(2) U.S. EPA, “Uses, Limitations, and Technical Basis of Procedures for Quantifying Relationships between Photochemical Oxidants and Precursors”;Nov 1977;EPA-450/
2-77-021k
Sudarshan Kumar
Environmental Science Department General Motors Research Laboratories Warren, Michigan 48090
S I R The regression and rollback equations presented in my note ( I ) were stated to be approximate and valid only within specified constraints of concentrations and NMHC/NO, ratios. These will be called “transition ratios” in this response to the comments by Kumar. Kumar acknowledged that the rollback equation had been stated to be applicable only within the specified constraints. Yet in his calculations he disregards the constraints and therefore incorrectly concludes that the equations are suspect and always overestimate the NMHC reductions. Of the 27 examples in Table I of Kumar’s comments, only two were in compliance with the constraints. For the two valid cases, the hydrocarbon reductions calculated by the rollback equation were within 10 and 15% of those determined by Kumar by using the standard EKMA diagram. The 15% difference of the hydrocarbon reduction corresponds to an equivalent difference of about 0.01 ppm ozone at the O,, level. This amount is within the realm of the term “approximate”. As stated by Kumar the equations do not allow for NO, inhibition of ozone production under any condition. The equations apply only within the transition ratios where ozone is a function of P1f2.They do not apply below the lower transition ratio where NO2 inhibition occurs. Nor do the equations apply above the upper transition ratio where ozone depends mainly on the NO, concentration. None of the examples or comments by Kumar addressed the area near or above the upper transition ratio, which is about 20 based on the standard EKMA diagram. Kumar also stated that when NMHC and NO, reductions are introduced, the resulting NMHC/NO, ratios generally are below the lower limit of the range of validity (below the lower transition ratio) of the regression equation. Obviously this was true for 25 of his 27 examples. However, the lower transition ratio will not be reached when the hydrocarbon control required is moderate and the initial ratio is high. It will not be reached when the control requirements are substantial, but the reductions of hydrocarbons and NO, are commensurate or nearly so. As stated in the note, the transition ratios differ depending on environmental chamber conditions and other factors, but to my knowledge the reason or cause of the differences has not been satisfactorily explained. Subsequent to publication of the note, a recent article by Spicer was reviewed to determine the transition ratios based on his environmental chamber study results (2). Within an ozone concentration range of 0.23-0.35 ppm the transition ratios appeared to be about 10 and 25. These are higher than those reported in the note. Glasson’s environmental chamber data were reviewed for the same purpose (3). On the basis of his data, the transition ratios appeared to be about 25 and 50 within an ozone concentration range of from 0.10 to 0.25 ppm. It was not possible to determine the ratios accurately because 568
Environ. Scl. Technol., Vol. 17, No. 9, 1983
the number of data points from Glasson’s studies were limited, as were those from Spicer’s studies. Thus the transition ratios determined from results of various chamber studies differ considerably, and these differences raise questions as to which if any transition ratio values are applicable to urban atmospheric situations. The lack of reliable ambient air NMHC data has hindered the determination of ozone-precursor relationships in the ambient air. However during a number of summer days in 1976 and 1977, reliable speciated hydrocarbon measurements were made in the ambient air at Los Angeles from 6:OO to 9:00 a.m. PDT (4, 5 ) . Los Angeles precusor-Pasadena ozone relationships were developed by using this data and indicate the following: (1) The daily maximum ozone concentration at Pasadena correlates well with the 6:OO-9:00 a.m. P112concentrations at Los Angeles. With a sample number of 36, the ozone-precursor correlation coefficient was 0.70 and is highly significant. (2) The mean NMHC/NO, ratio was 8.7, and the range was 5.3-14.3. The correlation coefficient of ozone as a function of the ratios was -0.02, and the ozone scatter was reasonably uniform throughout the range of the NMHC/NO, ratios. Thus, there is no indication of NO2 inhibition on ozone formation in the ambient air of the Los Angeles area. If inhibition occurs in urban atmospheres, it does so at ratios below 5.3. Registry No. Ozone, 10028-15-6;NO,, 11104-93-1. Literature Cited (1) Kinosian, J. R. Enuiron. Sci. Technol. 1982,16,880-883. (2) Spicer, C. W. Environ. Sci. Technol. 1983, 2, 112-120.
(3) Glasson, W . A. J.Air Pollut. Control Assoc. 1981,31,1169. (4) Mayrsohn, H.; Kuramato,M.; Crabtree,J. H.; Sothern, R. D.; Mano, H. S. ”AtmosphericHydrocarbon Concentrations, June-September 1976;”Research Division, CARB: Sacramento, CA, 1977. (5) Mayrsohn, H.; Kuramoto, M.; Crabtree,J. H.; Sothern,R. D.; Mano, H. S. ‘‘AtmosphericHydrocarbon Concentrations, June-September 1977”; Research Division, CARB: Sacramento, CA, 1978. John R. Klnoslan
California Air Resources Board Sacramento, California 95812
Comment on “Acid Preclpitatlon in Historical Perspective” and “Effects of Acid Precipitation”+ SIR The history of acid rain research has been detailed by Cowling (3) in a recent critical review. Cowling (3) states that the observations of Cronan and Schofield ( 4 ) and Ulrich et al. (27))among others, are of key importance to our understanding of acid rain effects. Technical issues supporting this perspective are discussed in another recent article by Glass et al. (7). In this correspondence, several aspects of these reports are examined, and some pertinent literature not cited by Cowling (3)are presented to provide a more complete view of the acid rain problem. Manuscript preparation supported by the U.S. Department of
Energy under Contract W-7405-eng-26 with Union Carbide Corp. Publication No. 2168, Environmental Sciences Division, ORNL.
0013-936X/83/0917-0568$01.50/0
0 1983 American Chemical Society
