0.5

0.73 0.79 0.70 0.66

3 to 1

3 2 0.5

0.70 0.71 0.68

4 to 1

2 1

0.65 0.58

8 to 1

2 1

0.61 0.62

12 to 1

3

0.56

16 to 1

2

0.48

Above 30 to 1

3,2,1

0.35

amounts of propylene consumed were linearly related to the initial concentration of propylene at a fixed reactant ratio. As the ratio of reactants decreased from near 2 to 1, the fraction of propylene consumed also decreased from 0.6 to 0.8 to about 0.30. An increase in ratio to the range above 30 to 1 also causes the fraction of propylene consumed to approach 0.30. The consumption of propylene in the static irradiation work was computed in terms of the time required for half-conversion of propylene to products. The results at an initial propylene concentration of 1 and 2 p.p.m. with initial nitrogen oxide levels varying from 0.25 to 3 p.p.m. can be represented by the equation t (C3H6) = 45(NO, 1). This linear decrease in half-conversion time must cease at somewhat below 0.25 p.p.m. of nitrogen oxide. The half-conversion time must minimize and then increase rapidly as the nitric oxide concentration approaches zero. In irradiation under static conditions at a fixed initial nitrogen oxide concentration of 0.25 p.p.m., the halfconversion times decreased slowly with decreasing initial propylene concentration as follows: 2 p.p.m., 60 minutes; 1 p.p.m., 60 minutes; 0.25 p.p.m., 90 minutes. Formation and Consumption of Nitrogen Dioxide. The formation of nitrogen dioxide, in terms of time required to reach nitrogen dioxide maximum concentration, is plotted in Figure 1 as a function of initial propylene Concentrations. The time required to reach peak nitrogen dioxide concentration increased both under static and dynamic conditions as the propylene concentration decreased ; under dynamic flow conditions, the times became essentially constant at higher propylene concentrations. The critical factor appears to be the ratio of propylene to nitrogen oxide, with the time required to reach peak nitrogen dioxide tending to increase more as the reactant ratio becomes smaller. The rates of formation of nitrogen dioxide show the inverse relationship, so they are decreasing with decreasing reactant ratio. The time required t o reach the nitrogen dioxide maximum, under both static and dynamic flow conditions at the 2-p.p.m. propylene level with initial nitrogen oxide levels ranging from 0.1 to 2 p.p.m. can be represented by the equation t max.(NOn) = 60(NO,) 7. This relationship breaks down when no definite maximum in nitrogen dioxide is observed, as at 2 p.p.m. of propylene and at 3 or more p.p.m. of nitrogen oxide. If the rate of nitrogen dioxide formation is computed either as the initial nitrogen oxide concentration divided by the time required to reach the nitrogen dioxide peak, or by the time required for the nitrogen dioxide to reach half the initial nitrogen oxide concentration, the rate is constant. The rate obtained is 0.205 p.p.m. minute-'. In the latter computation, the results of experiments at 2 p.p.m. of propylene and 3 p.p.m. of nitrogen oxide can be included, and the rate is the same as that for lower nitrogen oxide concentrations. If the nitrogen dioxide reaches a maximum, it then decreases as the reaction proceeds. The percentage decrease in the dynamic experiments was 60 to 80 at the higher propylene to nitrogen dioxide ratios, but as the reactant ratio approached 1 to 1, the percentage decreased rapidly. This 60 to 80 conversion is associated with the choice of a 120-minute average irradiation period. In the static irradiations with 6 hours of irradiation time, 90 to 98 of the nitrogen dioxide concentration was consumed by the end of the reaction at reactant ratios at or above 1 to 1. At a 2-to-3 ratio, the nitrogen dioxide concentration reached a plateau within 4 hours of irradiation

0STATIC, 0.25 ppm N I T R W N OXIDE -0- D Y N M K . 0 2S ppm NITROGEN ONDE

+

+

z

z

10

PROPYLENE CONCENTRATION. ppm

Figure 1. Time to nitrogen dioxide peak as function of propglene concentration under static and dynamic flow conditions

and remained constant for at least 6 hours of irradiation. The nitrogen dioxide dosages under static conditions for 2to 6-hour irradiations at 2 p.p.m, of propylene with 0.25, 1.0, 2.0, or 3.0 p.p.m. of nitrogen oxide are as follows: 0.22, 1.0, 1.75, and 1.65 p.p.m. and 0.35, 1.55, 5.3, and 9.0 p.p.m. per hour. Thus, for the 6-hour irradiations, the nitrogen dioxide dosages increase approximately with the 1.5 power of the nitrogen oxide concentration. Carbon Monoxide. Because of variations in ambient carbon monoxide in dilution air and of limitations in sensitivity, carbon monoxide was measured for several static irradiations at the higher propylene levels. The formation curve of carbon monoxide is shown in Figure 2. Carbon monoxide is produced much more slowly than formaldehyde or acetaldehyde during the early stages of the reaction. As the reaction proceeds, carbon monoxide increases rapidly until late in the reaction it becomes the most abundant product. Formaldehyde and acetaldehyde either level off or begin to decrease during the period carbon monoxide is increasing. Aldehydes. Formaldehyde and acetaldehyde are major products of the photooxidation of propylene in the presence of nitrogen oxides. During the very early stages of the reaction, these two aldehydes were the only two products detected (Figure 2). In the static irradiation experiments, first, the acetaldehyde and then the formaldehyde reached a maximum or leveled off and then decreased in concentration. The decreases in the aldehyde concentrations occurred after almost all of the propylene and at least 90% of the nitrogen oxide had been consumed. The concentrations of formaldehyde and acetaldehyde as they varied with nitrogen oxide concentration were measured at each of several propylene concentrations in the irradiations under dynamic conditions. Since the acetaldehyde and formaldehyde yields on the average were in a 1.0-to-1 ratio, only the acetaldehyde results are given for conditions approaching dynamic equilibrium for various nitrogen oxide levels in Figure 3 (formaldehyde values at 2-p.p.m. levels of nitrogen oxides are given in Figure 4). The aldehyde concentrations usually passed through a maximum, but this maximum tended to be broad and flat. These maxima occurred between 1.5 to l and 2.0 to l . The decrease in aldehyde concentration on either side of the maximum was slow. The aldehyde concentrations decreased to about 50 of their maximum conVolume 1, Number 11, November 1967 901

