1120
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
(9) E. K. Hodgson and I. Friedovich, Phofochem. Phofobiol., 18, 451-455 (1973). (10) R. B. Brundrett, D. F. Roswell, and E. H. White, J . Am. Chem. Soc., 94, 7536-7545 (1972). (11) H. D. K. Drew and F. H. Pearman, J . Chem. SOC.,566-592 (1937). (12) A. Spruit-Van Der Burg, Recuel, 8g, 1536-1544 (1950). (13) H. D. K. Drew and R. F. Garwood, J . Chem. Soc., 1841-1846 (1937). 64, 2644-2649 (14) E. H. Huntress and J. V. K. Gladding, J. Am. Chem. Soc., (1942). (15) K.-D. Gundermann and M. Drawert, Chem. Bet‘., 95, 2018-2026 (1962). (16)R. B. Brundrett and E. H. White, J . Am. Chem. Soc., 96, 7497-7502 (1974). (17) H. R. Schroeder, P. 0. Vogelhut, R. J. Carrico, R. C. Boguslaski, and R. T. Buckler, Anal. Chem., 48, 1933-1937 (1976). (18) H. R. Schroeder, R. C. Boguslaski, R. J. Carrico, and R. T. Buckler, in “Methods in Enzymology”, VoI. 57, M. DeLuca, Ed., Academic Press, New York, N.Y., 1978. (19) K. Puget and A. M. Michelson, Photmhem. Photobbl.,24, 499-501 (1976).
(20) P. B. Shevlin and H. A. Neufeld, J . Org. Chem., 35, 2178-2182 (1970). (21) T. G. Burdo and W. R. Seitz, Anal. Chem , 47, 1639-1643 (1975). (22) W. R. Seitz, J . Phys. Chem., 79, 101-106 (1975). (23) S.B. Brown, P. Jones, and A. Suggett, Trans. Faraday Soc.,64,986-993 (1968). (24) N. A. Brown, R. F. G. J. King, M. E. Shillcock, and S. B. Brown, Biochem. J., 137, 135-137 (1974). (25) H. D. K. Drew and F. H. Pearman, J . Chem. Soc., 26-33 (1937). (26) G.K. Radda, in “Methods in Membrane Biology”, Vol. 4,E. D. Koru, Ed., Plenum Press, New York, N.Y., 1975,p 140. (27) G. R. Fleming, A. W. E. Knight, J. M. Morris, R. J. S. Morrison, and G. W. Robinson, J . Am. Chem. Soc., 99, 4306-4311 (1977). (28) K.-D. Gundermann, W. Horstmann, and G. Bergmann, Ann. Chem., 684, 127-141 (1965).
RECEIVED for review February 23, 1978. Accepted April 10, 1978.
Proton Induced y-Ray Analysis of Atmospheric Aerosols for Carbon, Nitrogen, and Sulfur Composition Edward S. Macias” and C. David Radcliffe Department of Chemistry, Washington University, St. Louis, Missouri 63 130
Charles W. Lewis and Carole R. Sawicki Environmental Sciences Research Laboratory, U S . Environmental Protection Agency, Research Triangle Park, North Carolina 277 1 1
A technique for the simultaneous quantitative determination of carbon, nitrogen, and sulfur using in-beam y-ray spectrometry has been developed for use with atmospheric aerosol samples. Samples are collected on quartz filters, and the aerosol composltlon is determined by analyzing the y rays emitted following the inelastic scattering of 7.0-MeV protons. Samples are nondestructively irradiated for 1000 s in a helium atmosphere, are not subjected to reduced pressures, and can be used for subsequent analysis. Detection limits for atmospheric samples are in the pg/cm* range with a precision of 5 % . The technique is compared with several more conventional methods of analysis.
