are quite low. A recent modeling exercise adds credence to this hypothesis. Dodge (12) found t h a t an increase in precursor dilution from 3 to 2 0 % k resulted in a decrease of between 39 and 52% in ozone formation. Conclusion
In conclusion, this three-day study suggests that: the amount of naturally emitted hydrocarbon is low, these low levels cannot possibly contribute to the production of significant levels of ozone, and man-made pollutants arising from stationary sources, gasoline spillage, and automotive exhaust constitute the major portion of the ambient hydrocarbons in the Florida area. This study was done in May and was not necessarily representative of an entire year; however, these conclusions should be qualitatively valid throughout the year. Literature Cited (1) Teraniski, R., Lundin, R., McFadden, W., Mon, T., Shultz, T., Stevens, K., Wasserman, J., J . Agr. Food Chem., 14,447 (1966).
(2) Schultz, T., Teraniski, R., McFadden, W., Kilpatrick, P., Corse, J., J . Food Sci., 29,790 (1964).
(3) Teraniski, R., Schultz, T., McFadden, W., Lundin, R., Black, D., ibid., 28,541 (1963). (4) Altshuller, A. P., Bufalini, J. J., Enuiron. Sci. Technol., 5 , 39 (1971). (5) Environmental Protection Agency Contract No. 68-02-2298 to Washington State University, Final Rep. in preparation, 1977. (6) Lonneman, W., Kopczynski, S., Darley, P., Sutterfield, F., Enuiron. Sci. Technol., 8,229 (1974). (7) Lonneman, W. A., “Ozone and Hydrocarbon Measurements in Recent Oxidant Transport Studies”, in Int. Conf. on Photochemical Oxidant Pollution and Its Control Proceedings, p 211, EPA-600/ 3/77-001a, Jan. 1977. (8) Grimsrud, E. P., Westberg, H. H., Rasmussen, R. A,, Int. J. Chem. Kinet., Symp. No. 1, p 183 (1975). (9) Lonneman, W. A., Seila, R. A,, Meeks, S. A., Symposium on the 1975 Northeast Oxidant Transport Study, Research Triangle Park, N.C., Jan. 20-21,1975. (10) Rasmussen, R. A., Enuiron. Sci. Technol., 4,667 (1970). (11) Seila, R., Lonneman, W., Meeks, S., J . Enuiron. Sci. HealthEnuiron. Sci. Eng. A, 11 (2), 121 (1976). (12) Dodge, M. C., “Combined Use of Modeling Techniques and Smog Chamber Data to Derive Ozone-Precursor Relationships”, in Int. Conf. on Photochemical Oxidant Pollution and Its Control, p 881, EPA-600/3-77-001b, Jan. 1977. Receiued f o r review March 17,1977. Accepted October 20, 1977
Conductimetric and Pararosaniline Method SO2 Monitoring Uncertainties and Their Significance Richard S. Fein” and Bruce S. Bailey Texaco Research Center, Beacon, N.Y. 12508
Conductimetric and pararosaniline method 24-h average
Table 1. Data Base for Method Comparisons
SO2 determinations from routine monitoring in southern California are analyzed to estimate method uncertainties. Random measurement errors range to 0.02 and 0.006 ppm for the two methods, respectively. Total conductimetric method uncertainties range to 0.06 ppm. Hence, this method is not satisfactory for determining compliance with very low ambient air quality standards. Uncertainties in the accuracy of sulfur dioxide monitoring methods affect the level a t which standards for SO2 may be set and the confidence with which the compliance with the standards may be determined. If monitoring uncertainties are large, standards requiring continuous attainment of low ambient SO2 concentrations may not be appropriate because compliance cannot be determined unambiguously. This report analyzes the relative uncertainties associated with 24-h average SO2 concentrations reported for measurements by the conductimetric and pararosaniline methods in routine ambient air monitoring of low SO2 concentrations in southern California. The uncertainties include any data system errors in addition to analytical uncertainties. This analysis is of considerable practical import since the conductimetric method is the reference method for California’s present 0.05 ppm and previous 0.04 ppm SO2 standards. 24-Hour Average SO2 Data Analysis
Comparisons of the two methods were made for 416 days of validated data from four locations. Table I describes the data base. I t was assumed that the same air was sampled by both methods, since the addresses for the measurements by the two methods are the same ( 1 ) . Twenty-four hour average SO2 measurements by the conductimetric (California Reference) method are the arithmetic 0013-936X/78/0912-0463$01.00/0
0 1978 American Chemical Society
city
Monitor location Address
Anaheim Los Angeles San Bernardino San Diego
1010 S. Harbor 434 S. San Pedro 172 W. 3rd 1011 Island
Data pairs
Tlme perlod
160 101 130
1968-74 1970-74 1968-74 1973-74
25