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1.5

Figure 3. Acetaldehyde concentrations produced as function of nitrogen oxide concentration under dynamic flow conditions

centrations only a t nitrogen oxide levels below 0.05 p.p.m. (ratios in the range 25 to 1 to 50 to 1) and a t nitrogen oxide concentrations well in excess of the propylene concentration (much greater than 1 to 1). At a fixed reactant ratio, the aldehyde yields near dynamic equilibrium after correction for background of 0.03 p.p.m. are linearly related to either the initial propylene concentration YCH~CHO = 0.35 Ci[CaHs] or the propylene consumed YCH'CHO= (0.45 to 0.5) Ci[C3Hs].Very little, if any, significant difference exists in the acetaldehyde yields per parts per million of propylene consumed in the 1.5-to-1 to 840-1 range 902 Environmental

Science and Technology

of reactant ratios. At reactant ratios of 1 to 1 or below, however, the yields per parts per million of propylene consumed are greater. The variations in formaldehyde and acetaldehyde concentrations from the photooxidation of 2 p.p.m. of propylene at various nitrogen oxide levels were measured in both static and dynamic irradiations (Figures 4 and 5). The aldehyde concentrations were higher and somewhat less dependent on nitrogen oxide concentration in static irradiations than in dynamic irradiations. In the 6-hour static irradiations, there is only about a 10 decrease in formaldehyde concentration between 0.25 and

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Volume 1, Number 11, November 1967 903

3 p.p.m. of nitrogen oxide. Maxima in aldehyde yields occur at about a 1.5-to-1 reactant ratio in the dynamic irradiations. At the lower nitrogen oxide levels (0.25 p.p.m.), the differences in aldehyde yields in 2-hour compared with 6-hour irradiations are small because the reactions are rapid and maximum concentration levels of the aldehydes are almost reached in 2 hours. At high nitrogen oxide levels, the rates of reaction decrease, and the aldehydes either do not reach a maximum value or they reach a maximum later in the reaction. Consequently, the aldehyde concentrations after 6 hours of irradiation are higher than after 2 hours of irradiation at nitrogen oxide levels above about 1.5 p.p.m. Significant differences in yields occur in 2- and 6-hour irradiations for formaldehyde compared with acetaldehyde. These differences result from variations in the mechanism of formation and consumption of these two aldehydes. Oxidant. Total oxidant was measured by both the colorimetric 1 potassium iodide laboratory procedure and the Mast Oxidant instrument. The results from these two types of measurements were parallel, so the same relationships were obtained. Consequently, only the results of the colorimetric analysis have been plotted. The total oxidant values obtained colorimetrically were corrected for nitrogen dioxide (1 p.p.m. of NOe = 0.1 p.p.m. of 0,) and peroxyacetyl nitrate (1 p.p.m. of PAN = 0.4 p.p.m. of 03). The net oxidant consists of ozone plus a possible small contribution from peroxides other than peroxyacetyl nitrate. The oxidant did not start to form until the nitric oxide approached zero, and the nitrogen dioxide reached its maximum concentration. The net oxidant obtained, as equilibrium is approached under dynamic flow conditions, is plotted in Figure 6 as a func-

tion of propylene concentration at each of several fixed propylene to nitrogen oxide ratios. An approximately linear region of increasing oxidant with increasing propylene concentration occurs, followed by a leveling off of oxidant at higher propylene concentrations, at 3-to-1 and 4-to-1 reactant ratios. A similar plateau in oxidant concentration is not reached at a 2-to-1 ratio up to 3 p.p.m. of propylene. Additional experimental results available from this study indicate that the oxidant levels at various propylene concentrations increase slowly with decreasing reactant ratios between 12 to 1 and 2 to 1. Below about a 2-to-1 reactant ratio, a rapid decrease in oxidant occurs. At a 1-to-1 reactant ratio, decreasing the concentration of reactants results in some increase in oxidant concentration, perhaps indicating the reduction in the quenching effect of nitrogen oxides depends on reducing the absolute concentration of nitrogen oxide as well as on reactant ratio. The variation in net oxidant with changes in nitrogen oxide concentration are shown in Figure 7 under dynamic flow conditions. These oxidant concentration curves are relatively flat around the maximum between 1.5-to-1 and 340-1 ratios, but the oxidant levels decrease rapidly to zero when nitrogen oxide concentrations are increased to give ratios corresponding t o 1.5 to 1 to 1 to 1. Oxidant values of about 0.1 p.p.m. were obtained when the nitrogen oxide levels were less than 0.05 p.p.m. The background oxidant levels formed by irradiating the dilution air averaged 0.065 p.p.m. The net oxidant values under dynamic conditions were also plotted as a function of propylene concentration at several nitrogen oxide levels (Figure 8). The suppression of oxidant somewhat by higher propylene and particularly by nitrogen oxide levels (low propylene t o nitrogen oxide reactant ratios)

DYNAMIC FLOW CONDITIONS

**.*..*-...*

----

2 TO 1 REACTANT RATIO 3 TO 1 OR 4 TO 1 REACTAN: RATIO 1 TO 1 REACTANT RATIO

.o PROPYLENE CONCENTRATION, p p m

Figure 6. Oxidant concentrations produced as function of propylene concentration under dynamic flow conditions

