thiophosphates where cadmium may be an impurity in the original zinc used to prepare the additive. The same authors reported 20-90 gg/g cadmium in car tires as a probable result of use of zinc-diethylcarbonate (and associated cadmium contamination) in the process of vulcanization. Nickel and chromium are frequently used in chrome plating, and copper is a common constituent of piping and other components of engines and chassis. The presence of these elements in soils and vegetation along motorways is probably due to mechanical wear and tear which deposits these metals in the fine dust of the roadway and therefrom to soils and vegetation. The extent to which the above heavy metals (apart from lead) constitute a hazard to public health is a question somewhat outside the scope of the present work, but because of their known toxicity to humans, this question should be studied further. Acknowledgment The authors express their appreciation to J. E. Cox and J.
Brown of the Soil Bureau Substation DSIR, Mount Albert, Auckland, for their kindness in obtaining soil cores for the present survey and to H. H. McCoach for providing soils from a background area. Literature Cited (1) Cannon, H. L., Bowles, J. M., Science, 137,765 (1962). (2) Page, A. L., Ganje, T. J., Enuiron. Sci. Technol., 4, 140 (1970). (3) Ward, N. I., Brooks, R. R., Reeves, R. D., Enuiron. Pollut., 6,149 (1974). (4) Olson, K. W., Skogerboe, R. K., Enuiron. Sci. Technol., 9, 227 (1975). (5) Dedolph, R., Ter Haar, G., Holtzman, R., Lucas, H., ibid., 4,217 (1970). (6) Rabinowitz, M. B., Wetherill, G. W., ibid., 6,705 (1972). (7) Ward, N. I., Reeves, R. D., Brooks, R. R., N.Z. J. Sci., 18, 261 (1975). (8) Lagerwerff, J. V., Specht, A. W., Enuiron. Sci. Technol., 4,583 (1970). (9) World Health Organization, “Health Hazards of the Human Environment”, WHO, Geneva, Switzerland, 1972.
Received for review December 22,1976. Accepted M a y 2,1977.
CORRESPONDENCE
SIR: Dr. Dimitriades ( 1 )has made significant progress in the search for an oxidant control strategy based on emission reductions. Despite the questionable long-term reproducibility (common to all smog chambers) of his unpublished smog chamber data, he has included the effects of NO, and HC on the oxidant maxima. Further progress along these lines has included computer modeling complete with the effects of realistic light intensity (2, 3 ) . An ozone isopleth diagram generated by these studies is shown in Figure 1. The purpose of this letter is to point out that the apparent safe area increasing with increasing NO, (the isopleth lines of positive slope) is an artifact of the static chamber and model assumptions used, that if two further parameters, mixing and dilution are accounted for, the safe area disappears in terms of protecting the public. Consideration of rural hydrocarbons leads to the conclusion that only NO, control can reduce the area over which large-scale low-level ozone episodes occur. Reference 3 states: If t h e oxidation of NO by organics is delayed sufficiently so t h a t t h e s u n has passed its zenith before significant amounts o f NO2 are created, photodissociation o f NO2 will be reduced, and less ozone will be allowed t o accumulate. OZONE
ISOPLETHS
k
0 . .
0”
z
0
0.8
0.4
1.2
pprnC Figure 1. Ozone isopleth diagram redrawn from ref. 2 NMHC
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This time for oxidation of NO to NO2 is a significant factor in reducing the maximum ozone observed in chambers or in models. When NO, is measured a t a ground level station, vertical mixing up to an inversion layer brings down air bearing some ozone (and relatively little HC or NO,). If we assume 40 ppb 0 3 in this air from aloft, then all levels of NO less than enough to make 40 ppb in the mixed layer will be rapidly converted to NOz. This initial conversion of NO to NO2 by vertical mixing enables smog chemistry to start faster ( 4 ) even though 0 3 is used up. The effect of vertical mixing thus causes the atmosphere to start its smog chemistry at NO2 whereas NO is used in the chamber studies. This effect is particularly important if one uses a surface NO reading as the initial input, whereas in the whole mixing layer it may rapidly become NOz. Diffusion also is important in eliminating the apparent “safe” area; thus, in ref. 2:
A “worst case”approach was adopted to describe t h e dispersion of pollutants. A zero wind speed was assumed and t h e only dispersion of pollutants considered was t h a t caused by t h e daily lifting of t h e inversion layer. T h e worst mixing occurs along t h e W e s t Coast where there is only a 100-meter difference between t h e m e a n summer morning and afternoon mixing heights. Over a 9-hour period this corresponds to a 3 percent per hour dilution rate. Blumenthal et al. ( 5 )have shown experimentally that diffusion and mixing can give a worse “worst case” than the static system modeled. This effect can be demonstrated using Figure 1. Thus, in terms of dose, if one considered the point A (0, = 0.16 ppm) then lateral dilution below an inversion base with absolutely clean air can lead to B (0, = 0.10 pprn). However, the air mass is now twice as large; thus, on the average twice as many people are affected, and the violation of the standard, although not so severe, lasts longer for any given wind speed. Worse yet, clean rural air contains 0.2-0.4 ppm NMHC (3). Dilution by a factor of two with this air leads to the point C (0, = 0.17 ppm), a worse violation for twice the affected population for a longer time. If natural sources of NMHC are taken as 0.3 ppm C for clean rural background, and if we assume total control of man-made HC emissions, then oxidant formation is con-
strained to the dashed line on Figure i. On this line NO, concentrations are the only factors which control oxidant formation. Since the rural NO, background is close to zero (3, 4,6,7), any level of NO, will be diluted toward the point D on the HC axis. For any NO, input sufficient to exceed 0.06 ppm NO,, maximum oxidant potential from Figure 1is about 0.14 ppm 0 3 . Rural violations of the 0.08 ppm standard will occur wherever NO, exceeds about 0.04 ppm. This rural NO, concentration will only be encountered in areas near man-made sources (as shown in ref. 3).Only NO, reduction will control the area over which the violation occurs. Note, however, that urban HC dontrol can limit rural excursions to
