CORRESPONDENCE
SIR: I am obliged to comment upon Dr. Daly’s paper ( I ) concerning oxidant control strategies for Sydney. In his paper Daly applies both the Dodge isopleths and the 2/3 Post and Bilger isopleths ( 2 )to a joint precursor distribution which he claims is representative of conditions found in Sydney and draws conclusions concerning the relative merits of strategies involving the control of NMHC, NO,, or both NMHC and NO, emissions. Daly’s analysis fails on two counts in that (a) the incomplete joint precursor distribution used by Daly is not representative of Sydney and is based upon a biased data set and (b) the Dodge isopleths do not describe ozone formation in Sydney. The incomplete joint precursor distribution contains 419 data pairs and was published by me ( 2 ) as a guide to NMHC/NO, ratios and upper limit concentrations observed in Sydney air. It does not describe the frequency of occurrence of various precursor concentrations since it excludes most of the low-concentration data. The complete distribution published in detail elsewhere ( 3 )contains 836 simultaneous 0.5-h averaged observations of NMHC and NO, concentrations and has been shown to be representative of the airshed and is independent of sample site location. In earlier publications ( 3 , 4 )using the complete precursor distribution and the 2/3 Post and Bilger isopleths, I derived a predicted ozone distribution which compared very well with that obtained by the New South Wales State Pollution Control Commission’s monitor network. These predicted and observed distributions on a full 2-yr basis 1975-77 are shown in Figure 1. The ozone distribution predicted by Daly from the incomplete precursor distribution and the 2/3 Post and Bilger isopleths is also shown. Daly’s predicted ozone distribution is clearly not compatible with the observed ozone distribution and reflects the bias of the incomplete precursor distribution toward higher precursor concentrations. With his control strategy conclusions based upon an unrepresentative precursor distribution, and his analysis using both the correct 2/3 Post and Bilger isopleths and the Dodge model (which does not describe the Sydney airshed), it is not surprising that Daly’s results contradict those of Bilger and mine (3-7). Daly’s Figure 1 is reproduced here as Figure 2B. It shows that a 33% reduction in NO, emissions is as effective for ozone control as a 33% reduction in NMHC emissions. This result is derived from the incomplete joint precursor distribution and the wrong ozone formation model. Using the complete joint precursor distribution and the 2/3 Post and Bilger isopleths, one obtains the results shown in Figure 2A; viz, 33% reduction in NMHC emissions is far more effective for ozone control than a 33% reduction in NO, emissions. In fact, the correct analysis shows that a 33% reduction in NMHC emissions is as effective as a total 66% emission reduction made up of a 33% reduction in NO, and a 33% reduction in NMHC. Under these circumstances, Daly’s discussion of cost functions ( I ) becomes somewhat spurious as it can be seen that a policy of NO, emission reduction in conjunction with NMHC emission reduction would require that NO, emission reduction exhibit a negative cost function. Daly has published a more recent analysis (8) in which he accepts the use of the complete precursor distribution and uses my simple ozone formation model: 0 3 = 0.39([NOX][NMHC])0.36ppm ( 3 ) .This model is an approximation to the 2/3 Post and Bilger curved isopleth surface and is only valid within the ratio limits 5 < [NMHC]/[NO,] ppmC/ppm < 20. His use of this simple model with the complete precursor distribution shifted to simulate emission reductions unfor1532
Environmental Science & Technology
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Figure 1. Predictedand observed ozone concentration distributions for Sydney. (Distributions predicted from summertime precursor distributions and summertime 7 3 Post and Bilger isophlethsare compared with full-year monitored ozone distributions by assuming no ozone formation in the winter half of the year and halving the predicted summertime frequencies.) A
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Figure 2. Predicted summertime ozone concentration distributions showing effects of different precursor emission control strategies: (A) correct distribution and the 2/3 Post and Bilger isopleths; (B) Daly’s distributions based on the incomplete precursor distribution and the Dodge isopleths.
tunately again leads his analysis astray. When the complete precursor distribution is shifted to simulate reductions in emissions of either NMHC or NO,, the precursor distribution shifts out of the region of applicability of the simple model, and Daly’s conclusions regarding ozone control strategy, which are claimed to support the results of the paper presently under discussion, are again invalid. In conclusion, I wish to stress that Figures 1-3 of Daly’s paper ( I ) are based upon an incomplete and biased precursor distribution and consequently the discussion of these figures in the text of Daly’s paper leads to invalid conclusions with respect to control strategies applicable to Sydney. I refer the reader to the analysis based upon the complete precursor distribution and the proven isopleth model Post ( 3 ) which results in a clear case for NMHC emission reduction for ozone control in Sydney.
