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Comment 703 R. Jack Cornett CRUSIUS, J. 1992. Evaluating the mobility of 13’Cs, Lorna Chant and 239+240Pu *I”Pb from their distributions in lami- nated sediments. Ph.D. thesis, Columbia Univ. Chalk River Laboratories -, AND R. F. ANDERSON. 1991. Core compression Environmental Research Branch and surficial sediment loss of lake sediment of high porosity caused by gravity coring. Limnol. Oceanogr. Chalk River, Ontario KOJ 1JO 36: 1021-1031. CUMMING, B. F., J. R. GLEW, J. P. SMOL, R. B. DAVIS, References AND S. A. NORTON. 1993. Comment on “Core com- BLOMQVIST, S. 1985. Reliability of core sampling of soft pression and surficial sediment loss of lake sediments bottom sediment-an in situ study. Sedimentology of high porosity caused by gravity coring” (Crusius 32: 605-6 12. and Anderson). Limnol. Oceanogr. 38: 695-699. CHANT, L. J., AND R. J. CORNETT. 199 1. Smearing of DAVIS, R. B., AND OTHERS. 1984. 13’Csand *lOPb dating gravity core profiles in soft sediments. Limnol. Ocean- of sediments from soft-water lakes in New England ogr. 36: 1492-1498. (U.S.A.) and Scandinavia, a failure of 13’Cs dating. CORNET-T,R. J., B. A. RISTO, AND D. R. LEE. 1989. Mea- Chem. Geol. 44: 151-185. suring groundwater transport through lake sediments WRIGHT, H. E. 1993. Core compression. Limnol. Ocean- by advection and diffusion. Water Resour. Res. 25: ogr. 38: 699-70 1. 1815-1823. Limnol. Oceanogr., 38(3), 1993, 703-705 0 1993, by the American Society of Limnology and Oceanography, Inc. Solar ultraviolet radiation measurements by o-nitrobenzaldehyde actinometryl The use of the o-nitrobenzaldehyde acti- Discovery Bay should not be less than at Mc- nometer to measure solar UV radiation in clear Murdo, but similar to San Antonio Bay. marine waters has been described (Fleisch- Fleischmann’s measurements are based on mann 1989). The low diurnal UV flux reported the photoisomerization reaction of the con- at the surface of Discovery Bay, Jamaica version of o-nitrobenzaldehyde (ONB) to o-ni- (18”28’30”N, 77”24’3O”W), conflicts with our trosobenzoic acid (ONBA) (Ciamician and Sil- measurements in San Antonio Bay, Chile ber 1900). An ONB preparation (-20 pmol (33”36’S, 7 1”36’W), and with those of the NSF cm-2) in polymer films of poly(methy1 meth- UV spectroradiometer network in the Antarc- acrylate) is used as an actinometer (Coulbert tic region (Booth et al. 199 1). et al. 1980). The precise amount of ONB pres- In Table 1 we compare the solar UV flux in ent in the film before and after it has been both hemispheres. Surprisingly, Fleisch- exposed to solar radiation is calculated with a mann’s data are a tenth of our measurements, calibration curve of the IR absorption peak of although Discovery Bay is 15” closer to the the ONB at 1,530 cm-‘. equator than is San Antonio Bay, and a fifth The photoisomerization of ONB to ONBA of those determined at McMurdo Station, Ant- is a well-known reaction (Leighton and Lucy arctica. However, from the noon sun angle data 1934; Cowell and Pitts 1968 and references for these latitudes, the solar UV radiation at therein), and the quantum yield of the rear- rangement in the region of 3,100-4,3OOA is 0.50 in a solid, liquid, or gaseous solution l Accepted: 4 September 1992. (Leighton and Lucy 1934). Acknowledgments However two aspects must be considered We acknowledge the Departamento Tecnico de Inves- when using this kind of actinometer. The films tigacion de la Universidad de Chile (Grant 43282-9214) are not necessarily absorbing 100% of the in- and FONDECYT-Chile (Grant 1143-9 1) for financial sup- port. cident UV light, so a correction for transmit- We extend our acknowledgments to the referees for tance of the film must be made. In addition, comments and references. the absorbed light intensity must be extrapo- 704 Comment Table 1. UV photon flux intensity (pm01 quanta m-2 s-l). Place Latitude Solar zenith angle UV (A + B) Discovery Bay, Jamaica 18”28’30”N 42” (Dee 82) 3-5 20” (Mar 83) (Fleischmann 1989) San Antonio Bay, Chile 33”36’S 30” (Ott 91) 56-70 (this work) McMurdo Station, Antarctica 77”5 1‘S 75” (Ott 89) 21-26 (Booth et al. 199 l)* * Original data are 8 17.8 pW cm-2 for UV-A plus UV-B spectral regions and 3,8 11 pW cm-2 for the visible region (400-600 nm). lated to zero irradiation time to correct for sity of the