Environ. Sei. Technol. 1987, 27, 505-508

concentration and exposure time (Figure 4). The infrared spectra for the unexposed reagent and the

reaction product with ozone were identical. The mass spectra showed the parent peak and fragmentation pattern in the reaction product to be identical with those of the unreacted reagent. The 400-mHz NMR spectra of the unreacted and reacted reagent in deuteriated chloroform showed significant peak broadening in the latter, which indicated the presence of unpaired electrons. The ESR spectrum of the product gave decisive evidence for the presence of a stable free radical.

On the basis of the data obtained, a one-electron oxi- dation process is proposed:

1

The reaction is not sensitive to the relative humiditv of

The solid reagent is intended for use in passive moni- toring or warning devices by means of visual Comparison to prepared color standards. The stability on storage, the sensitivity and selectivity of the reagent, and the color stability of the reaction product make phenoxazine an ideal reagent for this type of analysis. The fact that no hu- mectant is required is another advantage. The very stable color produced is unusual for a free radical. Such color stability would permit long-term determinations of very low ozone concentrations.

Registry No. Os, 10028-15-6; phenoxazine, 135-67-1.

Li tera ture Cited (1) Lambert, J. L.; Beyad, M. H.; Paukstelis, J. V.; Chiang, Y.

C. Anal. Lett. 1981, 14, 663. (2) Lambert, J. L.; Beyad, M. H.; Paukstelis, J. V.; Chejlava,

M. J.; Chiang, Y. C. Anal. Chem. 1982,54, 1227. (3) Lambert, J. L.; Trump, E. L.; Paukstelis, J. V. Environ. Sci.

Technol., first of three notes in this issue. (4) Environmental Protection Agency Fed. Regist. 1971,36(84),

8196. (5) Lambert, J. L.; Paukstelis, J. V.; Liaw, Y.-L.; Chiang, Y.

C. Anal. Lett. 1984, 17, 1987.

the airstream, and the red-orange color Produced bY"ni- trogen dioxide is visually quite distinct from the dull brown color of the ozone reaction product.

Received for review March 10,1986. Accepted December 19,1986. This research was suupported in part by National Science Foundation Grant CHE-8311011.

Dependence of /[ 0,-O('D)] on the Choice of Extraterrestrial Solar Irradiance Data

John A. Rltter"

Atmospheric Sciences Division, NASA Langley Research Center, Hampton, Virginia 23665

Donald H. Stedman

Chemistry Department, University of Denver, Denver, Colorado 80208

Russell R. Dlckerson

Meteorology Department, University of Maryland, College Park, Maryland 20742

Thomas E. Blackburn

NASA Ames Research Center, Moffett Field, California 94035

Estimates of the photolysis frequency G) of tropospheric ozone (0,) to the excited singlet oxygen atom [O(lD)] depend on a knowledge of the extraterrestrial solar irra- diance in the wavelength region between 300 and 320 nm. A standard format is proposed that facilitates the inter- comparison of solar irradiance data sets in the 300-320-nm wavelength region so as to determine their appropriateness for use in calculating tropospheric values for j[O,-O(lD)]. Twelve data sets are thus compared, the results of which indicate that, with the exception of two data sets, the resulting j values are consistent with each other to within the accuracy of the measured data and thus are appro- priate for use in determining j values. Computed j values from all 12 data sets are tabulated and indicate in a relative sense the dependence of modeled j values on the choice of the solar irradiance data set used.

In troduct ion

A critical parameter in the modeling of photochemical processes in the troposphere on either the urban or global scale is the photolysis of 03:

Although most of the O(lD) atoms that are then pro- duced are quenched to the ground state of O(3P) by ni- trogen or oxygen, a significant quantity is still available for reaction with water to form hydroxyl radicals:

(2) Hydroxyl radicals have been shown to be important not

only in the removal of several trace gases including CO, CH,, C2Hs, Hz, HzS, and SO2 (1) but also in the formation of ozone (2) leading to photochemical smog. Therefore, due to the central role that the presence of O(lD) atoms has in controlling photochemical processes on several scales in the troposphere, an understanding of the uncertainties inherent in the calculation of j[O,-O(lD)] is essential.

