significant in the case of hydroperoxy radicals. It is more likely that smog chamber surfaces provide an effective site for the occurrence of heterogeneous reactions (e.g., metal surfaces and nitrogen oxides reaction) and act as a source of low level contaminants such as higher molecular weight hydrocarbons, aldehydes, and nitrogen compounds. These “wall effects” tend to increase the oxidant yields in low reactivity smog chamber experiments. A key factor to be considered when assessing the validity of Chameides and Stedman’s results is that they did not solve the kinetics model directly, but solved a model of the model. That is, they introduced a different representation of the chemical events by assuming that the two most important species, hydroxyl (OH) and hydroperoxy radicals (HOz), as well as others, were in photochemical equilibrium. This technique of dealing with stiff systems of differential equations is usually called the quasi-steady-state-assumption (QSSA), and it has been shown to be an unreliable approach for solving atmospheric HC and NO, kinetics systems (5). To determine if the QSSA made by Chameides and Stedman could have affected their predictions, we repeated the entire simulation, but used the widely accepted Gear algorithm for systems of stiff differential equations (6) to provide a direct numerical solution of the mechanism. The urban NO, NOz, and 03 levels at 03,06,09, and 12 h, as determined from their graphs, were used as the initial conditions for the mechanism presented by Chameides and Walker ( 4 ) .As in the Chameides and Stedman simulation, the rate constants were taken from Hampson and Garvin (7), and the concentrations of the following species were assumed to be constant: methane, 1.5 ppm; hydrogen, 0.5 ppm; and water vapor, 18 000 ppm. In addition, no urban hydrocarbons or carbon monoxide was “transported” out of the urban environment, in conformance with the assumptions of Chameides and Stedman. The results of these four simulations are presented as linear plots in Figure 1. They differ markedly from those of Chameides and Sted-
(10) Lonneman, W., discussion presented at conference on “1975 Oxidant Transport Studies by EPA”, Research Triangle Park, N.C., Jan. 20-21,1976.
B. Dimitriades M. C. Dodge J. J. Bufalini K. L. Demerjian A. P. Altshuller U.S. Environmental Protection Agency Office of Research and Development Environmental Sciences and Research Laboratory Research Triangle Park, N.C.2771 1
SIR: In a recent paper, Chameides and Stedman (1) postulated that oxides of nitrogen (NO,) transported from urban centers interact with the natural methane (CH4) oxidation cycle to yield ozone ( 0 3 ) concentrations above the National Ambient Air Quality Standard (NAAQS) of 0.08 ppm. A photochemical model was used to demonstrate the proposed effects. If their results and conclusions were valid, they would have a major impact on air pollution control strategies. The conclusions that “the highly reactive heavier (than methane) hydrocarbons do not play a role in the production of ozone in rural areas” would be of extreme importance because the current oxidant control strategy of EPA is to control nonmethane hydrocarbons (NMHC). NO, is controlled only to the extent necessary to meet the NAAQS for nitrogen dioxide (Nod. The results of Chameides and Stedman are contrary to experimental evidence for CH4 and NO, behavior (2) and to other modeling results ( 3 ) .They presented no evidence to support their statement that smog chamber experimental results for less reactive hydrocarbons are unreliable because of radical loss to the walls. In fact, Chameides and Walker ( 4 ) concluded that heterogeneous processes were not likely to be
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man (Figure 3, ref. l ) ,often by a factor of 10 in O3 concentrations. This was not unexpected, since under the QSSA the radical concentrations can be several orders of magnitude in error ( 5 ) .The very fast initial rise of O3and the relatively fast loss of NO, in their model are symptomatic of this type error. The impact of these four urban systems on a rural site 10 h downwind, assuming no further emissions or dilution, is shown in Figure 2. Chameides and Stedman (Figure 5, ref. 1 ) show a very rapid rise of O3 beginning a t 1300, whereas our results indicate a decrease below their 40-ppb background level and a small rise between 19 and 22 h. Figure 1indicates that this rise occurred, not because of O3 synthesis due to CHI, but because of the partial survival of the urban initial condition of -60 ppb 03. I t would thus appear that the results reported by Chameides and Stedman are artifacts of the QSSA; therefore, their conclusion that NMHC’s are not necessary for the formation of O3 above the NAAQS is unsupported. In addition to difficulties in the solution of the model, there are other aspects of their approach that require comment. The situation assumed by Chameides and Stedman was essentially that of a nonsteady-state plug-flow reactor 155 miles long with a 10-h residence time (a flow rate of 15 mph). This cannot be considered as characteristic of most rural high oxidant situations since trajectory analyses of air parcels arriving at a rural sampling site for high oxidant days vs. low oxidant days do not show any definite patterns. The rural high oxidant situation is further characterized by multiday persistence over a very large area, usually near the center of a high-pressure area with characteristically light winds. The NMHC’s range typically from 0.5 to 1.0 ppmC, and NO, from 6 to 10 ppb (8, 9). These characteristics suggest in situ oxidant formation rather than transport of oxidant from a nearby (i.e., 10-h time) urban center. Outdoor smog chamber work at the Research Triangle Institute and a t the University of North Carolina demonstrates that a 95% diluted and “spent” urban smog system can readily generate O3 above 0.08 ppm after 2 and 3 days of natural irradiation (10, 11). Sickles has shown that isopentane and NO, systems subjected to sunlight irradiations can exceed the 0 3 NAAQS with initial concentrations as low as 0.2 ppmC NMHC and 20 ppb NO2 (12). This system is also capable of “storing” more than 50% of the O3generated in a given day’s exposure. Most evidence indicates that the rural high oxidant situation may be NO, controlled, but hydrocarbons other than CH4 are required to achieve O3 greater than 0.08 ppm. The situation is clearly not as simple as that suggested by Chameides and Stedman.
