NOTES
Relative Photochemical Reactivity of Propane and n-Butane Leo Zafonte' and Frank Bonamassa Haagen-Smit Laboratories, Atmospheric Studies Branch, Research Division, State of California, Air Resources Board, 9528 Telstar Avenue, El Monte, Calif. 91731
A smog chamber study was undertaken to investigate the relative photochemical reactivity of propane and n-butane. The use of lower, more realistic initial nitric oxide concentrations and longer irradiation times in the photochemical reaction mixture permitted ozone concentrations to reach higher levels compared to the classical experiment utilizing an initial nitric oxide concentration of 1-2 ppm and a 6-h irradiation time. Thus, the ozone yields and, therefore, the relative photochemical reactivity of moderately reactive compounds such as propane are increased. The study indicates that hydrocarbon reactivity experiments should be carried out as closely as possible to conditions that occur in the atmosphere to determine realistic potentials for ozone production. In this report we examine the results of experiments carried out in the evacuable irradiation chamber of the Laboratories of the California Air Resources Board (ARB). These experiments are intended to give insight into the question of the relative organic photochemical reactivity of propane and n butane. The ARB has recently adopted a three-class hydrocarbon reactivity scale ( 1 )which places all paraffinic hydrocarbons with three or more carbons in Class I1 (moderate reactivity organics). Dimitriades (2) considers all paraffinic hydrocarbons with four or more carbons to be of moderate reactivity (Class I11 of a five-class scheme) but places propane in the nonreactive classification (Class I) along with methane and ethane. The classification of Dimitriades is based on older smog chamber studies that were carried out at unrealistically high nitrogen oxides concentrations (2) and for irradiation times that are significantly less than those which occur in the ambient atmosphere ( 3 ) .Therefore, there is a need to examine this aspect of reactivity with additional experiments. The implications of the experimental results for oxidant control strategies are considered.
Experimental Experiments were carried out in a borosilicate glass-lined evacuable chamber (see ref. 4 for details of chamber construction and operation) with a volume of 4.11 m3. The inner surface area is 17.7 m2, and the surface area-to-volumeratio is 4.31 m-l. The photochemical lighting system consists of three parallel Pyrex tubes that extend through the chamber and are oriented axially between the side walls and the chamber center axis. The UV light intensity inside the chamber yields dioxide photolysis, kl, of 0.2 min-l within the chamber which is less than that observed in the ambient atmosphere ( 3 ) . The half-life for ozone decay in the dark chamber is in excess of 50 h. Prior to each experiment the desired volumes of the hydrocarbon and the nitrogen oxide were premeasured and then injected into the evacuated chamber. Zero air (supplied by Scott Research) was then introduced as the diluent gas to a total pressure of 16 psi. Water sufficient to give an initial
relative humidity of 50% was introduced into the chamber with the diluent air as it is passed through a heated bubbler containing deionized distilled water. The chamber is customarily evacuated to lov6 torr between experiments to maintain day-to-day system integrity and to prevent the possible accumulation of reaction products on the interior surfaces. A 10-h irradiation of zero air alone yields less than 0.02 ppm of ozone. Sampling for oxides of nitrogen (TECO Corp. analyzer, Model 12-A) and ozone (Meloy Corp. analyzer, Model OA 325) is continuous. The ozone and nitrogen oxides analyzers are calibrated with a Monitor Labs Model 8500 calibrator. Con; centrations of ozone (up to 1.0 ppm) are simultaneously monitored during calibration with a DASIBI UV photometer (DASIBI Corp., Model 1003H) which has been standardized against a UV photometer. Hydrocarbons are analyzed by standard gas chromatographic techniques. The gases are sampled by means of Pyrex probes extending 2.5 ft beyond the interior walls of the chamber center axis. Each of the experiments was duplicated as similar reactant concentrations and yielded similar results. The data reported represent two single experiments.
Results and Discussion For these experiments it was decided to investigate the ozone yield from the irradiation of a nitric oxide/air mixture with each of the hydrocarbons individually. In this way the experiments would be comparable to similar experiments upon which Dimitriades (2) based his reactivity analysis. A major departure from the earlier studies, however, is that in the present work the initial nitric oxide concentrations of 0.2 ppm and the initial concentration of hydrocarbon of 7.5 ppmC are significantly lower, Le., the earlier studies involved nitric oxide concentrations in excess of 2 ppm and hydrocarbon concentrations in excess of 2 ppm compound. The initial concentrations used in these experiments were designed to approximate more closely realistic in terms of existing atmospheric conditions. The total irradiation time of 10 h also approximates more closely the 10-12 h of irradiation time available in the atmosphere ( 3 ) during the late spring and summer. Figures 1 and 2 give typical results for the irradiation of n-butane and propane, respectively, under the conditions described above. The most important observation is that both hydrocarbons produce substantial amounts of ozone during the 10-h irradiation period. Furthermore, the ozone yields during these irradiations are nearly peaked a t the end of the irradiation period, rather than earlier in the time period. Table I summarizes some of the parameters from the above experiments. The nitrogen dioxide maxima in both experiments are reached a t 2.5 and 3.5 h for the butane and propane experiments, respectively, and the peak value for the propane experiment (Figure 2) is only slightly lower than that for nbutane. After reaching a peak the nitrogen dioxide exhibits a steady decline in concentration, as expected. The onset of Volume 11, Number IO, October 1977
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z
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Table 1. Reaction Parameters for n-Butane and Propane Irradiations Compound
Initial concn, m Nitrica Hydro- %otrogen oxide carbonb dioxidea
n-Butane 0.19 Propane 0.19
7.4 7.8
0.01 0.02
maxa
0.17 0.16
Time to max, h
2.5 3.5
Max ozonea
0.22 0.15
a Concentration expressed in units ppm. Concentration expressed in ppmC units.