Many of the adverse effects of air pollution are due to atmospheric suspended particulate material (aerosols). For this reason much work has been done in the past few years to improve the techniques of analyzing these particles with the ultimate goals of understanding the complex processes leading to aerosol formation in the atmosphere and of assessing the effects. Instrumental elemental analysis methods such as neutron activation analysis ( I ) and x-ray fluorescence (2) are currently being used to analyze for many of the chemical elements with atomic numbers between those of sodium and lead. Except for sulfur, these heavier elements are present in only trace quantities in typical atmospheric aerosol samples. The four light elements, carbon, nitrogen, oxygen, and sulfur, account for most of the fine-particle mass ( 3 ) . At present, a convenient, fast, nondestructive and inexpensive technique to measure these abundant light elements is not in general use. These data are essential to determine the mass balance of atmospheric particulate matter and to understand the chemistry of these particles. In this paper, we report on the development of a new nondestructive method for the determination of carbon, nitrogen, and sulfur in atmospheric aerosols. The method is based on y-ray emission following the inelastic scattering of 0003-2700/78/0350-1120$01.00/0
protons. An earlier paper by Shabason and Cohen ( 4 ) also describes this technique. The method takes advantage of the large cross section of many light elements for excitation to a low-lying nuclear state. The resultant rapid y-ray emission is, in general, unique to a particular nuclide and thus can be used as a signature for the chemical element. This analytical method is rapid because the y-ray spectrum is recorded inbeam during a short irradiation. Filters on which aerosol samples have been collected are irradiated without pretreatment, which avoids errors introduced by sample dissolution required for the more conventional chemical analysis. The method measures the total elemental abundance in a sample, not just the fraction that dissolves in a particular solvent. Furthermore, the method analyzes the bulk sample rather than just the surface as measured in techniques such as ESCA. The y rays emitted from carbon, nitrogen, and sulfur are all of energy greater than 2 MeV (half thickness >16 g cm-2);therefore, no y sample absorption corrections are necessary. The proton beam passes through the sample losing less than 0.4 MeV in the entire filter and aerosol deposit, thus no proton absorption correction is necessary. Preliminary reports of the use of this method for atmospheric aerosols have appeared previously (5-10). This paper contains a detailed description of the method and a comparison with other analytical techniques.
EXPERIMENTAL Sample Collection. Samples of atmospheric aerosols used in this work were collected with manual dichotomous virtual impactor samplers (1.2). Such samplers fractionate the aerosol into two size classes, a “fine” fraction consisting of particles having aerodynamic diameters less than 3.5 pm, and a “course” fraction including particles between 3.5 and approximately 20 gm. This classification corresponds approximately to respirable and nonrespirableparticles. The two classes of particles are uniformly deposited on separate filters in each sample. Five samplers were operated simultaneously side by side. The use of replicate samplers generated matched samples which were used in extensive comparison between the y-ray and alternative analytical methods. Two samplers employed 37-mm diameter 0 1978 American
Chemical Society
10-
I
Sulfur
' /
i
n Plotter
Figure 1. Schematic diagram of the sample irradiation chamber and electronics used for carbon, sulfur, and nitrogen analysis in this study
Pallflex Tissuquartz 2500QAO filters, and three used 37-mm diameter Fluoropore FALP filters. The Tissuquartz filters were chosen for use with the light element analytical technique because these filters contain carbon, nitrogen, and sulfur in only trace amounts. The Fluoropore filters contain carbon as a major constituent ([C2F4],)but are very well suited to x-ray fluorescence analysis for sulfur. Both filters exhibit a high collection efficiency (>9870) for particles down to 0.034 km (12). With Fluoropore filters, the aerosol was collected in 29-mm diameter deposits. To decrease the importance of blank subtraction for Tissuquartz filters, the aerosol deposit diameter was reduced to 12.7 mm by employing special filter holders preceded by aluminum cylinders with tapered bores to guide the aerosol into the smaller area. The field sampling was done on the roof of a five-story building in the city center of Charleston, W. Va., for a 21-day period from August 25 to September 14,1976. The samplers operated at 14 L/min, with each sample collected over 24 h, corresponding to a nominal sample volume of 20 m3. By using either