LETTERS California smog Dear Sir: Dr. Pitts points out in a May feature article (€S&T, May 1977, p 456) that there is inadequate data for tighter controls on NO, and low reactivity hydrocarbons, but appears to favor their imposition on the basis of speculative harmful effects even though the cost is extremely high. He justifies NO, controls on the following bases: Downwind O3may be increased by increased overall NO, emissions. Nitrosamines are “suspected” carcinogens. Control of NO, decreases NOp levels. Reduction of NO, lowers secondary nitrate aerosols, nitric acid and PAN. The only known source of ozone is the photolysis of NO2. There appears to be no clear-cut evidence that downwind ozone was increased by higher NO, emissions. Such increases could be attributed to a combination of meteorology, limited and unreliable ozone data, and growth in reactive hydrocarbon (RHC) emissions in nearby Orange County. As Dr. Pitts points out, there is no evidence for the presence of nitrosamines in the Los Angeles atmosphere or, if they were present, that they would contribute to human cancer. Also, it is questionable that if they did form, whether realistic tighter NO, controls would decrease their level. The statement that decreases in NO, emissions would result in decreased NOz must consider the trade off involved in the rapid reaction (03 -I-NO O2 -t NO2). Ozone is 10-50 times as toxic as NOp (FederalRegister, Vol. 36,No. 105). The benefits derived from this reaction in the major population areas far outweigh the effects of NO2. The toxicity of NOp at current levels is extremely questionable. If reactive hydrocarbons are reduced by about 50% and present peak NO, values applied (0.2-0.3 ppm), only a minor part of the NO would be oxidized in the basin below the inversion layer. Also, ozone would be below present ambient standards according to Dr. Pitts’ isopleths. This decrease in RHC should be achieved by measures now in place such as old car phaseout and evaporative controls. Increased downwind NO emissions from future growth in the eastern end of the basin would also decrease local ozone levels because of reaction with ozone. If NO, is more tightly controlled to its practical limit (0.1 ppm), it would appear that a level of 0.15 ppm ozone would never be achieved. Regarding secondary nitrogen products (aerosols, PAN, nitric acid), these do not form significantly until NO is depleted. For fixed HC level and radiation, NO depletion
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time is proportional to initial NO level. A lower level of NO, to its practical limit (0.1 ppm) would increase the time available for secondary reactions and, thereby, be expected to increase secondary products. Dr. Pitts presents no evidence to support his view that realistic stricter NO, controls would produce lower levels of secondary nitrogen products. The argument is made that the photolysis of NO2 is the only known source of ozone. While this may be true, it is a misleading statement. The photolysis generates NO, which rapidly reacts with ozone, so that there is negligible net effect in producing ozone, which is proportional to the radiation level and the ratio of NO2/ NO. NO emissions serve to decrease this ratio. In the absence of HC’s this quasiequilibrium gives a very low ozone level. SAPRC’s chamber work with zero added NO, and 0.6 ppm NMHC produced an ozone level of 0.16 ppm in 6 h, showing that if NO2 is required, only the amounts present in unpolluted atmospheres are necessary for ozone formation. Certainly the reactivity of all organics is of concern and there is no doubt that low reactivity hydrocarbons make a minor contribution to NO oxidation and resultant ozone formation. However, Dr. Pitts appears to condone the current policy of regulatory agencies that regard all organics as equivalent on a weight basis, since they eventually will contribute to ozone formation somewhere. W.
B. lnnes
Upland, Calif. 91786
California smog (cont.) Dear Sir: The excerpts selected by Dr. lnnes from the feature (fS&T, May 1977, p 456) in many cases do not reflect what I actually wrote, nor do they necessarily reflect what I really believe. For example, statements such as: “appears to favor their imposition on the basis of speculative harmful effects even though the cost is extremely high” and “However, Dr. Pitts appears to condone the current policy of regulatory agencies that regard all organics as equivalent on a weight basis, since they eventually will contribute to ozone formation somewhere,” are straw men, contrary to the thrust of my article, and not worth discussing in this forum. Turning to matters of science, Dr. Innes’ statement that “There appears to be no clear cut evidence that downwind ozone was increased by higher NO, emissions” is clearly contradicted by the vast amount of information presently available on simulated and real polluted atmospheres. With respect to southern California, Dr. John Trijonis et ai. recently carried out a detailed analysis of air quality
and emission data and concluded that “the historical trends in ambient oxidant over the past decade represent, in effect, a test of an oxidant control strategy in which hydrocarbon emissions are reduced while increasing NO, emissions, thus reducing the hydrocarbon to NO, ratio. The net result has been a decrease in oxidant levels in western and central Los Angeles County and an increase in oxidant levels in the eastern South Coast Air Basin.” Furthermore, as I noted, the complex nature of the chemical and physical transformations during transport of polluted air parcels and the associated “downwind O3effect“ are not unique to southern California. Considering nitrosamines, my point clearly was that we need to know far more about the atmospheric reactions of their possible precursors, NO, and amines. Dr. lnnes has only to consult the recent EPA Star Series document “Scientific and Technical Assessment Report on Nitrosamines” to appreciate my concern and that of the scientific community. As far as NOp being the only known photochemical source of ozone, as well as a host of toxic nitrogenous compounds (e.g., PAN), Dr. Innes’ arguments fail to take into account the well-established fact that severe smog episodes can last for days, providing ample time for complete photooxidation of HC and NO, if not in the central city areas, then certainly in downwind suburbs miles away. Thus, the air quality of someone (or something), somewhere, will be further degraded by increased NO, emissions. Whether or not this is significant depends on the situation. Space limitations preclude a detailed discussion of several other of Dr. Innes’ arguments; I can only restress my belief that different strategies for NO, and hydrocarbon control may be required for different basins. These will depend upon the present ambient levels of HC and NO, in a given urban atmosphere, as well as, of course, on meteorological and topological conditions, projected growth, etc. Smog chamber isopleths provide one basis for assessing the relative merits of HC and NO, controls in reducing ozone in the central city and downwind. In conclusion, rather than relying on Dr. Innes’ interpretation of my views, and of our SAPRC chamber data, I would encourage readers to consult our original technical articles as well as my testimony to legislative bodies and control agencies. As are all scientists, we will be pleased to furnish reprints! James N. Pitts, Jr. Statewide Air Pollution Research Center
University of California Riverside, Calif. 92521
