Oxidation of Nitric Oxide in the Presence of Ultraviolet Light and Hydrocarbons J. J. Rufalini and A. P. Altshuller U. S . Department of Health, Education, and Welfare, National Air Pollution Control Administration. 4676 Columbia Parkway. Cincinnati, Ohio 45226
The osidation of nitric oxide has been investigated in the presence of 2.3-dimethyl-2-butene, 1-butene, 1,3,5-trimethylbenzene, and rz-butane. Nitrogen dioxide dosage curves are shown as a function of HC/NO, ratios with NO,r kept constant. The NOi, dosage increases very quickly with increasing concentrations of 2,3-dimethyl-2-butene and 1,3.5-trimethylbenzene. Greater concentrations of 1-butene and ri-butane are required to obtain similar increases in NO, dosage. Oxidant was observed only after maximum dosage had been achieved. N o oxidant was observed for the n-butane-NO, system under the conditions employed in this study. Neither the rate of oxidation of nitric oxide nor the rate of reaction of the hydrocarbon increased when water vapor was increased from 1.1 to 11 mm.
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he thermal Oxidation of nitric oxide has been studied extensively (Bodenstein and Wachenheim. 19 18: Johnston and Slentz. 1951; Treacy and Daniels, 1955; Glasson and Tuesday, 1963). The thermal oxidation in the presence of ultraviolet light has also been investigated and shown to be dependent upon the extent of oxidation (Bufalini and Stephens, 1965). Also, the ratio of nitric oxide to nitrogen dioxide has been shown to be constant at equilibrium depending upon the light intensity-ie.,
Samples Co. Air, nitrogen, and nitric oxide were from Matheson. Irradiations were carried out in the chamber previousl!, described at 25" = 1" C. (Altshuller and Cohen, 1963). All General Electric F 41T6BL blacklights were employed (E,,,,, = 3600 A , ) . A 72-1 borosilicate glass reaction vessel was used for the irradiations. An all-Teflon stirrer was placed on the bottom of the flask. Hydrocarbon concentrations were measured with a gas chromatograph equipped with a flame ionization detector. Two columns, one Carbowax 1540, the other SF-96, were employed for the separations. Nitrogen dioxide was measured by conventional colorimetric procedure. Nitric oxide was measured as the dioxide after conversion with sodium dichromate impregnated on glassfiber filter paper (Ripley, Clingenpell, et nl., 1964). The nitrogen oxide was kept constant at 2 p.p.ni. while the hydrocarbon concentrations were varied. The first-order dissociation constant for NO2, k,, was obtained by photodissociating the nitrogen dioxide in the parts-per-million range (v./v.) in nitrogen. The equation employed for this was d ( N O , ) / d t = -1.5 k,+(NO,). The primary quantum yield was taken as unity. The rate constant for the reaction, k , . is a measure of the light intensity. This was 0.24 niin.-l,
In the presence of reactive hydrocarbons, this equilibrium expression no longer applies. In this investigation. the effects of added hydrocarbons were investigated. It was desired to find just how little hydrocarbon was required to convert nitric oxide to nitrogen dioxide at a rate significantly different from the thermal rate. Experimental 1 -Butene and wbutane were Phillips pure grade. 1,3,5Trimethylbenzene (mesitylene) was obtained from K and K 1.ahoratories. 2.1-Diniethyl-2-butene from the Chemical
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Figure 1. Oxidation of nitric oxide in the absence and presence of ultraviolet light Volume 3, Number 5, May 1969 469
Results and Discussion Figure 1 shows theoretical values for dark and light reactions for the oxidation of nitric oxide. Values for the oxidation of nitric oxide in the presence of ultraviolet light were obtained from the integration of the equation (Bufalini and Stephens, 1965). r
c' Y
This figure shows that the simple assumption that ultraviolet light does not affect the thermal rate of nitric oxide oxidation is only valid a t short reaction times when the nitrogen dioxide is at very low concentrations. If the atmosphere were polluted with only oxides of nitrogen, the nitric oxide would oxidize during the period of darkness. In Figure 1 this condition would be described by one of the dark reaction curves but at much lower concentrations than these shown in this figure. A t sunrise, the light intensity begins to increase and factors expressed in the second term on the right of Equation 2 begin to contribute. Depending on the magnitude of the light intensity-Le., time of day, latitude, and amount of cloud coverage, and also on the amount of nitrogen dioxide present-the nitric oxide may be oxidized at a lower rate as shown by the lower curves in Figure 1 or nitric oxide may be formed by the dissociation of NO2 if the ratio is less than that prescribed by the equilibrium condition of Equation 1. The atmosphere polluted only with oxides of nitrogen can then be described by two curves, one for nitric oxide, the other for nitrogen dioxide. At night, the dioxide will increase t o a maximum while nitric oxide will decrease to a minimum. During daylight hours, the nitrogen dioxide will be at a minimum and the nitric oxide at a maximum. In the presence of hydrocarbons, the equilibrium described by Equation 1 in sunlight is destroyed. Oxidation of nitric oxide no longer follows the curves shown in Figure 1 but is much more rapid. The possible mechanisms for the photooxidation of nitric oxide in the presence of hydrocarbons have been reviewed by many investigators (Altshuller and Bufalini, 1965; Wayne, 1962;