904 Environmental Science and Technology

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Volume 1, Number 11, November 1967 905

oxidant dosage, but the same increase in nitrogen oxide concentration markedly increases the nitrogen dioxide dosage, Peroxyacetyl Nitrate. As with ozone or oxidant, peroxyacetyl nitrate did not form until the nitric oxide approached zero and the nitrogen dioxide reached a maximum concentration. The peroxyacetyl nitrate usually reached a plateau concentration in approaching equilibrium in dynamic flow and in most of the 6-hour static irradiations. The peroxyacetyl nitrate concentrations as dynamic equilibrium was approached in the dynamic flow experiments are plotted in Figure 10 as a function of nitrogen oxide concentration at each of several propylene concentrations. The curves reach maxima at reactant ratios between 2 t o 1 and 3 t o 1. The peroxyacetyl nitrate concentrations, as equilibrium conditions were approached in dynamic flow experiments, are plotted in Figure 11, as a function of propylene concentration. The concentrations decrease continuously with decreasing propylene concentration, and the peroxyacetyl nitrate concentrations are at or close t o zero a t reactant ratios of 1 t o 1 and below under dynamic irradiation conditions. These results differ from those obtained for ozone. No saturation effect of increasing propylene concentration was observed for peroxyacetyl nitrate as was observed for oxidant. This type of result is consistent with the organic nature of the peroxyacetyl nitrate molecule. In Figure 12, peroxyacetyl nitrate concentrations are plotted as a function of nitrogen oxide concentration at the 2-p.p.m. propylene level for static and dynamic conditions. The concentrations and the ratios of reactants at which maximum yields are obtained are nearly the same for both the 2-hour dynamic average irradiation period and the 2-hour static irradiations. A large increase in peroxyacetyl nitrate concentrations occurs a t the higher nitrogen oxide concentrations after 6-hour irradiations. The maximum occurs at a reactant ratio between 1 t o 1 and 2 t o 1 rather than 2 to 1 to 3 t o 1. This parallels closely the results obtained for ozone (or oxidant) under the same experimental conditions.

l

1

N I X O G E N OXIDE CONCENTRATION, ppm Figure 9. Oxidant concentrations produced as function of nitrogen oxide concentration under static and dynamic flow conditions

,906 Environmental Science and Technology

0 '.I-ROGEI.

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Figure 10. Peroxyacetyl nitrate concentrations produced as function of nitrogen oxide concentration under dynamic flow conditions

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Figure 11. Peroxyacetyl nitrate concentrations produced as function of propylene concentration under dynamic flow conditiom

Volume 1, Number 11, November 1967 907

N I T R O G E N O X I D E CONCENTRATION,

ppm

Figure 12. Peroxyacetyl nitrate concentrations produced as function of nitrogen oxide concentration under static and dynamic flow conditions

The ratio of net oxidant t o peroxyacetyl nitrate concentration near equilibrium in dynamic flow experiments ranges from 1.2 to 1 t o 3 to 1 and averages 2 t o 1. In static flow experiments, the ratio of net oxidant to peroxyacetyl nitrate ranges from 2 to 1 t o 3 t o 1. The peroxyacetyl nitrate dosages under static conditions for 6 hours of irradiation a t 2 p.p.m. of propylene with 0.25, 1.0, 2.0, and 3.0 p.p.m. of nitrogen oxide are as follows: 0.6, 2.0, 1.4, and 0.1 p.p.m. per hour. Thus, for the 6-hour irradiations, the peroxyacetyl nitrate dosages reach a maximum at a 2-to-1 ratio as do the net oxidant dosages. The peroxyacetyl nitrate dosages decrease with increasing concentration of nitrogen oxides above the 2-to-1 ratio of reactants at about the same rate as the net oxidant dosages. However, the maximum peroxyacetyl nitrate dosage is about one half of the net oxidant dosage. The peroxyacetyl nitrate dosage decreases more rapidly than the net oxidant dosage as the nitrogen oxide concentration is decreased towards zero. Other Products. The only other product that could be identified and analyzed was methyl nitrate. Although other lower-molecular-weight oxygenated products such as propylene oxide, acetone, or propionaldehyde could have been identified, there was no evidence of their presence as photooxidation products. Other products such as carbon dioxide o r formic acid could not be measured with the analytical procedures available. Even if analysis had been made for carbon dioxide, it would have been very difficult or impossible t o determine a trace of carbon dioxide as a photooxidation product in the large excess of carbon dioxide in the dilution air. The methyl nitrate yields were always very small; the concentrations varied from 0.000 to 0.02 p.p.m. The yield increased when concentrations of nitrogen oxide increased from 0.25 to 3 p.p.m. When the conditions were such that peroxyacetyl nitrate was formed, the ratios of peroxyacetyl nitrate t o 908 Environmental Science and Technology

methyl nitrate were in the range of 50 t o 1 t o 200 t o 1. Thus, methyl nitrate under all experimental conditions was a very minor product of the photooxidation reaction. Carbon Balance. The major carbon-containing products included formaldehyde, acetaldehyde, carbon monoxide, and peroxyacetyl nitrate; methyl nitrate was a minor product. Carbon monoxide analyses in static experiments were made a t initial concentrations of 2 p.p.m. propylene and 1 p.p.m. nitric oxide and also a t levels of 1 p.p.m. propylene and 0.25 p . p m nitric oxide. The averaged results for four experiments a t 2 p.p.m. propylene and 1 p.p.m. nitric oxide are given in Table I1 in terms of moles of the various products per mole of propylene consumed. At 60 minutes (and earlier), all of the two-carbon fragment was accounted for almost entirely as acetaldehyde. I n numerous other experiments at various reactant ratios in both static and dynamic irradiations, 0.9 i 0.1 mole of acetaldehyde was formed per mole of propylene consumed during the first 30 to 60 minutes of reaction before the nitrogen dioxide peak. I n the set of static experiments (Table 11),90 2 of the carbon is accounted for a t 60 minutes, which is the time in which the nitrogen dioxide reaches its maximum concentration. As the reaction continues, the carbon balance obtained becomes poorer. This trend does not necessarily indicate that an unidentified two- or three-carbon product is formed. In experiments under dynamic flow conditions, formaldehyde, acetaldehyde, and peroxyacetyl nitrate at various reactant ratios accounted for the following percentages of carbon consumed: 1-to-2 ratio, 100; 1-to-1 ratio, 90; 2-to-1 ratio t o 1640-1 ratio, 60 to 70; above 16-to-1 ratio, 60. If it had been practical to analyze for carbon monoxide in dynamic flow experiments, the carbon monoxide yields obtained should contribute significantly t o accounting for the carbon balance particularly a t 2-to-1 ratios and above.

