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
Table 1. Principal Reactions in NO,/HO,
--
System rate constant ppm-mln unltsa
reaction
+ hu NO + O(3P) O(3P) + O2 + M O3 + M + NO NO2 +
1. 2. 3.
NO2
4.
NO2 4- O(3P) NO f 0 2 NO2 4- O(3P) NO3 NO2 NO 4- O(3P) NO2 0 3 NO3 02 NO3 f NO 2N02 NO3 NO2 N2O5 NO2 NO3 N2O5 2HON02 N2O5 H20 NO NO2 H20 2HONO NO NO2 HONO HONO 0 3 hu O2 O('D) 0 3 hu 0 2 O(3P) o('D) M o ( 3 ~ )M O('D) H20 20H HONO 0 2 HO2 NO2 HO2 NO2 ---* HO2NO2 HOz NO2 HO2NO2 NO2 4- OH HO2 NO OH t NO HONO HONOp OH NO2 HONO hu -+ OH NO CO OH COP HO2 H20 NO2 OH HONO HO2 HO2 H202 0 2 OH HO2 H20 0 2 OH O3 HOP O2 HO2 0 3 OH f 202
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.
30.
03
+
0 2
+
-+
-+
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
-
+
-+
--
+
- --
+
+
+
-+
+
+
+
+
+
+ H20
re1
notes
variable exp(510/T) 3.9 X 3.1 X lo3 exp(-1450/Tj 1.34 x 104 3.6 x 103 5.6 X l o 2 exp(584/ T ) 1.8 X 102exp(-2450/T) 2.8 x 104 2.19 X 1O2exp(861/Tj 7.44 X iOi5exp(-i03i7/T) < i . 5 x 10-5 2.2 x 10-9 1.4 x 10-3 variable variable 3.0 X l o 4 exp(l07/T) 3.4 x 105