mean of all hourly data from the National Aerometric Data Bank (NADB) for those days with a t least 18 h of data. Twenty-four hour average SO2 measurements by the pararosaniline (Federal Reference) method were converted to ppm from the pg/m3 24-h average values in the NADB. Figures l a and I b show representative examples of the cumulative distribution of the measurements. Figure 2 shows the cumulative distributions of the paired differences between the SO2 measurements for the same day a t each site. ‘‘a Values” from nonparametric and parametric statistical tests in Table I1 indicate t h a t the two SO2 measurement methods are not equivalent a t any of the four locations ( 2 ) .The means and biases and their standard deviations are shown in Table 111;biases are relative to the pararosaniline method. Regressions summarized in Table IV show that the maximum bias a t 0.04 ppm SO2 ranges to f 0 . 0 4 ppm. Biases of this magnitude when measuring ambient air in the eastern US., Kawasaki, Japan, and California are reported in the literature (3-8). Usually, but not always, the conductimetric measurements exceed the pararosaniline measurements. The biases are attributed to the effect of interferring pollutants such as salt aerosols, acid mist, and ammonia with the nonspecific conductimetric method (9-12); however, the SOz-specific pararosaniline method can be biased low ( 1 3 ) . T h e random variability of the test methods was estimated by an analysis of variance. The analysis assumed equal coVolume 12, Number 4, April 1978
463
lOOL Y&m&TIMETRIC X CONDUCTIMETRIC M E T H O 0 PARAROSANILINE
0 X
0 PARAROSANILINE
MEMO
METHOD
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00 xxx 0
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0
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Figure l a . Frequency distributions of San Diego data
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.01
0.1
1
10 50 PER C E N T
9
90
Figure lb. Frequency distributions of San Bernardino data
variances between method errors and atmospheric SO2 concentrations for the two methods and zero covariance between method errors. The results indicated that testing errors of approximately f0.02 and f0.006 ppm were associated with the conductimetric and pararosaniline methods, respectively, a t the 99.7% (3 u) confidence level. Based on these estimates of bias and random testing error, a single 24-h SO2 determination by the conductimetric method may be in error relative to the pararosaniline method (Federal Reference) by as much as 0.06 ppm (0.04 ppm bias 0.02 ppm) a t the concentration level of the previous 0.04 ppm California standard. At higher SO2 concentrations, the maximum possible error is larger.
0 ANAHEIM
X LO5 ANGELES 0 SAN BERNARDINO
0 SAN DIEGO
+
Significance of Data Analysis
-,lo
' I
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10
" ' I , ' ' 50
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90
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1.9
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Figure 2. Frequency distributions of paired differences
The large uncertainties in routinely determined low-level 24-h average SO2 concentrations are important in setting a standard and in determining compliance with the standard. For setting a standard, a regulatory agency needs to consider realistic uncertainty limits (including measurement uncertainty) on the threshold SO2 level from each significant research study. Such consideration for all significant studies allows setting a standard that has an acceptable factor of safety and is not unduly over-stringent because of one or a few uncertain low thresholds. To reliably achieve a factor of safety while allowing some reasonably low level of SO2 emissions, the agency must specify a reference measurement method with small bias and random uncertainty. The nonspecific conductimetric method with uncertainties approaching f0.06 ppm is not suitable with low standards such as the present California 0.05 ppm 24-h standard. The need for small measurement method uncertainty is exemplified by the 101 days of data from Los Angeles. For these days the nonspecific conductimetric method indicates that three days exceed the previous 0.04 ppm standard and
one d a y exceeds the 0.05 ppm standard; the SOz-specific pararosaniline method indicates that all days comply with both standards with a factor of safety of a t least 40%. Thus, measurement method uncertainty leads to grossly conflicting determinations of compliance with the 0.04 and 0.05 ppm standards. Determination of the capability of a reference (or equivalent) monitoring method to determine compliance with a standard requires knowledge of the absolute uncertainties of the method under routine monitoring conditions. Such knowledge is obtained from carefully controlled monitoring experiments employing independent primary calibration standards. While the data available for this analysis did not include such primary calibration data, the comparative data available did permit reliable estimates to be made of the monitoring uncertainties of interest. The results of this work clearly indicate the large uncertainties associated with the conductimetric method and its unsuitability to determine compliance with a standard in the 0.04-0.05 ppm range.
Table II. a-Values for Method Comparisons Comparative property measured Blas Locatlon
Anaheim
Los Angeles San Bernardino San Diego
464
Frsq dlstrlbutlon KolmogorovSmlrnov