An appreciable fraction of the acetaldehyde was converted to formaldehyde and carbon monoxide. During the period between 240 and 300 minutes reaction time, propylene had already been almost entirely consumed, so the direct reaction of propylene does not contribute significantly t o the formaldehyde concentration. Nitrogen dioxide also is almost entirely consumed by this stage in the reaction so acetaldehyde is not consumed t o form peroxyacetyl nitrate. The formaldehyde concentration during this period was essentially constant, an indication that the rate of formaldehyde consumption is about equal t o the rate of acetaldehyde consumption. These considerations indicate that acetaldehyde was being consumed to form formaldehyde (plus other products) at least at 0.05 mole per hour (per mole of propylene initially present). When this correction for acetaldehyde consumed is made and the resulting acetsldehyde concentration is added to the peroxyacetyl nitrate present, 80 to 90 % of the two-carbon fragment can be accounted for throughout the 6-hour reaction period. Nitrogen Balance. The nitrogen oxide concentration (nitric oxide plus nitrogen dioxide) usually was constant a t its initial level until the nitrogen dioxide reached a maximum. In some 6-hour static irradiations in which no nitrogen dioxide maximum was reached, some loss of nitrogen oxide was observed late in the irradiation. After peroxyacetyl nitrate begins to be formed, the nitrogen balance depends on the initial level of nitrogen oxides. When the initial nitrogen oxide level was below 0.2 p.p.m., all of the nitrogen oxide consumed could be accounted for as peroxyacetyl nitrate; when initial nitrogen oxide level was between 0.2 and 0.5 p.p.m., 50 t o 90 % of the nitrogen oxide consumed could be accounted for as peroxyacetyl nitrate; when the initial level ranged between 0.5 and 1.5 p.p,m., only 35 t o 70% o f t h e nitrogen oxide consumed

appeared as peroxyacctyl nitrate. The exact fraction accounted for was also dependent on reactant ratio in the nitrogen oxide ranges listed. These conclusions apply to both the static and dynamic irradiations. The methyl nitrate accounted for only a negligible part of the nitrogen. No other organic nitrogen compound was identified. Light Intensity Effects. Measurements were made both under dynamic conditions and in a 6-hour static irradiation period. The initial propylene concentration was held a t 2 p.p.m. The light intensities used included full lights ( K , = 0.4 minute-'), one-half full lights, and one-third full lights. The results are listed in Table 111. The products, particularly oxidant and peroxyacetyl nitrate, were most sensitive to light intensity at the reactant ratio of 2 t o 1 under dynamic conditions. No oxidant or peroxyacetyl nitrate were formed at one-third lights, and aldehyde concentrations were reduced significantly. These reductions in yields were generally attributed t o a marked slowing down of the over-all reactions. For example, over four times as much propylene was consumed a t full-light intensity as at one-third lights. Although the reaction rates were greatly reduced, the yield of aldehydes, in parts per million per part per million of propylene consumed, actually increased by twofold. When irradiations were carried out under dynamic conditions, but at a reactant ratio of 8 t o 1, both concentrations and yields of products (parts-per-million per part-per-million propylene consumed) were much less sensitive t o reductions in light intensity. Oxidant levels were reduced by 60 %, whereas peroxyacetyl nitrate levels were reduced only 40%. The aldehyde levels were reduced only 20 t o 3 0 z . The yields, in partsper-million per part-per-million of propylene consumed, did not vary greatly with reductions in light intensity.

Table 11. Carbon Balance Measurements from Photooxidation of Propylene-Nitrogen Oxide Mixturesa Moles of Products per Mole of Propylene Consumed - Carbon Atoms Irradiation Peroxyacetyl Accounted for Time, Minutes Formaldehyde Acetaldehyde Carbon monoxide nitrate Methyl nitrate in Products, 60 0.64 0.23 0.02 0.005 90 0.90 120 0.52 0.64 0.35 0.17 0.004 83 180 0.40 0.51 0.48 0.22 0.004 78 240 0.39 0.46 0.60 0.23 0 003 79 300 0.38 0.40 0.72 0.19 0.004 76 360 0.28 0.37 0.83 0.20 0,003 75 Average of four experiments each with 2 p.p.m. propylene and 1 p.p.m. nitrogen oxide, under static conditions a n d with full light intensity.

Table 111. Chemical Composition at Various Ultraviolet Light Intensities Products, P.P.M./P.P.M. of Propylene Consumed Initial Concentration. P.P.M. Peroxyacetyl Propylene Nitric oxide Chamber Conditiona Light Level Oxidant nitrate Formaldehyde Acetaldehyde 2 1 D, 2 hr. AIT 1* 0.65 0.72 0.28 0.8 2 1 0.63 D, 2 hr. AIT '12 0.25 0.06 0.55 0.32 0.31 2 1 D, 2 hr. AIT '13 0.00 0.00 0.63 0.7 D, 2 hr. AIT 1 0.36 0.10 2 '14 0.52 0.51 0.07 2 '14 D, 2 hr. AIT '12 0.15 0.49 0.48 2 'I4 D, 2 hr. AIT '13 0.14 0.06 2 1 S, hr. 0.9 1 0.9 0.44 0.8 2 1 S, hr. 0.1 '13 0.5 0.8 0.17 ~~

a 6

D = dynamic conditions: S = static conditions; AIT Full lights ( K d = 0.4 minute-').

=

Average irradiation time.