automatic flow control or manual flow monitoring, sufficient information was available to calculate the actual value of each sampled volume to an acpracy of a few percent. Sample Irradiation. The filter samples were irradiated with protons in the external beam facility of the Washington University sector-focused cyclotron. The energy of protons striking the samples was 7.0 f 0.1 MeV. This bombardment energy is above the threshold for inelastic scattering to the first excited state of carbon, nitrogen, and sulfur and is a t a nonresonant portion of the excitation functions (13) to the first excited state of carbon (14). On passing through the filter, the beam loses 0.38 MeV which drops the beam energy below the effective threshold for inelastic scattering to the 3- state in l60.The threshold for the production of this state is 6.51 MeV but the cross section is very low, below 6.7 MeV (15), which acts as the effective threshold. This allows the normalization of the data to the oxygen peaks from the quartz filter as described in a later section. A filter segment with a maximum width of 10 mm was cut from each of the 37-mm diameter filter samples to be used as a filter blank. Both the segment and the remaining pieces of each filter were separately mounted on 50 X 50 mm cardboard photographic slide holders. The samples were irradiated for 1000 s a t a beam current of -20 nA in helium maintained a t 1.1 atm. The irradiation chamber, shown in Figure 1,incorporates a modified commerical 35-mm slide projector with a remote slide changing mechanism to automate sample changing. The chamber walls are stainless steel, and the top is Plexiglas. The identity of the sample being irradiated is monitored with a closed circuit television camera
In-beam 7-ray Spectrum x2
1122
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978 MET HI ON I NE CALI BRATI 0 N
NITROGEN
50 0
-
0
0
50
100
150
Nitrogen Mass ( pg/crn2)
1
V)
;I O O O t
i/i
SULFUR
1,
100 200 Sulfur Moss (pg/crn2) 6000 r = 0.99
0
250 500 750 Carbon Mass ( pg/crn2)
Figure 3. Calibration curves for carbon, nitrogen, and sulfur analysis
of atmospheric aerosols determined by the y-ray techniques. The normalized y-ray intensity is the ordinate. The elemental concentration of standards is the abscissa. The lines in each plot are the linear least squares fit to the data. The correlation coefficient ( r ) is given for each set of data
peak area is proportional to the proton beam intensity and the oxygen content of the filter. The latter is expected to be quite constant in these pure quartz filters. Note that the oxygen peak area is not a function of thickness of the filter. The oxygen content of the aerosol, which is less than 10% of the total oxygen in the sample, will not affect the normalization unless the relative oxygen content of the aerosol changes. A doubling of the oxygen content from 20 to 40% of the aerosol mass would affect the oxygen peak area by -5%. Filter blanks cut from the sample filters and methionine aerosol standards were irradiated under conditions identical to the atmospheric samples, thus yielding comparable detector count rates. y-ray spectra from all samples were analyzed in the same way. The normalized peak intensities from the filter blank spectra were subtracted from the atmospheric aerosol results. Average blank values (26 f 3 fig/cm2for carbon, 8 f 5 pg/cm2 for nitrogen and 90 f 5 pg/cm2 for sulfur) determined from segments cut from filters were subtracted from those aerosol samples for which no blank from the same filter was analyzed. The conversion of peak intensity into elemental mass was determined using the standard methionine aerosol samples as described below. Calibration. A calibration for nitrogen, sulfur, and carbon deposited on filters was carried out using standard methionine aerosol (C5HI1O2SN)samples. Methionine aerosol was generated from a 1%aqueous solution with an ultrasonic nebulizer. Using a TWOMASS (16)two stage sampler, fine particles (diameter 0.9). As a further check on the carbon calibration, several weighed 37-mm Nuclepore filters were also analyzed. Using the known carbon composition (75.6%), these results were compared with the methionine data and found to be in close agreement. The Nucleopore results are the highest mass points (-750 yg/cm2) in the carbon calibration curve shown in Figure 3. The amount
of a given element in an atmospheric sample was determined by multiplying the y-ray peak area from that element by the slope of the appropriate calibration curve (the intercepts of these curves were zero within experimental error). In subsequent experiments using the same filter medium, it would not be necessary to carry out as complete a calibration; only a few standard methionine samples need be run as a check on the system. As a check on the stability of the solid angle subtended by the Ge(Li) detector, a 226Rasource (tip = 1600 yr) was counted before and after each experiment.
