and Maaskant for their laboratory analyses and their willingness to offer facilities for the experiments with the water quality monitor.
by laboratory measurements which showed a variance of Eo a t a maximum of 2 mV in the temperature range 10-30 “ C . The sensitivity, selectivity, and response of the electrodes after the tests were as satisfactory as a t the start.
Literature Cited
Conclusion The plastic-membrane nitrate-selective electrode with a quaternary ammonium salt as ion-selective species has proved to be reliable in continuous on-stream analysis of aerobic surface water and aerobic effluents of sewage treatment plants. Under aerobic circumstances only interference by chloride can be expected, which in most cases will result in a positive deviation of 8 1412 W
0 co
*
10-
8-
642i
A
. . . . . . . . . . . . . . . . . . . . . . . . 0 1 2 3 4 5 6 7 8 9 1 O l l l . 2
Collection Rate ( p m o l e SO*/Hr) Figure 6. '*Oenrichment vs. collection rate of particulate sulfate
The collection rates of SO2 (Figure 5 ) apparently dropped to near zero a t all three sites immediately after the low-pressure cell, accompanied by thunderstorms, passed over the region on July 23-24. (The unusually high value for the Auburn sample on July 22 coincided with a northeast wind, blowing from the direction of a large power plant in Springfield, Ill.) Trajectory Analysis. Back trajectories a t 6-h intervals were calculated for the air masses that were present a t the three sampling sites during the overnight sample collections
Figure 7. Back trajectories (6-h intervals) from midnight, July 23-24, 1975
on July 23-24, which yielded the maximum 8lSO values and minimum collection rates, and during the daytime sample collections on July 26. which generally yielded minimum b180 values and relatively high collection rates. The results are shown in Figures 7 and 8. The wind fields used in the calculation of these back trajectories were produced from mean winds a t rawinsonde stations by modified inverse distance-squared objective analyses (11).The winds up to 2000 m above mean sea level were averaged for the 0000 GMT (Greenwich mean time) soundings (late afternoon), and the first two layers (500-600 m) were averaged for the 1200 GMT (early morning) soundings. Wind fields between soundings were produced by linear interpolation. This pattern of analysis simulated transport within a layer capped by a typical nocturnal inversion during the night and early morning and within a deep, well-mixed layer during the afternoon and early evening. Several other methods of producing transport winds were also examined, with generally similar back trajectories being produced for trajectories ending a t 0600 GMT on July 24,1975, but with considerable variation in back trajectories ending a t 1800 GMT on July 26,1975. I t is recognized that any one-layer back trajectory method has considerable uncertainty beyond 24 h, since the planetary boundary layer tends to be layered a t night (with marked wind shear) and well mixed during the afternoon. I t is felt that the method described above gives representative back trajectories, particularly for the first 24 h back in time. The back trajectories for July 24 (Figure 7) indicate that the air mass was probably very clean up to 24-36 h prior to sampling, as the air mass had moved down from Canada behind a cold front. For the final 24 h prior to sampling, the air mass should have received some pollutant loading from sources in the Ohio River Valley. The back trajectories for July 26 (Figure 8) indicate a very slow-moving air mass, with a gradual drift up from the south. The pollutant loading within the 72-h period of trajectory calculations should have come from sources in the lower Mississippi Valley. There also could have been considerable pollutant loading prior to the 72-h period, although no calculations were made because of increasing trajectory uncertainty. 6lSO in Ambient Water Vapor. In Figure 3, a general downward trend in I 8 0 enrichment in water vapor is evident for the three sites during the first 3 to 4 days of sampling. The meteorological changes that occurred on July 23-24, which coincided with abrupt reductions in the concentrations of particulate sulfate and SO2 and a corresponding increase in the l80del value of sulfate, had relatively little effect on the oxygen-isotope quality of the atmospheric water vapor.