0 0
01
Table II. Reactivity Comparison of Hydroxyl RadicalHydrocarbon Rate Constant , I
0
2
1
4
1
1
6
1
8
1
1
.
10
I R R A D I A T I O N TIME ( H o u r s )
Figure 1. Ozone production from n-butane/NO photooxidation in ARB
high-vacuum irradiation chamber
I R R A D I A T I O N TIME ( H o u r s )
Figure 2. Ozone production from propane/NO photooxidation in ARB
high-vacuum irradiation chamber
ozone formation occurs after approximately 50% of the NO has been converted to NOz. The maximum ozone for the nbutane experiment is 0.22 ppm after 10 h, compared to a value of 0.15 ppm for propane. Thus, the ozone yield for propane over the 10-h irradiation time is very significant compared to that for n-butane, establishing that propane must be considered to be a reactive compound in terms of ozone production. It is worthwhile to consider the reasons for the observed reactivity of propane in more detail. I t was indicated above that previous reactivity experiments used high concentrations of nitric oxide (>1ppm) in the initial reaction mixture. The use of such a high initial nitric oxide concentration in the previous studies would increase the time to reach peak nitrogen dioxide concentrations beyond the 2.5 h observed in these experiments. Because significant ozone formation does not occur until the nitrogen dioxide concentration has peaked, there is little remaining time in a 6-h irradiation period to observe ozone buildup. The atmosphere, on the other hand, provides a significantly longer 10-12-h irradiation time. Most of the earlier experiments upon which Dimitriades and others rely were carried out over a 6-h irradiation period. Thus, the 1016
Environmental Science & Technology
Compound
Ozone yield, ppm
Relative reactivity
Propane n-Butane
0.15 0.22
0.68
1.00
kHOa
2.2 X 3.0 X
Ref
(HO) b , c
5 6
1.2 X lo6 1.2X IO6
a Units: cc/mol/s. Units: mol/cc. Estimated from the total hydrocarbon loss over a 7-h period.
apparent reactivity based on maximum ozone yield for propane and other moderately reactive organics would then be severely diminished, compared to the reactivity that is consistent with realistic atmospheric air contaminant concentrations. Table I1 compares the maximum ozone yields obtained in these experiments with the relative rate constants for the reaction of hydroxyl radical with the test hydrocarbon. The ratio of the rate constants, 0.73, is in excellent agreement with the observed relative reactivity for propane of 0.68. The reactivity measurements were made a t equal ppmC concentrations while the rate constants are in volumetric units. Therefore, agreement may be fortuitous. These data would indicate, on the one hand, that for moderately reactive organics, the rate constant for the reaction of the organic compound with hydroxyl radicals could be a good measure of the compound's reactivity in terms of ozone yield. However, as the HO rate constant increases and the organic structure becomes more complex, it is not anticipated that a linear relationship will still be valid, because of expected complexities in the reaction mechanism. The last column in Table I1 gives the average hydroxyl radical concentrations calculated from the hydrocarbon loss over a 7-h irradiation period. In both cases a concentration of 1.2 X 106 mol/cc was calculated. This value is lower but in general agreement with hydroxyl radical concentrations determined from modeling calculations. Conclusions The results of this study show that propane will produce significant levels of ozone in smog chamber irradiations. Comparison of the relative reactivity of propane with n-butane indicates that the ARB classification of propane as a moderately reactive hydrocarbon is confirmed under conditions which approximate those in the atmosphere. The use of lower, more realistic initial nitric oxide concentrations and longer irradiation times in organic reactivity studies permits ozone concentrations to reach higher levels when irradiating HC/NO, mixtures containing low and moderately reactive organics, thus indicating more realistic potentials for ozone production. Acknowledgment The authors express their appreciation to D. Daymon for his aid in carrying out these experiments.