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, ,
Figure 2. NO2 dosage, per cent reaction of 2,3-dimethyI-2butene, and oxidant formation at various HCINO, ratios is onehour irradiation. Light markings are for 50% relative humidity; dark markings are for 5% 470 Environmental Science & Technology
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2
Figure 3. NO2 dosage, per cent reaction of 1-butene, and oxidant formation at various HCINO, ratios with three-hour irradiation. Light markings are for 58% relative humidity
Stephens, 1961; Leighton, 1961; Cvetanovic. 1964) and will not be discussed here. Figure 2 presents the data for 2,3-dimethyl-2-butenenitric oxide system. This figure shows the amount of hydrocarbon reacting, the NO2 dosage, and the amount of oxidant produced, all in 60-minute irradiations. Dosage in this manuscript is defined as the integrated area for the nitrogen dioxide on a plot of concentration vs. time in the presence of a hydrocarbon subtracted from the area for the nitrogen dioxide without the hydrocarbon. The latter values would be obtained from curves similar to those shown in Figure 1. The maximum dosage occurs at an HC/NO, ratio of 0.8. Oxidant is first observed at this maximum dosage. For 1-butene. the maximum dosdge occurs at an HC/NO, ratio of 2.5. as shown in Figure 3. These data are for 180-minute irradiations. The figure shows that generation of oxidant begins somewhere beyond this maximum dosage point. much like that for TME. For mesitylene with 180 minutes irradiation, the maximum dosage occurs at an HC/NO, ratio of approximntely 0.6. Generation of oxidant again begins beyond this maximum dosage. The data for mesitylene are shown in Figure 4. The maximum dosage for all three compounds-i.e., TME, 1-butene, and mesitylene-occurs when all of the nitric oxide has been oxidized to NOy and very little. if any, hydrocarbon remains. At higher HC/NO, ratios, not all of the hydrocarbon is consumed at the time when the nitric oxide is completely converted. The reactions that occur beyond the region of complete nitric o ~ i d eoxidation
Figure 4. S O : dosage, per cent of reaction of 1, 3, 5trimethylbenzene, and oxidant formation as a function of HCINO, ratios with three-hour irradiation
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i 5.0
Figure 5. NO2 dosage and per cent reaction of n-butane at HC ,'NO, ratios after three-hour irradiation. Light markings are for 50% relative humidity
and maximum dosage are similar to those that occur when hydrocarbons are photooxidized in the presence of nitrogen dioxide. The dosage decreases at higher HC/NO,. ratios because the oxides of nitrogen are consumed by formation of such products as peroxyacetyl nitriate ( P A N ) , alkyl nitrates, and perhaps molecular nitrogen (Bufalini and Purcell, 1965). Figure 5 shows the data for the n-butane-nitric oxide system in 180-minute irradiations. It is surprising that such a small amount of this hydrocarbon can accelerate the oxidation of nitric oxide while not undergoing very much reaction itself. After three hours irradiation, only 3.5% of the hydrocarbon had reacted at an HC/NO, ratio of 4.8. The benefits derived from control of hydrocarbons without controlling oxides of nitrogen will depend largely on the amounts and types of hydrocarbons remaining. If a reactive hydrocarbon is allowed to remain in the atmosphere, the data show that as little as 0.28 p.p.m. of it (2,3-dimethyl-2-butene) can double the dosage of nitrogen dioxide from the oxidation of 2 p.p.m. of nitric oxide. This occurs at an HC/NO, ratio of only 0.14. Mesitylene also causes the nitrogen dioxide dosage to double at approximately the same H C / N O , ratio but for longer irradiations (three hours). Unlike 2.3-dimethyl-2-butene and mesitylene, l-butene caused the nitrogen dioxide dosage to double at an H C / N O , ratio of 0.5. N-butane does not double the dosage until a ratio of 5 has been reached. If the concentrations are decreased to more realistic atmospheric levels of oxides of nitrogen. 0.1 to 0.5 p.p.m., the rate of oxidation of nitric oxide appears to increase even though the HC/NO, ratios are held constant (Altshuller and Cohen, 1964). During this work, the authors investigated the effect of water vapor on the role of oxidation of nitric oxide and on the photooxidation rates of hydrocarbons. This aspect was undertaken since it has been reported (Diniitriades, 1967) that an increase in relative humidity from 1.5% to 35.9% had increased the rate of nitric oxide oxidation by a factor of 2.5 in the presence of ethylene. The data obtained for the various compounds when the relative humidity was increased from 5 % to 50% are shown in Figures 2 , 3, and 5 . The data clearly show that no effect occurs either in oxidation of the hydrocarbon or in the oxidation of nitric oxide. It is difficult to account for an increase in reactivity by the addition of water vapor. The HO-H bond that requires scission has higher energy than is usually available from the free radicals frequently encountered in these systems. The observations reported previously may have resulted from very reactive walls on the reaction chamber. A large portion of the free radicals