The photolysis frequency 0') of any atmospheric mole- cule to a particular product channel is the integral of the product of solar actinic flux (I), absorption cross section (g), and quantum yield (4) to that channel. Thus, for the molecular species i, wavelength A, altitude z , temperature T, and zenith angle 8

O(lD) + H 2 0 - 20H

0013-936X/87/0921-0505$01.50/0 0 1987 American Chemical Society Environ. Sci. Technol., Vol. 21, No. 5, 1987 505

j&,T,@ = ~ " - I ( X , z , S ) u ~ ( X , ~ ~ ~ ( X , ~ mm dX (3)

See, for instance, Leighton (3) . The terms u and C$ are properties only of the molecule and the environmental temperature, while I is a function of the extraterrestrial solar irradiance data and the radiative characteristics of the atmosphere.

A rigorous calculation of I(X,z,O) necessitates the treat- ment of the processes of ozone absorption, molecular and particle scattering, and local albedo variations. A number of authors have used various approximations to model these processes (4-9). An attempt to compare our recent measurements of tropospheric j[O3-0(lD)] to model results revealed a considerable discrepancy among the modeled values (10).

From a modeling perspective, one is confronted with a plethora of solar irradiance data sets (11-22) for the wavelength region appropriate for calculations of j[O,-0- (lD)] at ground level, with only qualitative information as to how they are correlated with each other. This uncer- tainty plays an as yet unquantified part in the discrep- ancies noted by Dickerson et al. ( I O ) . Since eq 3 shows that the modeled j value is the result of an integration of a product of three terms over a finite wavelength interval, a study designed to isolate the influence that different sources of solar irradiance values have on this calculation should not be based on a simple comparison of spectra but on the total integrated j value resulting from the use of the various data sets. Such an analysis would yield in- formation that is needed to interpret the discrepancies noted by Dickerson et al. (10). Admittedly, the comparison of j values resulting from these different data sets requires that a model, with its inherent assumptions, be employed in calculating I in eq 3. Additional uncertainties would therefore be introduced due to the errors inherent in the spectral forms of u and 4 used. However, by calculating j values on the basis of these data sets (as opposed to a simple integration of the irradiance values alone), one is afforded the opportunity to contrast the resulting j values with direct measurements and, as a result, aid in the de- termination of an appropriate ensemble of irradiance data sets for use in such calculations. Although this calculation is known to be sensitive to other parameters (23), in this study we investigate the extent to which the discrepancies in extraterrestrial solar irradiance data can be explained by applying the same, admittedly arbitrary, model and boundary conditions to each data set. In this manner, many of the errors inherent to the model will cancel. The boundary conditions used were chosen to match the con- ditions under which two measurements of j[O,-O(lD)] were made (10, 24). These measured values and their associated confidence intervals are included as a point of reference from which a range of appropriateness of the various irradiance data sets appropriate for use in such calculations might be inferred.

Calculation Me thods The downward ( 2 ~ sr) looking component of j[O3-0(lD)]

due to both direct and diffuse solar radiation within the interval 300-320 nm has been calculated for a ground-level site (1800 msl) by using a simple model based on the method of Leighton (3) . Anderson and Meier (25) have indicated that the use of such a model is justified for processes involving A <320 nm and for the case of relatively small zenith angles.

The model employs a plane-parallel approximation of an aerosol-free atmosphere. The Rayleigh scattering spectral transmittance functions were calculated from the

506 Environ. Sci. Technol., Vol. 21, No. 5, 1987

l o l q k , I , I , I , I 1 , , I , , , , , , , 11.c

e

Wavelength, A ( nm)

Flgure 1. Spectral variation of variables required for the computation of j[O,-O('D)] via eq 3: (+) ozone absorption cross sections (0); (0) calculated actinic flux ( I ) ; (0) quantum yield (4).

300 305 310 315 320

Wavelength, ~ ( n m )

Figure 2. Spectral contribution to j[O,-O('D)] with the data from Figure 1. The integral under the curve is j [O,-O('D)]. Structure in the vicinity of the peak contribution is evident when a linear scale is used.

method described by Goody (26). Ozone was assumed to be the only absorbing gas in the atmosphere, and the spectral values of its absorption cross sections were taken from Moortgat and Warneck (27). The temperature-de- pendent quantum yield was calculated according to the method described in NASA/JPL (28).

The pertinent variables of the calculation were as fol- lows: temperature, 305.0 K; solar zenith angle, 21.5'; ozone column content, 0.315 atm cm. These values were chosen to match the conditions present during an actual mea- surement of j[O,-O(lD)] taken at solar noon on July 30, 1977, in Boulder, CO (10). The effective ozone column for direct sunlight was 0.315 atm cm/cos 21.5O = 0.339 atm cm. The model was then executed with exactly the same boundary conditions but with several different sources of extraterrestrial solar irradiance data. These data were adjusted to 1 AU and interpolated when necessary to ob- tain 0.5-nm averages over the interval of 300-320 nm.