Literature Cited (1) “Adoption of a System for the Classification of Organic Compounds According to Photochemical Reactivity”, Staff Report #76-3-4, State of California, Air Resources Board, El Monte, Calif., Feb. 19, 1976. ( 2 ) Dimitriades, B., “Proceedings of the Solvent Reactivity Conference”, EPA-65013-74-010, Nov. 1974. (3) Zafonte, L., Rieger, P. L., Holmes, J. R., Enuiron. Sci. Technol., 11 ( 5 ) ,483 (1977).
(4) Shikiya, J. M., Daymon, D., Faigin, H., “The Hi-Vacuum Irradiation Chamber”, Publ. No. DTS-76-19, State of California, Air Resources Board, El Monte, Calif. (5) Gorse, R. A., Volman, D. H., J . Photochem., 3, 115 (1974). (6) Doyle, G. J.,Lloyd, A. C., Darnall, K. R., Winer, A. M., Pitts, J. N., Jr., Enuiron. Sei. Technol., 9,237 (1975).
Received for review January 12, 1977. Accepted May 9, 1977.
Comparison of Levels of Trace Elements Extracted from Fly Ash and Levels Found in Effluent Waters from a Coal-Fired Power Plant David R. Dreesen’, Ernest S. Gladney, James W. Owens, Betty L. Perkins, Caroline L. Wienke, and Lawrence E. Wangen Environmental Studies Group, M.S. 490, Los Alamos Scientific Laboratory, P.O. Box 1663, Los Alamos, N.M. 87545 Trace elements were extracted from a coal-fired power plant electrostatic precipitator ash with nitric acid, hydrochloric acid, citric acid, redistilled water, and ammonium hydroxide as extractants. Effluent waters at this plant were sampled to assess the elevation of trace element concentrations compared with intake waters. The results showed a positive correlation between those elements most extractable by water (B, F, Mo, and Se) or acid (As, B, Cd, F, Mo, and Se) and those elements most elevated in effluent waters (As, B, F, Mo, and Se). The expanding use of coal for electric power generation in the southwestern US., along with the high ash content of these coals, will necessitate the disposal of massive amounts of coal ash. The possible contamination of waters and soils by such coal ash disposal requires investigation. The solubility behavior of trace elements in the alkaline waters of this region may indicate that different elements are of concern here than those found in more acidic aquatic environments. A study has been initiated to identify which trace elements are most extractable from ash and to compare them with those elements that are observed at elevated concentrations in effluent waters from a coal-fired power plant in the southwestern U.S.
Experimental Methods Electrostatic precipitator ash for the extraction experiment, influent waters, and effluent waters were collected a t a large coal-fired power plant at Fruitland, N.M. A flow diagram of water usage a t this plant is given as Figure 1. Influent and effluent waters were sampled at four locations: (A) cooling lake intake; (B) cooling lake outlet a t the base of the dam; (C) decant channel-effluent water from the ash settling pond; and (D) ash pond-surface water. Ash slurry from the venturi scrubbers is sent to the ash pond where the ash settles and the water is decanted. Surface water in the ash pond was sampled for comparison with ash pond effluent water in the decant channel. The outlet of the cooling reservoir was sampled for comparison with cooling lake intake and ash pond effluent, as well as to approximate scrubber intake water. Coal ash effluents discharged into the cooling lake (1) may also influence trace element concentrations in the outlet. Each sample was filtered in the field with a Whatman No. 41 prefilter, a 0.45-1m Millipore membrane filter, and a backing pad; the filtered samples were acidified in the field and later frozen. The sampling train and bottles were made from plastic materials.
Trace elements in the precipitator ash were extracted using an ash-to-extractantratio of 1:4. The extractants included 0.1 M H3C&07 (citric acid), 1.0 M HC1, 1.0 M “03, 0.1 M HN03, 0.01 M “OB, 0.001 M HN03, redistilled H20, and 0.1 M NH40H. The ash-extractant mixture was agitated for 3 h and then filtered through Whatman No. 41 followed by Whatman No. 542 filter paper. The filtrates were acidified and frozen in polyethylene bottles. Complete details of the extraction experiment are reported elsewhere (2).
Analytical Procedures Fluoride was measured in the sample solutions with an Orion Model 94-09A specific ion electrode and a Corning Model 112 digital volt meter ( 3 ) .The boron content was determined by thermal neutron capture prompt y-ray analysis ( 4 ) by evaporating 10 mL of the solutions to dryness on small polyethylene sheets. The capture y-ray facility at the Los Alamos Omega West Reactor was described by Jurney et al. (5). The remaining elements in solution were measured by flameless atomic absorption with a Perkin-Elmer Model 306 spectrophotometer equipped with a deuterium background corrector and a HGA-2000 graphite furnace. Perkin-Elmer electrodeless discharge lamps were used for As, Cd, and Se determinations, and Perkin-Elmer hollow cathode lamps were SAN JUAN R I V E R
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1977
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