Volume 1, Number 11, November 1967 909

Irradiations also were carried out under static conditions at the reactant ratio of 2 to 1 (Figure 13). With 6 hours of irradiation, the oxidant and peroxyacetyl nitrate concentrations a t one-third lights were still only 40 to 60 of those a t full-light intensity. With the same irradiation time, the aldehyde yields a t one-third lights reached those attained at full-light intensity. Because somewhat less propylene was consumed even in 6 hours at one-third lights than at full intensity, aldehyde yields (parts-per-million per part-per-million of propylene consumed) again were a little higher a t one-third lights. In these static irradiations, no oxidant or peroxyacetyl nitrate appeared for the first 3 hours (Figure 13). Aldehydes also formed slowly because propylene was consumed slowly. As a result, acetaldehyde reached only 4 0 z of its maximum concentration after 3 hours of irradiation. The static and dynamic irradiation periods cannot be related quantitatively. Qualitatively, however, the absence of oxidant and peroxyacetyl nitrate in the 2-hour average irradiation periods with one-third lights in the dynamic flow experiments can be directly attributed t o a slowing down of the reactions to such an extent that nitric oxide was always present in the system. Thus, if the dynamic irradiation period was longer, oxidant and peroxyacetyl nitrate should be formed. Comparisons with Other Investigations of the PropyleneNitric Oxide System. Because of differences in concentrations of the reactants, in light intensity, in temperature, and in surface conditions in the various investigations, detailed comparison of experimental results is difficult. Schuck and Doyle (1959) reported only a few experiments with the propylene-nitrogen oxide system; their formaldehyde and oxidant yields were in the same range as those of the present study. More recently, Glasson and Tuesday (1964) measured a propylene-nitric oxide system irradiated under static conditions, with 2, 1, and 0.5 p.p,m, of propylene and with several nitric oxide concentrations between 0.1 and 1 p.p.m. Propylene photooxidation and ozone formation rates were given rather than product yields; peroxyacetyl nitrate yields, however, were in parts per million. In general, the rates of reaction reported

3.0

I I

I

I

I

I

I

2 I REACTANT RATIO, STATIC CONDITION PROPYLENE, FULL LIGHTS

........

--- ACETALDEHYDE, L f

,

...-. .-

FULL LIGHTS OXIDANTS, FULL LIGHTS PROPYLENE, 0NE.THIRD LIGHTS ACETALDEHYDE, ONE-THIRD LIGHTS

IRRADIATION TIME,

hours

Figure 13. Propylene, acetaldehyde, and oxidant concentrations produced at various irradiation times at two light intensities under static conditions

910 Environmental Science and Technology

1

by Glasson and Tuesday (1964) were much lower than those in the present study. For example, in the present investigation the propylene photooxidation rates at the 2-p.p.m. level were about three times as high as those obtained by Glasson and Tuesday, and the ozone formation rates also were much higher. Glasson and Tuesday reported peroxyacetyl nitrate yields that were much smaller, and their curves of PAN yield cs. nitrogen oxide concentration were much different than those in the present study. They reported suppression of peroxyacetyl nitrate formation at 2 p.p.m. of propylene and 1 p.p.m. of nitric oxide, whereas in the present study the peroxyacetyl nitrate yield was at or near its maximum at these concentrations. Romanovsky and coworkers (Romanovsky, Ingels, et al., 1965) also reported measurements with the propyleneenitrogen oxide system under static irradiation conditions. Although most of the work done was with higher reactant concentrations than those used in the present study, comparisons can be made at the 2-p.p.m. propylene level. The agrement between the results in regard t o the propylene, nitric oxide, and nitrogen dioxide half lives is satisfactory. For example, the propylene half lives in their study with irradiation of 2 p.p.m. of propylene and 3,2, or 1 p.p.m. of nitrogen oxides were 170, 130, and 95 minutes, respectively. In the present work, the corresponding times were 180, 145, and 90 minutes. The formaldehyde yields of 0.4 to 0.45 p.p.m. per part-per-million of propylene initially present were similar in both studies. The oxidant concentration levels in the present study were somewhat higher. For example. Romanovsky and coworkers (1966), using colorimetric KI analysis, found that 0.5 p.p.m. of oxidant formed after irradiation of 2 p,p.m. of propylene and 1 p.p.m. of nitrogen oxide; 0.9 p.p.m. of oxidant was measured in the present investigation. Their lower oxidant yields may have resulted from lower light intensities or higher rates of decay of the oxidant at surfaces of their chamber. Despite these differences in absolute concentrations of oxidant, the curves representing oxidant as a function of nitrogen oxide concentration are similar. The oxidant peak under static conditions a t the 2-p.p.m. propylene level was at reactant ratios of 2 to 1 to 1 to 1 in both studies. Comparison of Results on Oxidant Yields as Function of Nitrogen Oxide Concentration. The measurement of oxidant. has received more widespread consideration than any other aspect of the irradiated hydrocarbon-nitrogen oxide systems Methods of measurement vary, but results have been reported for five olefin- and one paraffin-nitrogen oxide system by various investigators (Altshuller and Cohen, 1964; Glasson and Tuesday, 1964; Haagen-Smit and Fox, 1954 and 1956; Leighton, 1961; Romanovsky, Ingels, et al., 1965; Stephens, Hanst, et a/., 1956; Tuesday, 1961). Most of the studies indicate that the maximum should be observed in the 1-to-1 to 3-to-1 range of reactant ratios. These studies cover hydrocarbon ranges from 0.5 t o 10 p.p.m. of hydrocarbon and from 0.04 t o 15 p.p.m. of nitrogen oxide. The oxidant maximum in the present study under both static and dynamic conditions falls within this 1-to-1 to 340-1 range of reactant ratios. In some of the measurements made by Haagen-Smit and Fox on the 2butene-nitrogen dioxide system, the oxidant maximum was observed at a ratio as low as 0.3 t o 1 (Haagen-Smit and Fox, 1956); the oxidant maxima in the remainder of the work of Haagen-Smit and Fox occur in the 1-to-1 t o 3-to-1 range. The results of Glasson and Tuesday for three olefins with nitric