DISCUSS ION Comparison with Other Analysis Methods. This y-ray method was compared with several other techniques in order to look for systematic discrepancies between methods. These intercomparisons used either the identical atmospheric filter samples previously measured with the (nondestructive) y-ray method or samples collected simultaneously from one of the replicate samplers, described in an earlier section. T h e analytical measurements were done by different individuals in three locations, each without knowledge of the others' results. The details and results of the intercomparisons were as follows. Dohrmann DC-50 Carbon Analyzer. The DC-50 Analyzer (Envirotech Corp., Santa Clara, Calif. 95050) can be used to measure either total organic carbon or total carbon. The system operates by means of sample pyrolysis using MnOz, reduction of carbonaceous compounds to methane, and quantitative determination of methane with a flame ionization detector. The instrument was operated in the total carbon mode, and as such should respond quantitatively t o all carbonaceous materials having a combustion temperature less than 850 "C. It was experimentally verified that calcium carbonate (decomposition temperature = 825 "C) was quantitatively analyzed. Since the instrument is designed and marketed for use in water analysis, two principal modifications of both hardware and procedure were made to enable the analysis of aerosol laden filters directly, without prior extractions. These were (a) change in integration time (from 6 to 8 min) for the sample injection and analysis period, and (b) optimization of the oxidation step to accommodate solid samples. The calibration was done with potassium hydrogen phthalate. Filter sections of 0.238-cm diameter were cut from both the aerosol-containing and clean areas (test samples and blanks, respectively) of each Tissuquartz filter. An average of five test samples and three blanks from each filter were separately analyzed. T h e amount of blank-corrected carbon for each filter, in pg/cm2, and its standard deviation were calculated from the individual aliquots taken from the filter. The average carbon blank and its standard deviation, based on 70 aliquots taken from 22 filters, was 20.4 f 3.5 yg/cm2. The comparison of the Dohrmann and y-ray results for carbon are shown in Figure 4a. Approximately two thirds of the measurements shown were made on identical filters for the two methods, while the remainder were duplicate samples. No systematic differences between results from the two classes of filters were observed, indicating that any differences between the two nominally identical samplers were small. T h e comparison of Figure 4a corresponds to an average deviation of 7.3% between the results of the two methods ( Z n l ( x l x2) /x1l/n).
Model 240 Elemental Analyzer. A series of measurements on Tissuquartz filters using a Model 240 Elemental Analyzer (Perkin-Elmer Corp., Norwalk, Conn. 06856) was performed by Battelle Columbus Laboratories, Columbus, Ohio 43201. This instrument is intended for the measurement of carbon, nitrogen, and hydrogen in solid test samples. The sample is combusted in an oxygen atmosphere at approximately 860 "C, with subsequent thermal conductivity detection of the products C02, N2, and H20. T h e instrument
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1078
1123
Carbon
-150-
B
3 100a U
50/ 0 '
u 5 0 100 150 200 C ( u g /crn2)
150
"0 Nitrogen
50
100
150
Flgure 4. Method intercomparison of atmospheric fine particle samples. In each plot the results of the y-ray method Is the abscissa. The
ordinate Is the result of the Dohrmann device in frame a, the Perkin-Elmer device In frames b and c, and x-ray fluorescence In frame d. The line in each plot is the linear least squares fit to the data was calibrated with acetanilide and otherwise operated in a manner essentially the same as that given previously (17). The samples were in the form of 10.3-mm diameter aliquots punched from the 12.7-mm diameter aerosol deposit on each Tissuquartz filter. All of the filters were identical to those previously measured with the y-ray method. The comparison between the Perkin-Elmer and y-ray results for carbon is shown in Figure 4b. The corresponding comparison for nitrogen is given in Figure 4c. The average difference between the results of the Perkin-Elmer and y-ray methods for the measurements shown corresponds to 16% for carbon and 11% for nitrogen. The likely explanation of the poorer agreement for the carbon comparison lies in the method of blank subtraction for the Perkin-Elmer measurements. Average blank values (21 4 pg/cm2 for carbon and 3.2 f 0.6 pg/cm2 for nitrogen) based on six blanks punched from three of the filters were used for the blank correction, instead of individual values. Because the average nitrogen blank was much smaller than the carbon blank, the nitrogen results would be correspondingly less affected by blank variability with this procedure, which is consistent with what is actually observed. Filter blank values from the y-ray method, the Dohrmann instrument, and the Perkin-Elmer instrument agree within the variability between filter samples and the uncertainties of the methods. X-Ray Fluorescence (XRF). To obtain a comparison for sulfur measurements, an XRF analysis was performed. Since thick (8 mg/cm2) Tissuquartz filters are not suitable for this type of analysis, the Fluoropore filters collected simultaneously during the same field sampling run were used for this purpose. A description of the energy dispersive pulsed XRF system used has been given elsewhere (18). The XRF and y-ray comparison of sulfur results is given in Figure 4d. The XRF sulfur measurements shown are actually averages of the results from three samplers. Since the aerosol deposit areas were different for the Fluoropore and Tissuquartz filters (6.60 and 1.27 cm2,respectively), the XRF measurements were multiplied by the area ratio. Thus a line of unit slope represents perfect agreement between the two methods, as with previous figures. The average difference between the results of the methods is 23%. It should be recalled that this comparison had more possibility for vari-
*
,,50
75
100
(pglcrn?)
