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Irradiance and Beam Transmittance Measurements Off the West Coast of the Americas

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Irradiance and Beam Transmittance Measurements Off the West Coast of the Americas VOL. 84, NO. C1 JOURNAL OF GEOPHYSICAL RESEARCH JANUARY 20, 1979 Irradiance and Beam Transmittance Measurements off the West Coast of the Americas RICHARD W. SPINRAD,J. RONALD V. ZANEVELD, AND HASONG PAK Schoolof Oceanography,Oregon State University, Corvallis, Oregon 97331 Measurementsof total irradiance versusdepth and beam transmissionversus depth were made at stationsnear shore along the west coast of the North and South American continents.The water typesat each station were optically classifiedaccording to the systemof Jerlov (1976), thus providingadditional information for the descriptionof the distribution of the world's ocean water types. In addition, the parameterk/c, wherek is the irradianceattenuation coefficient and c is the beamattenuation coefficient, has been shown to be a useful parameterfor determiningthe relative particle concentrationsof ocean water. INTRODUCTION mately 75 m. At two stations(8 and 9), equipmentmalfunc- Optical classificationof oceanwater is an important means tions preventedmeasurements from being taken. of distinguishing water types. Jerlov [1951] presented a The irradiance meter usedhad a flat opal glassdiffuser as a method of classificationaccording to spectraltransmittance of cosinecollector and containeda signallog amplifierto provide downward irradiance at high solar altitude. Downward irra- an outputsignal between +4 V dc. The spectralresponse of the diance is defined as the radiant flux on an infinitesimal element irradiance meter is shown in Figure 3. The transmissivity of the upper face (0 ø-180ø) of a horizontal surfacecontaining meter consistedessentially of a light-emitting diode (wave- the point being considered,divided by the area of that element length = 650 nm), collimatinglenses, and a photodiode.The [Jerlov, 1976]. Jerlov's [1951] classificationdefined three dif- optical path length of the meter was 0.25 m. ferent oceanic water types and five coastal water types, but The irradiance meter indicated values of the logarithm of further experimentation [Jerlov, 1964; Aas, 1967, 1969; Mat- the irradiance versusdepth. From this information the value suike, 1967,1973; Matsuike and Sasaki, 1968;Hetjerslev, 1973, of the irradiance attenuation coefficient k could be calculated 1974a, b; Matsuike and Kishino, 1973; Shimura and Ichimura, as follows [Jerlov, 1976]: 1973; Morel and Caloumenos,1974; Rutkovskaya and Khalems- a OogE) a Oog E) kiy, 1974] has shownthat the world's oceanwaters may better k - • be classifiedby 10 different curvesof irradiance transmittance dz /Xz versusdepth (Figure 1 showsthe approximatecurves of irra- where E is the measuredirradiance and z is the depth below diance versusdepth for each of the water types from Jerlov the ocean surface. Variations in solar elevation due to changes [1976]). Unfortunately, measurementsof irradiance pene- in latitude were considered in the calculation of k. tration are lacking in many areas of the world. The transmissivitymeter indicatedthe ratio of the radiant In this experiment, irradiance penetration measurements flux transmitted through 0.25 m of seawater to the incident were made in conjunction with measurementsof the water radiant flux. The total attenuation coefficient c can be obtained transmissivity.These measurementsyield a parameter which, from the measuredtransmissivity as follows [Jerlov, 1976]' when used with the irradiance attenuation coefficient, can define water types even more clearly than the irradiance atten- T = e -or uation coefficientalone. The more parametersused to identify where T is the percenttransmission, c is the total attenuation water types, the more accuratelythe water type can be de- coefficient,and r is the geometricalpath length of the meter. scribed.It is shownin this paper that the method of water type In this experiment,r = 0.25 m; therefore identificationby irradiance penetration measurementsalone may yield untrue conclusionsabout the similaritiesin particle PERCENT OF SURFACE IRRADIANCE content or yellow matter content of the two water masses. I% I0% I00% That is, two water massesmay have nearly identicalvalues of , 0 an irradiance attenuation coefficient(thereby classifyingthem as the samewater type, optically, accordingto Jerlov [1976]) but may in fact have very differentprofiles of light transmis- sion versusdepth. An explanationfor this discrepancyis pre- sented. EXPERIMENTAL PROCEDURE AND RESULTS 4o • Measurementswith the irradiance meter and transmissivity meter were taken once a day at 1200hours local time approxi- mately 250 milesapart from Newport, Oregon,to Chimbote, Peru (Figure 2). Observationswere madeto depthsof approxi- i i i i i i i i i i This paperis not subjectto U.S. copyright.Published in 1979by the American GeophysicalUnion. Fig. 1. Irradiance transmittanceversus depth for 10 water types. Paper number 8C0944. 