Discussion
Regionality of Particulate Sulfates. The concerted variations observed in the I 8 0 contents and in the concen-
Figure 8. Back trajectories (6-h intervals) from noon, July 26, 1975
trations of particulate sulfates a t St. Louis and the two rural sites, Auburn and Glasgow, during the 6-day sampling period were apparently controlled by regional meteorological phenomena. The concerted peaking a t all three sites (Figures 2 and 4) and the back trajectories of the air masses that were present a t the times of these maxima and minima (Figures 7 and 8) were not commensurate with localized transport of urban pollutants from St. Louis to the other two sites, nor from any other local source to all three sites. However, these results, which demonstrate the regional influence that meteorology can have upon the transport of particulate sulfates, are not necessarily representative of results which might otherwise be obtained under other environmental conditions. I t may be noted that the perturbation which appeared in the collection-rate data for Auburn on about July 26 (Figure 4) corresponded to a back-and-forth trajectory of the encompassing air mass, as shown in Figure 8. Origin and Mechanism of Formation of Particulate Sulfate. If the relatively high l8O content of the sulfate of low concentration in the air mass that covered the region of the three sampling sites on the night of July 23-24 was characteristic of sulfate which was transported fairly rapidly from Canada via the trajectories shown in Figure 7, and if the relatively low l8O content of the more concentrated sulfate in the air masses that covered the sample sites on July 26 was characteristic of sulfate which was formed and transported by sluggishly moving air in the trajectories shown in Figure 8, different mechanisms of formation may be indicated for the sulfates of the two different origins. Previous work ( 3 ) has shown that sulfate formed by hydrolysis-oxidation mechanisms, in which isotopic equilibration occurs between HSOs- and water before oxidation to sulfate, is heavily influenced by the lS0content of the hydrolyzing water. Such sulfate may differ significantly in ' 8 0 content from sulfate formed by gas-phase oxidation of SO2 in the atmosphere, followed by hydrolysis to form S042-. Huebert (12) has suggested that the results of this experiment support the hypothesis that homogeneous, gas-phase oxidation of SO2 takes place relatively slowly in the atmosphere a t all times, producing sulfates of relatively high $ 8 0 , and that sulfate formation by heterogeneous, hydrolysisoxidation mechanisms occurs intermittently, causing extra loading of the atmosphere with sulfates of lower P O . Accordingly, dry air masses may be characterized by particulate sulfates of low concentration and high P O , and moist air masses, by sulfates of higher concentrations and lower 6180.
Oxygen isotope studies need to be extended to the examination of particulate sulfates which are unambiguously formed by atmospheric oxidation-hydrolysis mechanisms, to determine whether such sulfates are indeed characteristically higher in lSO content than sulfates formed in the atmosphere Volume 12, Number 13, December 1978
1397
by hydrolysis-oxidation mechanisms. The particulate sulfates examined in this experiment did not appear to be isotopically coupled with the ambient water vapor in the air masses from which they were sampled. The lack of correlation between the temporal variations of P O of sulfate (Figure 2) and of water vapor (Figure 3) may indicate one or both of the following conditions: (1)the absence of a predominant secondary mechanism of formation in which the dl*O of sulfate would be expected to vary directly with the 6ls0 of water vapor and (2) the formation of the sulfate prior to association with the ambient water vapor that was sampled in the air mass. Furthermore, a significant fraction of the water vapor in the air a t the sampling points may have originated from localized evaporation and evapotranspiration within the respective vicinities of the samplers. We have routinely collected particulate sulfates on 3 consecutive days of each month for 2 years a t Argonne, Ill. The composite results (to be reported in a later paper) show no significant correlation between P O and concentration. Possibly, the strong negative correlations found in the present experiment are observable only in consecutively collected daily samples rather than in samples which are collected a t several-day intervals over a period of several months. There may be other hypotheses to explain the inverse relationship between 6l80 and concentration of particulate sulfates observed in this experiment. We hope that the reported observations will stimulate further investigation and discussion.