oxide indicate maxima for oxidant a t reactant ratios of 6 t o 1 t o 20 t o 1 (Glasson and Tuesday, 1964). Glasson and Tuesday reported rates of oxidant formation, whereas the other investigators reported oxidant yields. Comparison with Automobile Exhaust Irradiation Studies. Propylene was selected for this study as a representative hydrocarbon; however, even in a consideration of automobile exhaust the selection of a single hydrocarbon as representative is difficult. After the start of the study, several reactivity scales were constructed to give average reactivities of hydrocarbon emissions (Altshuller, 1966; Caplan, 1965). On a scale developed in this laboratory (Altshuller, 1966), exhaust from automobiles with uncontrolled or controlled emissions has an average reactivity significantly less than that of propylene. The computed reactivity falls between that of toluene and ethylene. O n a nitrogen dioxide formation rate scale (Caplan, 1965), average reactivities of automobile exhaust fall between those of ethylene and propylene. Thus, both scales suggest that automobile exhaust is somewhat less reactive than propylene. Despite this limitation, no other single hydrocarbon can be suggested t o more closely simulate the chnracteristics of a n irradiated automobile exhaust mixture. Irradiated automobile exhaust has been investigated in the same system used in the present study (Korth, Rose, ef NI., 1964; Leach, Leng, et a/., 1964). The average reactivity ofthese mixtures would be about the same as those discussed above. At the highest concentration level used, the molar concentration of olefins, aromatics (except benzene), and four-carbon and higher paraffins was 1.6 p.p.m. The inclusion of higher-molecular-weight hydrocarbons not analyzed would bring the concentration near 2 p.p.m. The ratios given in the original work were ratios of carbon parts per million to nitrogen oxide, and these ratios ranged from 1.5 t o 1 t o 24 t o 1. In terms of molar concentrations, these ratios would be in the range of 0.25 to 1 t o 4 t o 1. At ratios a t or below 0.5 t o 1, n o oxidant was formed, and n o nitrogen dioxide was consumed. Low concentrations of oxidant-i.e., below 0.20 p.p.m.-and some nitrogen dioxide conversions were observed at 1-to-1 ratios. This finding does not agree well with those of the present study, which show very little if any oxidant or peroxyacetyl nitrate at 1-to-1 ratios. Such a result is difficult t o explain in view of the lower average reactivity of automobile exhaust, if additivity of effects is accepted as a valid postulate. At higher ratios, the results are in better general agreement. In the automobile exhaust study, oxidant levels at higher reactant ratios ranged from 0.2 t o 0.5 p.p.m.; in the present study, oxidant levels in dynamic irradiations at higher ratios ranged from 0.2 t o 0.65 p.p.m. In the automobile exhaust study, nitrogen dioxide conversions at the higher reactant ratios reached 5 5 t o 65 %; in the present study, 60 t o 80 %. Significant yields of aldehydes are obtained only from olefins and aromatics, with the yields of aliphatic aldehydes from aromatics about half those from the olefins (Altshuller, Klosterman, et d., 1966). In the auto exhaust study, a t the highest concentration levels, only about 0.5 p.p.m. was olefin and another 0.5 p.p.m. aromatic (Leach, Leng, et al., 1964). Based on these approximations, the irradiated automobile exhaust might be expected t o form about as much aldehyde as does 0.75 p.p.m. of propylene. In irradiated automobile exhaust, the formaldehyde reached 0.3 p.p.m. and aliphatic aldehydes 0.5 p.p.m. The formaldehyde yield from 0.75 p.p.m. of propylene would be about 0.4 p.p.m., whereas the aldehyde

yield would be about 0.6 p.p.m. Consequently, the use of such assumptions leads t o reasonable agreement. The same irradiated automobile exhaust mixtures have been satisfactorily simulated by mixtures containing several olefinic and aromatic hydrocarbons. Some synergistic effects were observed in such mixtures (Altshuller, Klosterman, et a/., 1966). A comparison of the reactant ratios at which maximum oxidant is obtained is difficult. In the irradiated automobile exhaust study, the measurements a t the two lower hydrocarbon levels stopped a t ratios corresponding t o 2 t o 1 in the present study. Since this ratio is just in the region of the maximum, it is impossible t o know whether the maximum was attained in this or in a higher reactant ratio region. At the highest hydrocarbon concentration, measurements extended to a 440-1 reactant ratio. No maximum was attained a t a ratio lower than 4 to 1, so these results d o not agree with those obtained in the present work. The difference may relate t o the effect of the complex hydrocarbon mixture, as compared with a single hydrocarbon component, on oxidant formation as a function of nitrogen oxides. Stoichiometry in the Propylene-Nitric Oxide System. Earlier measurements of the number of molecules of nitrogen dioxide formed per molecule of ethylene consumed suggested that chains of considerable length may occur as a result of free radical reactions (Altshuller and Cohen, 1964). In the present investigstion, the ratio of molecules of nitrogen dioxide formed t o molecules of propylene consumed was computed (Table IV). The ratios varied from 1 to 8. As in the ethylenenitric oxide system, higher ratios occurred at the lower reactant concentrations and lower ratios of propylene to nitric oxide. These ratios are likely t o be minimum values (Altshuller and Cohen, 1964). Consequently, the present results support the previous work in that they indicate the presence of chains of appreciable length, particularly early in the reactions. Mechanism of Reaction. In the very early stages of the propylene-nitrogen oxide reaction, only acetaldehyde and formaldehyde are measured as significant products. Later in the reaction, increasing yields of carbon monoxide and peroxyacetyl nitrate are measured. Methyl nitrate always is a very minor product of reaction. In the late stages of the 6-hour static reactions, acetaldehyde and formaldehyde are being consumed while concentrations of carbon monoxide continue t o increase. Acetaldehyde photooxidizes to form formaldehyde as a major product. As a result, formaldehyde

Table IV. Stoichiometry in the Propylene-Nitric Oxide System Ratio of Molecules of Nitrogen Dioxide Formed to Propylene Consumed Initial Concentrations, P.P.M. t = 1 1 ~ f = Propylene Nitric oxide t max. f max. At t max.a 0.25 0.25 3 0.25 2.0 8 1. o 0.25 2 1 1. o 2.0 3 2.5 2.0 0.25 1.5 2.0 1. o 3 2.5 2.0 3.0 2.5 a

Time at which nitrogen dioxide reaches maximum concentration.