Figure 5. Method lntercomparison of atrnospherlc coarse partlcle samples. The results of the y-ray method is the abscissa. The results of the other methods is the ordinate. The line of slope 1 is drawn to show expected results for perfect agreement
Slope: 1.Ot.1
0'
25 Carbon
ability than the previous ones. Namely, samples were obtained with four individual samplers, using two different filter media, and collected in two different sized aersosol deposit areas. The comparisons shown in Figure 4,a-d may be briefly summarized by observing that to within about 5% there is no evidence of systematic discrepancies in any of the four methods investigated. Coarse Particle Comparisons. The previous results are based on measurements performed on filters containing the fine aerosol fraction only. For the filters containing the coarse particle aerosol fraction, of the three elements, only carbon was present in sufficient quantity to allow quantitative intermethod comparison. The same sample preparation and procedures used previously with the Dohrmann and Perkin-Elmer instruments were again employed. The comparison between measurements from these instruments and the y-ray method for carbon in the coarse fraction is shown in Figure 5. The comparison is strikingly different from that for the fine fraction in that the carbon values measured by the combustion methods are systematically smaller than the y-ray values by about one third. T o investigate the possibility that a significant amount of the carbon in the coarse fraction might be refractory and thus unaffected by the approximately 850 OC operating temperatures of the Dohrmann and Perkin-Elmer instruments, an additional set of measurements was performed (at Battelle Columbus Laboratories). These were done with a third combustion instrument, a Model 734-100 Low Carbon Analyzer (Leco Corp., St. Joseph, Mich. 49085), whose combustion temperature is about 2000 "C. The test sample and blank preparations were identical to those used with the Perkin-Elmer instrument. The Leco results are included in Figure 5 , and show the same systematically lower values as seen with the two lower temperature combustion instruments. Two observations suggest that the systematic discrepancy may be due to aerosol deposit nonuniformity of the coarse fraction. First, those filters for which any deposit nonuniformity was visible often showed a darker central region. In addition, because of the size of the proton beam spot employed (approximately 6-mm diameter), the y-ray method sampled only the inner 40% of the deposit area used in the combustion methods. These two observations taken together could have contributed to the y-ray method giving systematically higher results. The problem of nonuniform deposits is one of several that can cause difficulty in dealing with the coarse aerosol fraction (another is the lack of adhesion and consequent mechanical fragility of the aerosol deposit). Thus while the discrepancy apparent in Figure 5 is presumed to reflect sampling and/or handling problems rather than an analytical defect, the matter obviously deserves future investigation.
1124
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
Precision a n d Accuracy. The precision of the method depends on the stability of the electronic system, the counting statistics, and the stability of the proton beam. Peaks are well separated, making peak area calculations straightforward. The electronic system was designed to be very stable even a t high count rates. Using irradiation times of 1000 s in analyzing the 24-h samples of this study, the precision of the method was 5 % . The accuracy of the method is difficult to assess without certified absolute standards or accepted analytical procedures with which to compare. The comparison with other techniques described previously indicates that the method is a t least as accurate as alternative methods. Analysis of the methionine calibration standards indicates that the total accuracy of the method is a t least 10%.