that are produced in the photooxidation of hydrocarbons may be destroyed by the walls of such a chamber. Introducing a large quantity of water vapor may deactivate these active sites on the walls of the reactor by the adsorption of the highly polar water molecules. This would enable free radicals to have a longer lifetime and subsequently to react further with the hydrocarbon. Reactivity of the hydrocarbon and oxidation of nitric oxide should therefore increase. Since there was no observable increase in the rate, the borosilicate glass chamber was either conditioned for free radicals-Le.. from prior irradiations-or the glass system is less reactive and does not need prior conditioning.
Conclusions Assuming that all hydrocarbons can be effectively removed, the condition described earlier with two curves for the oxides of nitrogen will prevail. The question that may be asked is "Do oxides of nitrogen in themselves constitute a health hazard?" Currently available information on toxicity of nitric oxide indicates that it does not constitute a problem at the levels present in ambient air. Nitrogen dioxide, however, is definitely toxic to both plants and animals. Some limited studies have shown that prolonged exposures to nitrogen dioxide at levels of 0.5 to 1.0 p.p.m. can be detrimental to the health of animals. There is also evidence that long-term exposures of plants to NO2 concentrations below 1 p.p.m. lead to growth suppression and chlorosis (California Department of Public Health, 1966). Whether any benefits are derived from the control of oxides of nitrogen without controlling hydrocarbons is questionable. Altshuller, Cohen, et al. ( 1967) have shown that hydrocarbons photooxidize in the presence of aliphatic aldehydes. Their observations suggest that rates of reaction and product yields in atmospheric reactions may not approach zero even when nitrogen oxides are present at very low concentrations. A recent study (Altshuller. Kopczynski, et al., 1968) has shown that the so-called unreactive hydrocarbons such as n-butane and propane form a large amount of oxidant at high HC/NO, ratios. Once the hydrocarbon is reduced (at constant nitrogen oxide concentration) to the level at which oxidant is not formed, further hydrocarbon reduction causes a significant reduction in nitrogen dioxide dosage. Thus, both suppression of oxidant and marked reduction of nitrogen dioxide dosage is accomplished by hydrocarbon control alone. The nitrogen dioxide dosage at best can be reduced by hydrocarbon control to about the dosage associated with thermal reaction. However, even at a very high atmospheric level of 1 p . p m of nitric oxide, oxidation in the absence of hydrocarbon and without ventilation should only produce 0.2 p.p.m. of nitrogen dioxide after 12 hours of reaction in sunlight and darkness. Therefore, a high level of control of hydrocarbons should be effective in both control of oxidant and nitrogen dioxide. Nitrogen oxide control will produce further reduction in nitrogen dioxide dosage, but if nitrogen oxides are controlled too extensively, oxidant formation could occur again. The validity of the above discussion will depend on how well laboratory experiments can be extrapolated to actual atmospheric conditions. Only definitive stuaies of actual polluted urban atmospheres will determine the correctness of these model laboratory results. Volume 3, Number 5, May 1969 471
Literature Cited Altshuller, A. P., Bufalini, J. J., Photochenz. Photobiol. 4, 97-146 (1965). Altshuller, A. P., Cohen, I. R., Air Water Pollution 7, 787 (1963). Altshuller, A. P., Cohen, I. R., Air Water Pollution 8, 611 (1964). Altshuller, A. P.. Cohen, I. R., Purcell, T. C., Science 150, 1161 (1967). Altshuller, A. P., Kopczynski, S. L., Wilson, D., Lonneman, W., Sutterfield, F. D., 61st Annual Meeting, Air Pollution Control Assoc., St. Paul, Minn., June 1968. Altshuller, A. P., Kopczynski, S. L., Lonneman, W. A.. SCI. TECHNOL. 2, Becker, T. L., Wilson, D. L. ENVIRON. 696 (1968). Bodenstein, M.. Wachenheim, L., Z . Angew Chem. 31, 145 (1918). Bufalini. J. J.. Stephens. E. R., Air Water Pollution 9, 123 (1965). Bufalini. J. J.. Purcell. T. C., Science 150, 1161 (1965). California Department of Public Health, Bureau of Air Sanitation. Berkeley. Calif., "The Oxides of Nitrogen in Air Pollution." January 1966. Cvetanovic. R. J.. J . Air Polllition Control Ascoc. 14, 209 (1964).