Volume 1, Number 11, November 1967 911

continues t o increase in concentration and then levels off while the acetaldehyde itself is decreasing in concentration. In the latter stages of the static reaction, propylene is no longer available as a significant primary source of formaldehyde. Since the acetaldehyde is not being photooxidized at a rate sufficient t o maintain the formaldehyde concentration, the formaldehyde also begins to decrease in concentration. In the early stages of the reaction, acetsldehyde is formed on slightly less than a molz-per-mol? basis, as propylene is consumed. The yield of formaldehyde is lower, but these two products can account for up to 95 of the carbon atoms consumed (Table 11). The electrophilic attack of oxygen atoms from nitrogen dioxide photolysis must be important in this system, as it is in other such photooxidation rzactions. The reactions involved, however, are quite different fro:n those investigatzd in detail by Cvetanovic (1963). The bulk of his work was done in the absence of added oxygen molxul:s. If Cvetanovic's scheme is followed for propylsne-atomic oxygen reactions, propylene oxide, acetone, and propionaldehyde would be expected as major products, yet none of these substances are present a t detectable levels. Stephens (1961) postulated that oxygen molecules react with the intermediate biradical, and the subsequent intermediate is similar to that resulting from ozone attack with resulting decomposition to aldehyde (or ketone) and zwitterion. Cvetanovic pointed out, however, that the lifttime of the biradical is too short t o make its reaction with oxygen likely (Cvetanovic, 1964). Instead, he suggests reaction of the oxygen molecule with the excitcd molxular species. If this aspect of the mechanism is accepted, the fate of the excited propylene oxide and propionaldehyde molecules under the conditions of the present work must be decomposition rather than stabilization. Decomposition of each of these molecules must lead to acetaldehyde as shown below for propylene oxide. H H

I

CH3C-CH2*

+

I:]

CHaC-CHz

0 2 +

\/ 0

+

+

+

-

CH3CHO H2C-0-0 (1) Decomposition of the one-carbon zwitterion to hydrogen and carbon dioxide, although possible as a minor pathway, will not explain the yield of formaldehyde. If the zwitterion reacts with oxygen before decomposing, then the following reaction can be written.

+

-

+

+

H2C-0-0 Os -+CH20 0 3 (2) Cvetanovic (1963) suggested that a small percentage of biradicals may undergo pressure-independent fragmentation to free radicals. In the case of propylene, methyl and acyl radicals would be generated. These radicals should play a part in the rapid photooxidation of nitric oxide t o nitrogen dioxide. CH302 NO + CH30 NO2 (3) Another source of acyl radicals is as a result of hydrogen abstraction from acetaldehyde. If the structure C H &2-00-N02

+

+

I1 0

is accepted for the peroxyacetyl nitrate, then it can form 912 Environmental Science and Technology

by reaction of the acetyl radical with oxygen followed by reaction of the acetyl radical with oxygen followed by reaction with CH3C 0 2 + CH3C-02 I1 ,I (4) 0 0

+

+ NO2

CH3C-02 I'

-+

CH3C-02N02

(5)

1:

0

0

The problem still remains as to why peroxyacetyl nitrate is not formed until nitric oxide is at very low concentration levels. A possibility is the reaction of the peroxyacetyl radical with nitric oxide rather than with nitrogen dioxide. CH3C-02 NO CH3C-0 NO2 (6) If Reaction 6 is much more rapid than Reaction 5 , then the suppression of Reaction 5 while nitric oxide is present could be explained. The fate of C H 3 0 and C H 3 C 0 2must also be considered. The reaction of methoxy radicals with oxygen CH30 02 + CH20 HO? (7) would account for methoxy radicals in terms of a product that is present. This reaction also serves as a chain-carrying step. HOz NO + NOp HO (8) Acylate radicals may decompose to form additional methyl radicals and carbon dioxide. CH3C-0 + CH3. CO? (9)

+

+

-+

+

+

+

+

+

0 The decreasingly poor accounting for nitrogen atoms as the nitric oxide concentration increases also may be related t o the previous reactions. The higher the level of nitric oxide molecules the greater the importance of Reaction 6 and the loss of peroxyacetyl radicals via Reactions 6 and 9, thus suppressing Reaction 5 . The low yield of methyl nitrate hardly seems related to a very slow rate of reaction of methoxy radical with nitrogen dioxide. Instead, methoxy radicals may be unavailable for reaction. This is reasonably explained by reactions such as 7 being much more important than reaction of methoxy radical with nitrogen dioxide CH3O NO? CHgONO? (10) because of the much greater abundance of oxygen molecules. The form in which nitrogen atoms are present must still be considered if the initial sum of nitric oxide and nitrogen dioxide concentrations is not accounted for by peroxyacetyl nitrate and methyl nitrate. Another reaction that would consume both methoxy radicals and nitric oxide is CH3O NO -+ CHZO HNO (1 1) which would be in competition with Reaction 6. Reaction 11 also can be used to explain the inhibition of the over-all reactions by excess nitric oxide. The possible importance of this reaction and related reactions was discussed previously by Altshuller and Cohen (1964) and by Tuesday (1961), Stransz and Gunning (1964) also discussed the importance of H N O in the photolysis of formaldehyde with nitric oxide. At high concentrations of reactant, these investigators measured nitrogen and nitrous oxide as products. They attributed these products to the decomposition of (HNO& and the reaction of H N O with NO. Bufalini and Purcell(l965) have shown that molecular nitrogen is a reaction product of the ultraviolet irradiation

+

+

-+

+

of ethylene with nitrogen dioxide in oxygen-inert gas mixtures. In a larger-scale study such as the present one, it was not practical t o test for molecular nitrogen as a reaction product. However, the previous work suggests this as a reasonable explanation for the lack of nitrogen balance. Some losses may occur, however, by wall reactions. Carbon monoxide forms very slowly early in the reaction, but the yield increases as the aldehyde concentrations build up. Furthermore, the carbon monoxide yield continues t o increase rapidly late in the reaction when almost all of the propylene and nitrogen oxide have been consumed. These results strongly suggest that carbon monoxide is in large part or entirely a product of the secondary photooxidation of formaldehyde and acetaldehyde. Carbon monoxide is formed as a major product from the photooxidation of propionaldehyde in the parts-permillion range even in the absence of nitrogen oxides (Altshuller, Cohen, et ul., 1966). There is no reason why acetaldehyde and formaldehyde photooxidation should not also contribute t o the formation of carbon monoxide. However, small quantities of nitrogen oxide are present even late in the reaction. As a result, it is not possible to separate the direct photooxidation of the aldehydes from the nitrogen oxide-induced photooxidation of these aldehydes. Reaction by ozone with propylene can be assumed t o follow the Criegee mechanism, although recent studies suggest that this mechanism is not sufficient to explain ozonide formation from some types of olefins (Murray, Youssefyeh, era/., 1966). H?C=CHCHa

+

0 3

-+

H HyC-C-CH3

!