0013-936X/80/0914-1532$01,00/0 @ 1980 American Chemical Society
Literature Cited (1) Daly, N. Enuiron. Sci. Technol. 13, 1979, 1373-76. (2) Post, K.; Bilger, R. W. Atmos. Enuiron. 1978, 12, 1857-65. (3) Post, K. Atmos. Enuiron 1979,13, 783-90. (4) Post, K.; Bilger, R. W. Sydney Oxidant Study, Annual Report for the University of Sydney for 1977-78. Charles Kolling Research Laboratory Technical Note ER-29, Sept 1978. (5) Post, K. “Ozone Concentration Distributions for Sydney”,Proceedings, “Air Pollution into the Eighties-The National Quest for Emission Controls”; clean Air Society of Australia and New Zealand: Canberra, Oct 1979. (6) Bilger, R. W. Enuiron. Sci. Technol. 1978,12, 937-40. (7) Bilger, R. W. “The Hydrocarbon Route to Control of Photo-
SIR: The paper entitled “Use of Frequency Distributions of Potential Ozone in Evaluating Oxidant Controls” describes a method for combining photochemical models and real airshed data to provide frequency distributions of potential ozone. The method is exemplified by analysis of the published data for the Sydney airshed ( I ) . Post accepts the technique but rejects the results of the analysis (2).The rejection is based on the assertions that the technique does not use either a representative precursor distribution or a proven model (2). These assertions are examined below. The data analyzed consisted of -400 sets of 0.5-h averaged concentrations of NMHC and NO, concentrations originally described by Post as “typical of the morning precursor measurements made in Sydney on both trajectory and nontrajectory days” ( 3 ) .Subsequently, Post has published a more complete set of concentrations measured by the SPCC of New South Wales and Sydney University which contains 836 0.5-h averaged concentrations of NMHC and NO, ( 4 ) , and he points out that the values originally analyzed exclude most of the low-concentration data (2).The data originally excluded by Post were those which did not satisfy one of the following conditions: (1)The maximum recorded oxone concentration was greater than 5.0 pphm. (2) The maximum recorded NO, concentration was greater than 5.0 pphm. (3) The maximum recorded NMHC concentration was greater than 0.5 ppm. The bias built into the original set by the method of selection chosen by Post is a bias toward the precursor sets more likely to lead to oxidant problem days. Accordingly the distributions of potential ozone relate to such days and are those formable “for days of high oxidant or precursor concentrations” ( I ) . The identification of selection criteria for concentrations enables the threshold for these oxidant problems to be estimated. On the basis of the modified Dodge model, the limit of 0.05 ppm NO, and the limit of 0.5 ppm NMHC are, within the precision of the models, close to a limit of 0.12 ppm ozone. The discarding of concentrations below either of these precursor limits approximates the discarding of precursor sets with an ozone potential of less than -0.12 ppm of ozone. Post asserts (2) that the potential ozone distributions are not based on a proven model. They are as discussed fully ( I ) , based on the Dodge model both in its original form and in the 2/3 modified form used by Post and Post and Bilger (2,3). The factor */3 used by Post and Bilger ( 3 )is an empirical correction used in the attempt to improve the fit of ozone values calculated from Dodge to 16 sets of NMHC, NO,, and ozone data measured during the Sydney Oxidants Study. The sets of precursor concentrations have NMHC/NO, ratios in the range 5-20 and are presented ( 3 ) as validation of the Dodge isopleths providing that the ozone concentrations calculated from Dodge are multiplied by 0.67. No test of the goodness of fit is reported, and no verification of the model outside the NMHC/NO, range of 5-20 is established. Adoption of these 0013-936X/80/0914-1533$01.00/0
chemical Smog”;Symposium Proceedings of the Proposed Controls on the Evaporation of Solvents and the Storage and Transfer of Volatile Organic Liquids; Clean Air Society of Australia and New Zealand: Sydney, March 1978. (8) Daly, N. S.; Fuller, G. J. “Evaluating Options for Controls”, Proceedings of the Conference on Air Pollution into the Eighties-The National Quest for Emission Controls; Clean Air Society of Australia and New Zealand: Canberra, Oct 1979. Keith Post Department of Mechanical Engineering University of Sydney New South Wales 2006, Australia
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Figure 1. Predicted and observed ozone distributions in the Sydney Basin: ( - -)values measured across the SPCC network, 1975-77; (0) distribution predicted by Post; ( 0 )distribution predictable from potential
ozone (see text).
“modified” isopleths is in fact adoption of the Dodge model, since the isopleths are assumed to be parallel to the Dodge isopleths a t all times, especially outside the NMHC/NO, range of 5-20. The isopleths of the Dodge model were regarded originally as applicable to the evaluation of relative ozone rather than absolute concentrations ( 5 ) . Post applies the modified model to calculating absolute concentrations of ozone (2) by using conditions considerably different from those in the original model. The assumptions in the treatment are set down elsewhere (6). The technique of evaluating distributions of potential ozone by using airshed data treats the problem as one of controlling the ambient concentrations of NMHC and NO, in such a way as to minimize the frequency with which the oxidant has the potential for exceeding the stated goal ( I ) . Thus the ozone potential of each set of precursor concentrations is calculated as if existing airshed conditions were akin to those of the smog chamber. In practice the meteorological conditions favoring oxidant formation occurs only at random intervals so that ont would expect that the frequency with which potential ozone exceeds a stated value to be greater than the frequency with which ozone actually exceeded that value. This basic postulate is not appreciated by Post (21,who maintains that “the predicted ozone distribution is clearly not compatible with the observed ozone distribution and reflects the bias of the incomplete precursor distribution toward higher precursor concentrations.” In fact, it is more likely to reflect that meteorological conditions do not favor oxidant formation on every occasion. When the distribution of potential ozone is subjected to the same operations as those assumed by Post, it leads to a distribution which agrees with the distribution of ozone reported by Ferrari et al. (7). The previous analysis
@ 1980 American Chemical Society
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14, Number 12, December 1980
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