CONCLUSIONS The method can be extended to include most light elements. The analysis of oxygen in atmospheric aerosols is of particular interest in the study of the material balance of fine particles (3,5). The high oxygen content in the quartz filters used in this work precludes the analysis of oxygen in the particulate material. However, collection on filters with low oxygen content would avoid these difficulties. The method is sensitive enough for use with higher time resolution sampling as short as 1-4 h (5). An alternative nuclear method of light element analysis employs the detection of elastically scattered protons (PESA) (19). The detection of y rays following the inelastic scattering of protons as described in this paper has some advantages over the detection of elastically scattered protons. The y-ray technique is not restricted to very thin samples, and does not expose samples to vacuum as does the proton elastic scattering method. The PESA technique is more sensitive, but for typical atmospheric aerosol samples both methods have adequate sensitivity. ACKNOWLEDGMENT The authors are indebted to Frank Huber of Battelle Columbus Laboratories for his measurements using the Perkin-Elmer and Leco analyzers, and to Dwight Rickel of Northrop Services, Inc., for the XRF analyses. The authors thank Roland Head and the entire Washington University cyclotron staff for help during this work. Special thanks go
to Alfred G.LeComte and Richard Goldworm for help with sample preparation and irradiation. The assistance of Daniel Hitt in programming the data acquisition and analysis system is gratefully acknowledged. We also thank Rudolph B. Husar and Richard Delumyea for many helpful discussions regarding this project.
LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11)
(12) (13) (14) (15) (16) (17) (18) (19)
W. H. Zoiier and G. E. Gordon, Anal. Chem., 42, 257 (1970). T. G. Dzubay and R. K. Stevens, Envlron. Scl. Technol., @,663 (1975). S.K. Friedlander, Envlron. Scl. Techno!., 7 , 235 (1973). L. Shabason and B. L. Cohen, Anal. Chem., 45, 284 (1973). E. S. Maclas, "Proceedings of the 6th Material Research Symposium: Methods and Standards of Environmental Measurement", U t / . Bur. Sfand. ( U . S . ) , Spec. fubl., 464, 179 (1977). C. W. Lewis, T. G. Dzubay, and P. W. Llsowski, presented at the 172nd National meeting of the American Chemical Society, Division of Environmental Chemistry, San Francisco, Calif., 1976. E. S. Macias, presented at the 172nd National Meeting of the American Chemical Society, Division of Nuclear Chemistry, San Francisco, Calif., 1976. E. S. Macias, C. D. Radciiffe, and B. Smith, Bull. Am. fhys. Soc., 21, 1007 (1976). C. W. Lewis, C. R. Sawicki, and E. S. Macias, presented at the 173rd National Meeting of the American Chemical Society, Division of Environmental Chemistry, New Orleans, La., 1977. E. S. Macias and C. W. Lewis, presented at the 173rd National Meeting of the American Chemical Society, Division of Environmental Chemistry, New Orleans, La., 1977. T. G. Dzubay, R. K. Stevens, and C. M. Peterson, "X-Ray Fluorescence Anabsis of Environmental Samples", T. G. Dzubay, Ed., Ann Arbor Science, Ann Arbor, Mich., 1977, pp 95-105. B. Y. H. Liu and 0. A. Kuhimey, "X-Ray Fluorescence Analysis of Environmental Samples", T. G. Dzubay, Ed., Ann Arbor Science, Ann Arbor, MlCh., 1977, DD 107-119. H. S. Adams,'J. D.Fox, N. P. Heydenburg, and G. M. Temmer, fhys. Rev., 124, 1899 (1961). F. Boreii, P. N. Shrivastava, 8. B. Kinsey, and V. D. Mistry, fhys. Rev., 174. 1221 (19681. F. Ajzeiieig-ieiove, m i . m y s . A , 166, 86 (1971). E. S.Macias and R. B. Husar, Envlron. Scl. Technol., 10, 904 (1976). R. K. Patterson, Anal. Chem., 45, 605 (1973). J. M. Jakievic, D. A. Landis, and F. S. Gouiding, "Energy dispersive x-ray fluorescence spectrometry using pulsed x-ray excitation," Adv. X-Ray Anal., 10, 253 (1976). J. Wliliam Nelson, X-Ray Fluorescence Analysis of Environmental Samples," T. G. Dzubay, Ed., Ann Arbor Science, Ann Arbor, Mich., 1977, pp 19-34. Also: B. L. Cohen and R. A. Moyer, Anal. Chem., 43, 123 (1971).
RECEIVED for review December 27, 1977. Accepted April 3, 1978. This work has been supported in part by the U S . Environmental Protection Agency, Atmospheric Instrumentation Branch, under grant R803115.