spectrophotometer lamp is low absorption due to internal filtering by the enough to change the optical density of the ONBA product. In Fig. 1 we compare the ab- actinometer solution < 0.00 1 absorbance units sorption spectra of ONB and ONBA in meth- for every scan of the spectrum. Furthermore, anol. we analyzed the kinetic behavior of the ONB We have used the method developed by under solar irradiation and observed that the Fleischmann; however, we have made the ex- filter effect due to the ONBA product cannot perimental determinations in solution instead be neglected over 120 s of cloudless solar ir- of in solid phase. We prepared 0.25 mM ONB radiation for ONB concentrations ~0.5 mM. solutions in methanol and these actinometers, The polymer films prepared by Fleischmann sealed quartz cells, were exposed to solar ra- contain ONB in amounts on the order of 1 diation for 30 s. To correct for the visible light pmol cm-*, which are equivalent to 100 mM contribution to the photochemistry reaction, solutions for 0.1 -mm-wide polymer films. This we used cells containing UV filters as a ref- high concentration of the actinometer would erence. Our measurements were made in a Cary induce an artifact in the radiation measure- 17 spectrophotometer at 320 nm. The inten- ments. For example, if we consider a 12% ONB photoconversion, the ONBA photoproduced - 8I will present the same transmittance as the re- mainder ONB at 320 nm. The percentage of UV-B reaching the earth’s surface is x0.3% of total solar radiation at the top of atmosphere (T. P. Coohill unpubl.), 6- therefore we can expect EUV : EPAR ratios near 0.1, as can be seen from the data reported by Booth et al. 199 1. From solar UV irradiance reported by Fleischmann, we calculate an m 41 EUV : EPAR ratio of 0.0008, which is 0.004 times the extraterrestrial ratio (Frederick et al. ‘9 1989). Furthermore, if we consider that the u solar n-radiances determined by Fleischmann involve UV-A plus UV-B, we find that these 2 data are lower than the UV-B irradiance alone determined at Melbourne, Australia (37”47’S), in December 1987 (6-8 pmol quanta m-* s-l) I (Roy et al. 1990). New reported spectral irra- diances for the UV region at sea level in the 220 280 340 400 Antarctic region also agree with our present Wavelength (nm) observations (Smith et al. 1992). Fig. 1. Absorption spectra (M cm-‘) of o-nitroben- Finally, we consider the use of ONB as an zaldehyde (ONB) and o-nitrosobenzoic acid (ONBA) in actinometer appropriate for solar radiation methanol at room temperature. measurements, but the filter effect by the Comment 705 ONBA, the transmittance of the solution, and CIAMICIAN, G., AND P. SILBER. 1900. Chemische Licht- wirkungen. Berichte 33: 29 1l-29 13. the photochemical reaction induced by the vis- COULBERT,~.D.,A. GUFTA,AND J.N. PITTS. 1980. UV ible range between 4,000 and 4,300A must be actinometer film. NASA Tech. Briefs 165. considered in final data processing. G. COWELL, W., AND J. N. Prrrs. 1968. Photochemical studies in rigid matrices. 2. A study of the photo- Raiil G. E. Morales chemical reactivity of antracene in polystyrene and Gregnrio P. Java the development of an o-nitrobenzaldehyde actinom- eter in poly(methylmethacrylate). J. Am. Chem. Sot. Laboratory of Luminescence and 90: 1106-l 110. Molecular Structure E. FLEISCHMANN, M. 1989. The measurement and pen- Department of Chemistry etration of ultraviolet radiation into tropical marine Faculty of Sciences, University of Chile water. Limnol. Oceanogr. 34: 1623-l 629. FREDERICK, J.E.,H.E. SNELL,ANDE.K. HAYWOOD. 1989. Casilla 6 5 3, Santiago Solar ultraviolet radiation at the earth’s surface. Pho- tochem. Sergio Cabrera LEIGHTON,P.Photobiol.F.50: 443-450. A., AND A. LUCY. 1934. The photoiso- Department of Cellular Biology and Genetics merization of o-nitrobenzaldehydes. 1. Photochemi- cal results. J. Chem. Phys. 2: 756-759. Faculty of Medicine, University of Chile Rov,C. R.,H. P. GIES, AND G.ELLIOTT. 1990. Ozone depletion. Nature 347: 235-236. References SMITH, R. C., AND OTHERS. 1992. Ozone depletion: Ul- BOOTH, C. R., T. LUCAS, J. YEH, AND D. NEUSCHULER. traviolet radiation and phytoplankton biology in Ant- 199 1. Antarctic UV spectroradiometer monitoring arctic waters. Science 255: 9 52-9 58. program. Biospherical Inst. Tech. Ref. 90-3b.
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