Literature Cited (1) Cunningham, P. T., Holt, B. D., “l80Analysis in the Study of
Atmospheric Sulfate Aerosols”, pp 19-43, Chemical Engineering
Division Environmental Chemistry Annual Report, July 1974June 1975, ANL-75-51, Argonne National Laboratory, Argonne, Ill., 1975. (2) Holt, B. D., Engelkemeir, A. G., Johnson, S. A., Cunningham, P. T., “Oxygen Isotopy in the Formation of Sulfate Aerosols”, pp 683-92, Proceedings of the Second International Conference on Stable Isotopes, Oak Brook, Ill., 1975, CONF-751027, USERDA, 1976. (3) Holt, B. D., Cunningham, P. T., Engelkemeir, A. G., “Application of Oxygen-18 Analysis to the Study of Atmospheric Sulfate Formation”, in Robinson, B. W. (Comp. and Ed.), “Stable Isotopes in the Earth Sciences”, pp 105-9, New Zealand DSIR Bulletin 220, 1978. (4) Lloyd, R. M., Science, 156, 1228 (1967). (5) Holt, B. D., Anal. Chem., 19,1664 (1977). (6) Coutant, R. W., Enuiron. Sci. Technol. 11.873 (1977). (7) Pierson, W. R., Hammerle, R. H., Brachaczek, W. W., Anal. Chem., 48,1808 (1976). (8) “Sulfur Dioxide Interferences in the Measurement of Ambient Particulate Sulfates”, Final Report from Radian Corp. to Electric Power Research Institute, EPRI 262, Jan. 1976. (9) Appel, B. R., Kothney, E. L., Hoffer, E. M., Wesolowski, J . J., “Comparison of Wet Chemical and Instrumental Methods for Measuring Airborne Sulfate”, U S . Environmental Protection Agency Report No. EPA-600/7-77-128, Nov. 1977. (10) Forrest, J., Newman, L., Atmos. Enuiron., 7,561 (1973). (11) Inman, R. L., “Operational Objective Analysis Schemes”, National Severe Storms Forecast Center, Circular No. 10, National Severe Storms Laboratory, Norman, Okla., 1970. (12) Huebert, B., National Center for Atmospheric Research, Boulder, Colo., private communication.
Received for review December 9, 1977. Accepted July 5 , 1978. This work u:as performed under the auspices of the U.S. Department of Energy, Division oXBasic Energy Sciences. Support for the analysis of the samples was provided by the U.S. Environmental Protection Agency, Environmental Sciences Research Laboratory, Aerosol Research Branch.
Continued Development of a Kinetic Mechanism for Photochemical Smog Andrew H. Falls and John H. Seinfeld” Department of Chemical Engineering, California Institute of Technology, Pasadena, Calif. 9 1125
A kinetic mechanism for photochemical smog is developed to incorporate recent new information on rate constants and mechanisms. The predictions of the mechanism are compared with smog chamber data on propylene, n-butane, and propyleneln-butane systems. Areas of uncertainty are delineated, and the influence of these uncertainties on the predictions of the mechanism is discussed. There has been considerable effort devoted to the development of chemical reaction mechanisms for photochemical air pollution (1-10). The strategy in mechanism development has generally been to proceed from the detailed chemistry of specific hydrocarbon-NO, systems, such as those containing propylene or n -butane, to generalized chemistry that might be capable of representing an atmospheric system. Although many important areas of uncertainty still exist, some success has been achieved in simulating smog chamber data for hydrocarbon systems such as ethylene, propylene, and n-butane (8).
For atmospheric simulations, two approaches to mechanisms have been taken: surrogate mechanisms-mechanisms in which organic species in a particular class, e.g., olefins, are represented by one or more members of that class, e.g., propylene (7); and lumped mechanisms-mechanisms in which organic species are grouped according to a common basis such 1398
Environmental Science 8 Technology
as structure or reactivity (2,3,5,6,8,9).In general, surrogate mechanisms tend to be more lengthy than lumped mechanisms because, within a surrogate mechanism, each individual species is treated as a separate chemical entity. Consequently, because of the computational requirements associated with calculating chemistry and transport simultaneously, surrogate mechanisms are inappropriate for use in atmospheric models that include an adequate treatment of meteorology. Considerable new information relating to rate constants and mechanisms of reactions influential in photochemical smog has recently been obtained ( 1 1 ) .In addition, extensive smog chamber data from the Statewide Air Pollution Research Center (SAPRC) of the University of California, Riverside, are now available for mechanism evaluation (12).In this paper a new lumped chemical mechanism for photochemical smog that includes the latest information on relevant rate constants and mechanisms is developed. Substantial changes have been made from the mechanism of Hecht et al. ( 5 ) reflecting this new information. With respect to the simulation of atmospheric systems, one major area of uncertainty still exists, namely the chemistry of aromatic species. Much research is currently being directed toward the atmospheric chemistry of aromatics (13),although sufficient information is not now available to include aromatic species in this study. Consequently, attention is focused on olefins, paraffins, and aldehydes. 0013-936X/78/0912-1398$01 .OO/O @ 1978 American Chemical Society