Resul ts and Discussion As indicated in eq 3, the actinic flux, cross section, and

quantum yield are needed to calculate j values. The spectral values used in this study for the latter two quantities are shown in Figure 1 along with values of the actinic flux that were derived from the irradiance data of Mental1 et al. (16) with the above-mentioned model. This figure shows the rapidly decreasing values of the ozone cross sections and quantum yields contrasted against the

Table I. Comparison of 277 - j[O,-O(lD)] Calculations with Various Solar Irradiance Data with Measurements

reference j[O,-O(lD)], s-l

Mount and Rottman (11)' 2.41 x 10-5

Mount and Rottman (13) 2.55 x 10-5 Mount and Rottman (14) 2:58 x 10-5 Simon (15) 2.73 x 10-5 Mentall et al. (16) 2.75 x 10-5

Heath (18) 2.83 x 10-5

Nicolet (20) 2.89 x 10-5 De Luisi (21) 2.90 x 10-5 Ackerman (22) 3.18 x 10-5 Dickerson et al. (IO) (3.6 i 1.1) x 10-5b

Broadfoot (12) 2.53 X

Arvesen et al. (17) 2.77 X

Thekaekara (19) 2.88 X

Blackburn (24) (2.54 f 0.25) X

"Recalibrated data as per Mount and Rottman (13). bThe error bars (f) given on the measured values are estimates of the 95% confidence interval including an estimate of absolute accuracy.

increasing and fluctuating values of actinic flux. Figure 2 shows the results of the calculation of the distribution of j[O3-0(lD)] for each wavelength interval calculated with the data in Figure 1.

Integrating the model results to obtain the total value of j[O,-O(lD)] for each of the 12 sources of extraterrestrial solar irradiances gives the values shown in Table I. Two measured j[O,-O(lD)] values are also shown. The value given for Dickerson et al. (10) was the peak value measured on July 30, 1977. The value given for Blackburn (24) is an interpolation of measured results, based on an im- provement of the technique of Dickerson et al. (IO), for a 0.339 atm cm effective ozone column. The latter rep- resents a j[O,-O(lD)] value with an average measured aerosol extinction coefficient of 7500 = 0.27 f 0.13.

The irradiance data of Nicolet (20) were analytically determined from the Planck function assuming a black- body temperature of 5550 K. Although the data of Mount and Rottman (10 ,13 ,14 ) do not extend completely to 320 nm, j[03-O(1D)] values determined with these data are estimated to be greater than 99% of the expected value. The data of Arvesen et al. (1 7) have been shifted by -0.4 nm as suggested by Broadfoot (12) and scaled by 0.987 as suggested by Nicolet (29). Since many of the data sets used in this study do not extend below 300 nm, this was chosen to be the lower limit of the integral in eq 3. The resulting truncation error, incurred by neglecting the contribution from the interval 297.5-300.0 nm, was de- termined to be only *3% of the value given in Table I. The use of a more sophisticated model would undoubtedly alter the values shown in Table I somewhat, although it is not expected that the results, in a relative sense, would change by an appreciable amount.

Discounting the value given in Table I for Nicolet (since it was obtained from an arbitrary selection of blackbody temperature), an analysis of the results shown in Table I gives an average j of (2.74 * 0.41) X s-l for the re- maining 11 data sets, where the error limits represent two standard deviations. By applying the principles of small sampling theory (T test), we can expect the 1 % confidence limits of the mean j value to be (2.74 f 0.21) X s-l. Further inspection of the results shown in Table I indicates that the range of j values calculated with the different data sets is 30% of the lowest calculated j value. The results also show that the calculated j values fall as much as 33% below the mean value measured by Dickerson et al. (IO), although most of the values lie within the lower error limit of their measurements. The mean value of j[O3-0(lD)]

4-

+.

t o I +

ol? 00 + ' 0 n,

OOO*' 0'

"7 0 .7+ + o

h i = 0.5 nm

0 0

O O o 0 j

Wavelength, A (nm)

Figure 3. Wavelength-dependent contributions of j [O,-O('D)] nor- malized to Mentall et al. (76) for various solar irradiance data: (0) Broadfoot (72); (+) Arvesen et al. (77); (A) Ackerrnan (22).

reported by Blackburn (24) however lies within the spread of the computed values shown in Table I.