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Literature Cited (1) Chameides, W. L., Stedman, D. H., Enuiron. Sci. Technol., 10, 150-53 (1976). (2) Heuss, J. M., 68th Annual Meeting of the Air Pollution Control
Assoc., Paper 75-16.1,Boston, Mass., 1975.
(3) Demerjian, K. L., Kerr, J. A., Calvert, J. G., in “Advances in Environmental Sciences and Technology”, J. N. Pitts and R. L.. Metcalf, Eds., Vol4, pp 104-08, Wiley, New York, N.Y., 1974. (4) Chameides, W. L., Walker, XC.G., J . Geophys. Res., 81,413-20
(1976). (5) Farrow, L. A., Edelson, D., Int. J . Chem. Kinet., VI, 787-800
(1974). (6) Gear, C. W., “Numerical Initial Value Problems in Ordinary Differential Equations”, Chap. 11,Prentice Hall, Englewood Cliffs, N.J., 1971. (7) Hampson,R. F., Garvin, D., NBS Technical Note 866, Nat. Bur.
Stand.,Washington, D.C., 1975.
(8) Johnson, D. R., Bach, W. D., Jr., Decker, C. E., Hamilton, H. L., Jr., Matus, L. K., Ripperton, L. A., Royal, T. M., Worth, J.J.B., RTI Project No. 41v-764,106, Research Triangle Institute, North Carolina, 1973. (9) Johnson, D. R., Decker, C. E., Eaton, W. C., Hamilton, H. L., Jr., White, J. H., Whitehorne, D. H., EPA-450/3-74-034, Research
Trianale Institute, North Carolina, 1974. (10) Ripperton, L. A., Eaton, W. C., Sickles, J. E., 11, Final Report, EPA Contract No. 68-02-1296, 1976. (11) Jeffries. H. E.. Fox, D. L.. Kamens. R. M.. EPA-650/3-75-011. University of North Carolina, Chapel Hill, N.C., 1975. (12) Sickles, J. E., 11, PhD thesis, University of North Carolina, Chapel Hill, N.C., 1976. H. E. Jeffries M. Saeger
Department of Environmental Sciences and Engineering University of North Carolina Chapel Hill, N.C. 27514
SIR: Dimitriades et al. and Jeffries and Saeger have argued against our mechanism (1) for the production of high levels of ozone in rural areas. Their objection is largely based on clear discrepancies between our model results and theirs. They suggest that the fault lies in our use of the quasi-steadystate-assumption, which, they claim, tends to yield unrealistically high radical concentrations. They also argue that our results are contrary to smog chamber data. However, we believe our predictions can be justified by comparison with measurements and the predictions of others. Truly unpolluted air may not be found in Dearborn, Mich.; however, our model predicts OH radical concentrations in agreement not only with Wang et al. (2) but also, when the proper input data are used, with the unpolluted upper tropospheric measurements of Davis ( 3 ) .If, as Wang et al. have observed, OH is present in the summer daytime atmosphere a t concentrations of the order of 107 ~ m - significant ~ , quantities of 0 3 could be generated from our assumed concentrations of CH4, CO, and H2 of 1.5, 0.65, and 0.5 ppm, respectively. While CO is a product of the oxidation of CH4, we recognize that a CO concentration of 0.65 ppm, appropriate for the continental U.S. ( 4 ) , represents an anthropogenic input. At these concentrations H02 and CH302 radicals are produced a t a combined rate of about 5-10 ppb h-l, as shown in Table I. For NO concentrations of the order of 50 ppb, this production will result in an equivalent O3 production rate of about 5-10 ppb h-I as predicted in our model calculations (11.
While smog chamber data would appear to contradict our calculations, Demerjian et al. ( 5 ) point out that significant problems arise in any attempt to interpret smog chamber data. This may arise partly from unreproducibility, since no published data are yet available which show either that any one chamber can quantitatively repeat the same data a year or two later, or, more importantly, that any two smog chambers can Volume IO, Number 9, September 1976
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