355 35• SPINRADET AL.: IRRADIANCEMEASUREMENTS 120' I10ø I00 ø 90 ø 80 ø • 2/z/z2 (I) / 40 ø 30 ø 20 ø -'-z/,(/.• • Eli)2/26 '1)"-"-) • •'•.2/28 (I o) 2/24 •e. 3/2 (Io) Fig.2. Mapshowing locations anddates ofstations taken. The water types are shown inparentheses. T = -o. 25c transmissivitywith depth indicated that for a givendecrease in or transmission(corresponding to an increase in c) theirradiance attenuationcoefficient k increased proportionally. Therefore, c = -4In T neglectingslight variations (less than 5%), the valuesof k/c The high degreeof accuracyobtained from the c meteris werequite constant with depthbelow the 3 or 4 m of surface attributedto the precisecollimation of the mainbeam and the smallsolid angle of detection[see Bartz et al., 1978]. TABLE 1. Irradianceand Transmissivity Results From All Stations In Table 1 the measuredvalues of k, T, and c and the Percent quantity k/c are shownfor eachof the stations.Variations in Station k, m-• Transmission c, m-• k/c 125 -- I 0.063 81.0 0.843 7.5 X 10-2 2 0.021 82.0 0.794 2.64 X 10-2 3 0.031 88.5 0.489 6.37 X 10-2 4 0.055 85.5 0.627 8.78 X 10-2 5 0.022 82.9 0.750 2.98 X 10-2 6 0.029 85.5 0.628 4.66 X 10-2 7 0.032 83.0 0.745 4.34 X 10-2 10 0.053 82.5 0.770 6.89 X 10-2 11 0.039 82.5 0.770 5.07 X 10-2 • 50 12 0.013 84.0 0.697 1.92X 10-2 13 0.024 82.0 0.794 3.02 X 10-2 14 0.029 57.3 2.23 1.32 X 10-2 G• 25 15 0.023 21.0 6.24 0.37 X 10-2 16 0.068 50.8 2.713 2.52 X 10-2 17 0.023 62.0 1.91 1.21 X 10-2 o i 18 0.034 83.5 0.721 4.73 X 10-2 400 600 800 I000 1200 19 0.027 51.8 2.63 1.02 X 10-2 WAVELENGTH (rim) 20 0.043 77.3 1.03 4.19 X 10-2 Fig. 3. Spectralresponse of the irradiancemeter. 21 0.052 71.3 1.35 3.83 X 10-2 SPINRAD ET AL.: IRRADIANCE MEASUREMENTS 357 TABLE 2. Classificationof Water Types by IrradianceAttenuation RELATIVEIRRADIANCE (Normalized to Imw/cm2-'at surface) Coefficient k o. ool O.Ol o.i i.o o Water Type Stations of [Jerlov,1976] k, m-• This Water Type I 0.016 2,12 la 0.025 5, 13, 15, 17, 19 ,-- .30-- lb 0.030 3,6,7,14 II 0.035 11,20, 18 E III 0.055 4, 10,21 1 0.068 1,16 3 0.095 5 0.154 C3 60-- / •/• TA6 7 0.223 9 0.282 ,;'/ ../ 90-- water. Table 2 showsthe valuesof k usedto definewater types optically [Jerlov, 1976]. This table also showsthe classification Fig. 5. Irradiance profiles with depth at stations6 and 14. of the water typesfrom this experimentas identifiedsolely by the irradiance attenuation coefficient k. OBSERVATIONS AND CONCLUSIONS In Figure4 the valuesof k for eachstation have been plotted As Table 1 shows,a given water column may have a similar against the valuesof c for the samestation. Constant values of or identical value of k as another water column, but the value k/c are shown in the figure, and the values of k for various of c may be quite different.This is also seenin Figures5 and 6, water types are shown along the abscissa. which show typical profilesof irradiance and transmissivityat stations6 and 14. An important factor to consideris that c, in this experiment,was measuredat a singlewavelength of light (650 nm), whereas k was measured for the range of wave- lengthsfrom approximately400 nm to 1000 nm. The fact that ,o * oOoO o'ø o'øo.øo9o the value of c at 650 nm may be different for two water columns having identical valuesof k is an indication that the types of material in the two samplesmay differ significantly. The existence of yellow matter in the seawater could not be detectedby the transmissivitymeter alone, since the absorp- tion of light by yellow matter is negligiblefor light of wave- oo length 650 nm [Jerlov, 1976]. However, particulatelight atten- uation at that wavelengthis quite significant[Burt, 1958]. At the shorter wavelengthsof light, both particulate matter and yellow substancecontribute significantlyto light attenuation. The parameter k/c is a usefulindication of relative amounts of particulate matter and yellow substancein seawater.The higher the value of k/c is, the more yellow matter or the less particulate content there is in the sample. The value of k for • •.o wavelengthsof 400-1000 nm may be the same for a particle- laden sample as for a sampleof water containingmuch yellow z matter. The two sampleswould be quite different in nature and so should be classifiedas such. The use of a transmissivity % TRANSMISSION OF 650-nm LIGHT 40 60 80 I00 o i i i I i ' I I I I I "% 0.1 I Io lb • 1112I 3 i [ i i i ! I [ [ STA./4// / / SE4.6 / / IRRADIANCE ATTENUATION COEFFICIENT , / / / k (xlO-2'm-I) / i i Fig.
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