It is also of interest to compare the spectral distribution of the j values resulting from a few of the selected data sets. Such a comparison would be analogous to contrasting the spectra of their respective solar irradiance values since the same model has been applied to all of the data sets. Although it is common to compare irradiance data in this manner (30), methods of this type may lead to ambiguous conclusions in regard to the appropriateness of a particular data set for use in calculating j values. Figure 3 shows such a comparison and correctly predicts that the j values stemming from the data of Ackerman (22) will be higher than that of Mentall et al. (16)) while the data of Broadfoot (12) will give lower j values. It is also noted, with the aid of Table I, that in spite of the scatter shown by the ratioed data of Arvesen et al. (17) the total integrated j value is almost identical with that obtained from the data of Mentall et al. (16). This illustrates the subtle but im- portant point that it is the total integrated j values that should be intercompared and not their respective spectral values. Another such example is presented in the work of Simon (30) in which it is stated that the spectral irra- diance values of Broadfoot (12) and Arvesen et al. (17) are discrepant by as much as 40%. Such information is cer- tainly useful for many applications, but from the per- spective of one who is trying to ascertain which, if either, of these data sets is appropriate for calculating j values, the question still remains. However, as a result of the analysis of the j values shown in Table I, it is shown that the respective j values resulting from the data of Broadfoot (12) and Arvesen et al. (17) are not as discrepant as one might have thought. Albeit the data of Arvesen et al. (17) used in this study have been shifted and scaled, the dif- ference would still be on the order of only 10%.

As mentioned previously, the range of calculated j values is approximately 30% of the lowest entry. This variation is difficult to explain in view of the fact that the stated accuracies of many of the individual measurements vary from *3% to f15% (31). It is doubtful that solar varia- bility could be a major contributing factor in this wave- length region (18) . Differences in calibration techniques (16) and reference standards (31) are more likely to be the cause of the variations shown in Table I. Due to this uncertainty, it is impossible to identify any particular data set as being the correct one. As such, the focus of this work

507 Environ. Sci. Technol., Vol. 21, No. 5, 1987

Environ. Sci. Techno/. 1987, 21, 508-511

centers on the dispersion of the values and not on the mean value itself. Inspection of Table I indicates that there is a significant portion of the range of calculated values where the confidence limits of both measurement techniques overlap. However, on the basis of applying the concepts of small sampling theory to the calculated j values shown in Table I, it would appear that the data sets of Ackerman (22) and Mount and Rottman (11) can be rejected at the 1 % significance level. The exclusion of these two data sets places the range of j values within 15% of the lowest value, which is a reasonable range in view of the accuracies of irradiance values quoted by the respective investigators.

Conc 1 us ions The results of this study, as shown in Table I, indicate

the dependence of calculated j values on the choice of available solar irradiance data sets. This study has shown that, with the exception of the data sets of Ackerman (22) and Mount and Rottman (111, the data sets investigated vary with a magnitude that is .consistent with the reported measurement accuracy ascribed to solar irradiance mea- surements in this wavelength region, and as such, any of these remaining data sets is appropriate for use in com- puting j[O,-O(lD)]. Our study indicates that the com- parison of future irradiances, for use in calculating j - [O,-O(lD)], would be facilitated by comparing the asso- ciated j values against like quantities from existing data sets.

Registry No. 03, 10028-15-6; 0, 17778-80-2.

Literature Cited Logan, J. A.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J . Geophys. Res. C: Oceans Atmos. 1981,86,7210-7254. Fishman, J. Nature (London) 1978, 274, 855. Leighton, P. A. Photochemistry of Air Pollution; Academic: New York, 1961. Shettle, E. P.; Green, A. E. S. Appl. Opt. 1974, 13, 1567. Luther, F. M.; Gelinas, R. J. J. Geophys. Res. 1976,81,1125. Isaksen, I. S. A.; Midtbo, K. H.; Sunde, J.; Crutzen, P. J. Geophys. Norv. 1977, 31, 11. Luther, F. M.; Wuebbles, D. J.; Duewer, W. H.; Change, J. S. J . Geophys. Res. C: Oceans Atmos. 1978, 83, 3563-3570.

(8) Augustsson, T. R.; Levine, J. S. Atmos. Environ. 1982,16,

(9) Thompson, A. M. J. Geophys. Res. D: Atmos. 1984, 89,

(10) Dickerson, R. R.; Stedhman, D. H.; Delany, A. C. J . Geo-

(11) Mount, G. H.; Rottman, G. J. J. Geophys. Res. C: Oceans

(12) Broadfoot, A. L. Astrophys. J . 1972, 173, 681-689. (13) Mount, G. H.; Rottman, G. J. J. Geophys. Res. C: Oceans

(14) Mount, G. H.; Rottman, G. J. J. Geophys. Res. C: Oceans

(15) Simon, P. C. Bull. Mem. Acad. R. Med. Belg. 1975, 6,

(16) Mentall, J. E.; Frederick, J. E.; Herman, J. R. J. Geophys.

