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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.

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.),
                                                                                          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)
                                                                                          (Roy et al. 1990). New reported spectral irra-
                                                                                          diances for the UV region at sea level in the
                    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-
                                     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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