Dimitriades, B.: J . Air Pollution Control Assoc. 17, 460 (1967). Glasson, W. A., Tuesday. C. S., J . A m . Chem. SOC.8 5 , 2901 (1963). Johnston, H . S.: Slentz, L. W., J . A m . Chem. SOC.73, 2948 (1951). Leighton, P. A., Physical Chemistry: Vol. IX, Thotochemical Aspects of Air Pollution," Academic Press, New York, 1961. Ripley, D . L.. Clingenpeel, M. M., Hurn, R. W., Air Water Pollzrfion 8, 455 (1964). Stephens, E. R., in "Chemical Reactions in the Lower and Upper Atmosphere." Cadle, R. D., Ed., p. 51, Interscience, New York, 1961. Treacy. J. C.: Daniels, F.. J . ' 4 m . Chem. SOC. 77, 2033 ( 1955). Wayne. I_. G.. "The Chemistry of Urban Atmospheres." Tech. Prog. Rep. 111, Los Angeles Air Pollution Control District. December, 1962.
Received for review Airgust 21, 1968. Accepted January 23, 1969. itlention of cornnzercial products does not constitiite endorsement b j the Public Health Service.
Determination of Lead in Airborne Particulates in Chicago and Cook County, Illinois. by Atomic Absorption Spectroscopy Carole D. Burnham, Carl E. Moore, and Eugene Kanabrocki Chemistry Department, Loyola University, Chicago. Ill. 60626
Don M. Hattori City of Chicago, Department of Air Pollution Control. Chicago, Ill.
A simple precise procedure for determining the lead content of suspended particulate samples collected from the air uses atomic absorption spectroscopy. It is necessary to utilize the standard additions technique to overcome matrix effects. Analyses of 38 samples collected in the Chicago and Cook County area on March 31, 1966, as a part of the National Air Sampling Network, yielded values from 0.10 to 3.18 pg. of Pb per cubic meter of air. Results obtained by the referee method of polarography showed substantial agreement with atomic absorption values. Possibilities of employing atomic absorption for the determination of other metals found in suspended particulates are currently being investigated.
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lthough the field of air pollution is attracting increasing public and scientific attention, there is a notable lack of information on the nature, quantity, and seasonal variations of pollutants in various localities. Without these data, the establishment of sensible legal standards for the prevention and control of atmospheric contamination is difficult. if not impossible. 472 Environmental Science & Technology
It is the purpose of these studies to develop simple, in expensive methods and procedures for the routine determination of metals contained in the particulate matter collected at air sampling stations in widely different environments. Since unique and complex conditions exist at each geographical location, matrix interferences in any given sample are difficult to predict or duplicate. thus posing the need for an analytical technique relatively free from chemical or spectral interferences. The technique of atomic absorption spectroscopy (Walsh, 1955) is an obvious choice for the analysis of many metals. Use of the atomic absorption methodology would allow the small Eow-budget laboratory to evaluate field samples and accumulate data on purely local conditions, in contrast to the large spectrographic equipment which is currently in good use (Keenan and Holtz. 1964) at the larger air pollution labs but which is outside both the budget and technical capabilities of the smaller laboratory. It is always preferable to develop analytical methods using actual field samples, particularly for spectrochemical methods in which matrix effects play a significant role. The present technique was thus evolved using particulate samples obtained on March 31, 1966, at 38 stations located throughout Chicago and Cook County. Ill. These samples were collected routinely and were made available for investigation by the City of Chicago. Department of Air Pollution Control. As a