(17) Arvesen, J. C.; Griffin, R. N.; Pearson, B. D., Jr. Appl. Opt.

(18) Heath, D. F. J. Geophys. Res. 1973, 78, 2779-2792. (19) Thekaekara, M. P. Appl. Opt. 1974,13, 518-522. (20) Nicolet, M. Rev. Geophys. Space Phys. 1975,13,593-636. (21) De Luisi, J. J. J. Geophys. Res. 1975, 80, 345-354. (22) Ackerman, M. Mesospheric Models and Related Mea-

surements; Reidel: Dordrecht, Holland, 1971; pp 149-159. (23) Butler, D. Geophys. Res. Lett. 1978,5, 769-772. (24) Blackburn, T. E. Ph.D. Dissertation, The University of

(25) Anderson, D. E.; Meier, R. E. Appl. Opt. 1979, 18,

(26) Goody, R. M. Atmospheric Radiation; Clarendon: Oxford, 1964.

(27) Moortgat, G. K.; Warneck, P. 2. Naturforsch. A: Phys. Phys. Chem. Kosmophys. 1975,30, 835-844.

(28) NASA/JPL “Chemical Kinetic and Photochemical Data for Use in Stratospheric Modelling”; Evaluation No. 4, Publication 81-3; Jet Propulsion Laboratory: Pasadena, CA, 1981.

1373-1380.

1341-1349.

phys. Res. C: Oceans Atmos. 1982,87,4933-4946.

Atmos. 1981,86, 9193-9198.

Atmos. 1983,88, 5403-5410.

Atmos. 1983,88, 6807-6811.

399-409.

Res. C: Oceans Atmos. 1981,86, 9881-9884.

1969, a, 2215-2232.

Michigan, Ann Arbor, MI, 1984.

1955-1960.

(29) Nicolet, M. Planet. Space Sei. 1981, 29, 951-974. (30) Simon, P. C. Aeron. Acta A 1980,211, 1. (31) NASA/FAA/NOAA “The Stratosphere 1981 Theory and

Measurements”; Report No. 11; World Meteorological Organization: Geneva, Switzerland, 1981.

Received for review November 18, 1985. Revised manuscript received September 10, 1986. Accepted October 23, 1986.

Reduction of Trihalomethanes in a Water-Photolysis Systemt

Chee K. Tan and Tsen C. Wang”

Department of Chemical and Environmental Engineering, Harbor Branch Oceanographic, Inc., Fort Pierce, Florida 33450

Hydrogen generated from a heterogeneous photo- catalyzed water-photolysis system [Pt(colloid)/Ru- ( ~ ~ Y ) ~ + / M V ~ + / E D T A ] with visible light irradiation is used to reduce trihalomethanes. The conversion of trihalo- niethanes to low molecular weight hydrocarbons such as methane is demonstrated in this study.

In troduct ion

Conventional sterilization of water with chlorine forms trihalomethanes and other chlorinated hydrocarbons in the water (1-4). Trihalomethanes (THM) are a group of compounds consisting of chloroform (CI-ICI,), bromodi- chloroniethane (CHC12Br), chlorodibromomethane (CH-

ClBr2), and bromoform (CHBr,). They are suspected of being carcinogenic compounds (5 ,6) . Methods suggested for removing THM from water include adsorption on powdered or granulated activated carbon (3, aeration, and coagulation of precursors prior to chlorination (8-1 I).

Oxidation of chloroform to carbon dioxide and hydro- chloric acid in an aqueous solution (10-200 ppm) by photoassisted heterogeneous Ti02 catalysts with near-UV illumination was studied (12, 13). Hoke (14) reported that trihalomethanes at 150-250 pg/L could be reduced by hydrogen with zinc as a catalyst; the degradation rate was faster at a low pH and negligible a t a high pH.

Gratzel (15-23) has reported that metal-catalyzed water-photolysis systems generate hydrogen from water with visible irradiation. With Gratzel’s water-photolysis system, hydrogenation of unsaturated organic compounds and hydrogenolysis of benzyl chloride Were carried out at room temperature and pressure (24).

”This paper is Harbor Branch Oceanographic, Inc., Contribution No. 564.

508 Environ. Sci. Technol., Vol. 21, No. 5, 1987 0013-936X/87/0921-0508$01.50/0 0 1987 American Chemical Society