SIR: Dr. Stedman’s comments are understood to be essentially as follows: The questionable long-term reproducibility of Dimitriades’ “unpublished” data makes it inappropriate to determine effects of NO, and HC on oxidant maxima. The upper part of the isopleth diagram (Dr. Stedman’s Figure 1) depicting a beneficial effect from NO, increase is an artifact of the static chamber and model assumptions used; mixing and dilution should be considered. Dilutiodmixing with air containing O3 or natural hydrocarbons could increase oxidant or population exposure or both. Only NO, control can reduce the large areas over which lesser violations of oxidant air quality standard occur. In response to the first comment, Dimitriades’ data have been previously published both in a scientific journal ( I )and in much more detail as a US.Bureau of Mines Report of Investigations (2). The data were obtained in an experimental test program completed within a period of approximately 6 months. The published reports include graphs from which the precision-related data scatter can be judged. This author disagrees with Dr. Stedman’s other comments. Those comments appear to reflect a misunderstanding of the intended applicability of the O3 isopleths. These isopleth diagrams are intended for use upon the urban 0 3 problem only-as was clearly explained in the Dimitriades articles ( 3 , 4)-and not upon the rural problem. Thus, increase of the NO, emissions in urban areas with low HC-to-NO, ratio will have a beneficial effect on the oxidant-related air quality. The effect of such NO, increase upon the downwind rural areas will not necessarily be beneficial. Dr. Stedman’s comments regarding the roles of natural hydrocarbons, of O3 from aloft, and of the NO, factor upon rural air quality have some merit but are in some respects incorrect and certainly far from delineating the effects of all factors that have a role in this extremely complex problem. A discussion of the rural oxidant problem, however, is not relevant to the subject here. This author agrees with Mr. Hovey’s comments. It is indeed true that the Dimitriades model ( 3 , 4 )has only “local” validity, that is, is applicable only in situations in which oxidant/Os is produced from local emissions. Situations in which the problem is caused largely by oxidant/03 transported in the region call for different, as yet not quantified regional models.
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Alternatively, the Dimitriades model could be used but only after simplifying assumptions are made. Mr. Ford’s first comment apparently pertains to the NOzto-precursor relationship, a subject of peripheral interest only in the Dimitriades article ( 3 ) .Whether the ambient precursor pollutant concentrations are or are not log-normally distributed is irrelevant to the subject, as it is the possibility that “the annual maximum 3-hourly concentration of NO2 may exceed the annual average by more than 10 times”. For the purpose of calculating oxidant-related control requirements by t h e Dimitriades model, it is necessary that the NO2-dictated NO, control requirements first be determined. To determine such requirements, one may assume a 1:lrelationship between NO, and NO2-as was assumed by Dimitriades 13) for illustrative purposes-or any other relationship. In response to Mr. Ford’s questions on the identification of the precursor sources responsible for the observed oxidant problem, there are aerometric techniques by which the oxidant contribution of the local emissions can be estimated. The sources responsible for the transported-in oxidant cannot be pinpointed, a problem for which the solution probably lies in development of area-wide control strategies. Questions of such nature are treated in forthcoming EPA documents including some new control guidelines (5) and the newly revised Air Quality Criteria for Photochemical Oxidants and Oxidant Precursors. Literature Cited (1) Dimitriades, B., Enuiron. Sci. Technol., 6,253 (Mar. 1972).
(2) Dimitriades, B., “On the Function of Hydrocarbon and Nitrogen Oxides in Photochemical Smog Formation”, U.S. Bureau of Mines, RI7433, Sept. 1970. (3) Dimitriades, B., Enuiron. Sci. Technol., 11,80 (Jan. 1977). (4) Dimitriades, B., “An Alternative to the Appendix-J Method for Calculating Oxidant- and NO*-Related Control Requirements”, International Conference on Photochemical Oxidant Pollution and Its Control Proceedings, EPA-600/3-77-001b, US.Environmental Protection Agency, Research Triangle Park, N.C., Jan. 1977. ( 5 ) Fed. Regist., 42,9202 (Feb. 15, 1977).
Basil Dimitriades Environmental Sciences Research Laboratory U S . Environmental Protection Agency Research Triangle Park, N.C. 2771 1