I

0

-

0

\/ 0

CH3CHO

+

-+ HE-0-0

The increase in the percentage of carbon atoms unaccounted for as products must also be considered. Decomposition of the one-carbon zwitterion may occur to some extent with the formation of carbon dioxide. Similarly, the decomposition of the acylate radical results in carbon dioxide as a product. If these two reactions increase in relative importance as the irradiation proceeds, carbon dioxide could account for much of the discrepancy in the carbon-mass balance. In addition, hydrogen abstraction by peroxymethyl radicals could contribute t o at least a small yield of methyl hydroperoxide late in the reaction. In formaldehyde photooxidation, hydrogen peroxide is formed as a product (Cohen, Purcell, et ul.. 1967). Because a small yield of methyl hydroperoxide also may form, it is possible that the oxidant even after correction of peroxyacetyl nitrate and nitrogen dioxide consists not only of ozone but of small amounts of these peroxides. Analytical procedures to determine such products are being developed (Cohen, Purcell, et ul., 1967), but they were not available during the present investigation. In the mechanism discussed thus far, only reactions of atomic oxygen and of ozone with the propylene molecule have been considered. The over-all rate of consumption of a hydrocarbon in photooxidation reactions may not be accounted for by atomic oxygen and ozone attack alone (Altshuller and Bufalini, 1965; Altshuller and Cohen, 1964). Computations of the contributions of atomic oxygen and ozone reactions have been made for a number of the experiments conducted under static conditions. The atomic oxygen concentration has been computed on the basis of the revisions in rate constants discussed by Schuck, Stephens, et al. (1966). The propyleneatomic oxygen rate constant also has been adjusted. The resulting expression used t o compute the averaged rate of consumption in parts per million per minute is as follows:

-

(12)

Table V. Rates of Reaction of Propylene with Atomic Oxygen and Ozone Compared with Over-all Rate of Consumption of Propylene Rate, P.P.M., Minute-' Initial Concentration, P.P.M., Irradiation Ratio obsd. ____ Obsd.' Calcd.b to calcd. rates Propylene Nitrogen oxide Time, Minutes 0.27 0,0032 0.012 30 1 0.25 0.29 0.0024 0.0083 60 0.25 1 0.45 0.0018 0.0040 95 1 0.25 0.35 0.0016 0.0046 30' 1 2 0.55 0.0029 0.0053 65' 1 2 0.84 0.0032 0.0038 90' 1 2 0.23 0.024 0.0055 35 2 0.25 0.35 0,010 0.0035 80 2 0.25 0.36 0.0018 0.005 135 2 0.25 0.40 0.0008 0.0019 240 2 0.25 0.05 0.00045 0.0093 3Y 1 2 0.37 0.016 0.006 115 1 2 0.60 0.0015 0,009 1 200 2 0.15 0.0012 0.008 2 90' 2 0.30 0.010 0.003 145 2 2 0.40 0.0014 0.0035 265 2 2 0.12 0.0013 0.011 170' 3 2 0.53 0.0016 0.003 245' 3 2 b c

Propylene consumption rate averaged over 10- to 30-minute interval. Sum of rates of reaction of propylene with atomic oxygen and ozone. Before nitrogen dioxide maximum.

Volume 1, Number 11, November 1967 913

The higher rate constant of 5 X l o 3liter mole-’ second-’ (1.1 X p.p.m.-l minute-’) was selected for the propyleneozone reaction (Altshuller and Bufalini, 1965). The results of these computations are compared with the observed rates of reaction of propylene in Table V. Two experimental conditions were considered. One of these was in the early stages of the reactions when the propyleneozone reaction did not contribute significantly. Under these conditions, the ratio of calculated t o observed rates varied from 0.05 to 0.84. The other condition was later in these reactions when the propylene-atomic oxygen reaction did not contribute significantly. Under this second set of conditions, the ratio of calculated to observed rates varied from 0.23 to 0.60. The ratio was largest when the observed rate was lowLe., a t or below 0.005 p.p.m, minute-’. The rate was low when the nitrogen oxide was in excess and when most of the propylene had been consumed, which was late in the reactions. Apparently, these results can only be explained by postulating one or more species aside from atomic oxygen and ozone capable of reacting with propylene. The observed and calculated results cannot be brought into agreement by an upward revision in rate constants. Any large increase in constants would result in ratios of greater than 1 for some of the experimental conditions included in Table V. Both free radicals and zwitterions can be suggested as species capable of reacting with the olefin. The available experimental evidence indicates, however, that the direct reaction of zwitterions with olefins is less probable (Altshuller and Bufah i , 1965) than their reaction with oxygen or direct decomposition. The alternative possibility is direct reaction of peroxyalkyl, hydroperoxyl, or peroxyacetyl free radicals with the propylene molecule. When peroxyacetyl nitrate is being formed rapidly, it does not seem likely that a significant fraction of the peroxyacetyl radicals could be available for reaction with the propylene. Peroxyalkyl and hydroperoxyl radicals are likely attacking species throughout the reaction. The available experimental results are of little assistance in determining the relative reactivity of these species. The products of attack by these species on propylene do not appear t o be readily distinguishable from those associated with atomic oxygen or ozone attack in the presence of oxygen. CH302

+ H*C=CHCH3

+

I

CHIYHCH3 O-?

L

I1 0-7 I

’

I

II

CH~J 0

CH31

Reactions 14 and 15, followed by Reaction 1, result in much the same over-all sequence of reaction steps Acknowledgment

The authors acknowledge the assistance of T. A. Bellar

914 Environmental Science and Technology

in the analytical method for determination of peroxyacetyl nitrate. Literature Cited

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