Envlron. Sci. Technol. 1983, 17, 64
Comment on “Approach to Forecasting Daily Maximum Ozone Levels in St. Louis” SIR: The article by Prior et al. (I) which appeared in the April 1981 issue of Environmental Science and Technology makes some significant technical contributions that merit discussion. The authors’ statement that “a test of the effectiveness o f . [a] . forecasting model is to examine the ability of the model to predict ozone episodes” is true. They should have been more emphatic-it is the test of any ozone model. The fact that they clearly recognize “ozone episodes” as being distinct and important events and endeavor to deal with them is in itself a significant contribution. In too many cases there has been a tendency to relegate episodic days to the realm of “outliers”and simply ignore them. Yet from both theoretical and legal standpoints they are the ballgame. I also believe that the authors’ finding that the best regression variable for predicting daily maximum ozone is the ozone concentration at 9:00 A.M. is important. I have approached the same problem by looking for a correlation between maximum daily downwind plume ozone with maximum daily upwind ozone or “side” ozone (i.e., adjacent rural maxima) with some success. It is my thesis that the atmospheric photochemical process is limited by the supply of free radicals with sufficient energy to attack hydrocarbons and sustain a chain oxidation process. Ozone via several mechanisms can yield such radicals. Therefore, the supply of input ozone can control the level of photochemical activity and the maximum concentration of secondary ozone that develops downwind. Input ozone either can be of stratospheric origin or can be transported from elsewhere. This thesis in no way exonerates anthropogenic pollutants as also being essential for the generation of high levels of downwind plume ozone. This view does, however, change one’s perception of the factors that limit the ozone on any given day. The almost total failure of episodic ozone maxima to correlate with either hydrocarbon or NO, precursor concentrations in numerous studies simply points out that some other variable controls. It also explains the near total failure of hydrocarbons abatement to reduce ozone. If hydrocarbons are 10-foldin excess, reducing the excess to 5-fold will have little or no effect. This is particularly true if the real limitation on the process is free-radical population as controlled by input ozone. The finding of Prior et al. that 900 A.M. ozone is the best independent predictive variable is certainly consistent with this thesis. Ozone at 9:00 A.M. is a measure of “input ozone”. I do take issue with these authors when they suggest that a St. Louis episode might be attributed to the passage of incoming air over “the massive emission sources of the Ohio Valley”. While there is probably validity in claiming the Ohio Valley to possess massive emission sources of TSP or SOz, the claim is far less supportable for NO, or hydrocarbons.
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64
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Environ. Sci. Technol., Vol. 17, No. 1, 1963
Blaming ozone episodes on transport has been popular recently. Thus Minnesota partisans blame a 140 ppb level observed in 1980 on Chicago, and authors speaking for the Northeast blame everything West-even Texas; and now Prior et al. in analyzing St. Louis point the finger at the Ohio Valley. Of course these views are politically popular but to date are supported by little scientifically solid evidence. Few authors appear to have a true perspective of the vastness of the continental atmosphere when compared to that of the urban areas or of the potential of the atmospheric cleansing and dilution processes. Anthropogenic hydrocarbons and NO, simply cannot be transported for several hundred miles over one or more days elapsed time and arrive at concentrations capable of generating ozone levels significantly above the current standard such as found in and downwind of urban areas. Neither can ozone at such concentrations be transported for such distances in the surface boundary layer. Dilution and destruction serve to substantially obliterate ozone plumes within 100 miles or less. Thus, except for urban areas in close proximity, one must accept the fact that high levels of photochemical ozone are derived from local pollutants. In pollution-free atmospheres well above the surface of the earth, ozone itself is long lived. Thus ozone can at times survive along distance transport-but not at major concentrations. There is no way atmospheric dilution processes can be avoided. Such transported ozone, while insignificant in concentration when compared to that found during episodes in downwind urban plumes, can be significant when compared to minimum ozone background levels. Thus transported ozone could increase a background of 50 ppb to perhaps 80 or even 100 ppb. This ozone in turn would increase background free-radical concentrations and the intensity of the photochemical smog process in polluted areas. Thus claimed instances of ozone transport may in reality be initiation phenomena. The thesis that the influx of stratospheric ozone initiates surface photochemistry via free-radical processes has received more attention and rests on more substantial evidence. It is probable that widespread ozone episodes are the consequence of the widespread influx of stratospheric ozone behind cold fronts and in the back side of highpressure air masses, not merely due to the direct transport of ozone. Registry No. O,, 10028-15-6.
Literature Cited (1) Prior, E.J.; Schiess, J. R.; McDougal, D. S . Enuiron. Sci. Technol. 1981, 15, 430.
Harry M. Walker
Monsanto Chemical Intermediates Co. Alvin, Texas 77511
0013-936X/83/0917-0064$01.50/0
0 1982 American Chemical Society
