A New Way to Study Water-Vapor Absorption Coefficient

Vol.7 No.2Marine Science BulletinOct. 2005A New Way to Study Water- Vapor Absorption CoefficientZHOU Liangming (周良明), GUO Peifang (郭佩芳), TAN Yingyu (谭映宇)Department ofOceanography, Ocean University of China, Qingdao 266003, ChinaAbstract In the visible spectrum, the atmospheric attenuations to sunlight mainly includeaerosol scattering, atmospheric molecule Rayleigh scattering and ozone absorption, while in thenear- infrared spectrum (from 650 nm to 1 000 nm), we must take water-vapor absorption intoaccount. Based on the atmospheric correction theory, using spectrum irradiance data measuredby Instantaneous Ground spectrometer, ozone content measured by Microtops II ozone monitor,water- vapor content and aerosol optical thickness measured by sun photometer, we give a newway to study water-vapor absorption to sunlight, and the result shows that the main peak valuesof water- vapor absorption coefficients are 0.025 cmi ', 0.073 cm ', 0.124 cm' , 0.090 cmi , 0.141cm' and 0.417 cm', which respectively lie at 692 nm, 725 nm, 761 nm, 818 nm, 912 nm and937 nm.Keywords atmospheric attenuation, water-vapor, ozone， aerosol scattering, RayleighscatteringIntroductionAtmospheric correction is one of the most important problems for satellite ocean color remotesensing, and the key to atmospheric correction is to acquire the ingredients of atmosphere, from contentto distibution. For ocean color remote sensing, the range of spectrum we concern is from 400 nm to 760nm, so we mainly take into account the Rayleigh scattring, aerosol scattering and ozone absorptionwhen computing atmospheric correction.The influence of water-vapor absorption in the near-infrared spectrum causes an apparent spectralvariation, which will lead to an error in atmospheric correction. This effect is dependent on thewater-vapor content of the atmosphere. If the water-vapor content is known, a simple procedure isprovided to remove the degradation of the atmospheric correction. Uncertainty in the water-vaporcontent will limit the accuracy of the correction algorithm.The absorption of water vapor is lttle in the visible spectrum, while in the near infrared spectrum, itis large by comparison. The optical thickness of water-vapor absorption is proportional to its content.Generally, the optical thickness of water-vapor absorption may be presented asIw=k_U中国煤化工(1)CNMHGwhere kw is the absorption coefficient (in cm "), and U is tune waler-vapor comtent uIn cm).Received on June 9, 200568Marine Science BulletinVol.7The Instantaneous Ground spectrometer (ISI921VF) can measure spectral irradiance from 380 nmto 1 000 nm. The spectral information of 650 nm to 1 000 nm thus gives a chance to study water-vaporabsorption.1 Atmospheric Attenuation in the Visible Spectrum1.1 Ozone absorptionThe value of ozone absorption optical thickness T。is related to the ozone absorption coefficientKQ(h) by Gordon (1995) "U。(2)To =Koz 1000where h is in centimeter, and U。is the ozone content in milliatmosphere centimeter (Dobson Units),which can be measured by Microtops II ozone monitor.The ozone absorption coffcient K。(2) was taken from Nicolet (1981) [2]K。=(2.1349x10*)e-0.14052(3)1.2 Aerosol scatteringThe aerosol is initially assumed to be nonabsorbing and to be scattering. About 90 % of the aerosolis confined to a layer of 2 km of thickness near the sea surface with about 80 % of the Rayleighscattering molecules above the aerosol. The aerosol optical properties in the real coastal environmentcan vary significantly.The aerosol optical thickness T。 varies with wavelength according to Sturm (1981)B1r。()=r。(h)(h1q)-B(4)where the exponent β is assigned to be 0 or 1.Microtops II sun photometer can observe t。 at 440 nm, 500 nm, 675 nm, 870 nm and 1 020 nm.According to these five Tg, we can compute τ。with all wave length using(4).1.3 Rayleigh scatteringThe Rayleigh optical thickness being directly proportional to the ground pressure, the actual valueof τp is computed according to Sturm (1981)r(,z)=0.008792-40 p(5)where a is in micrometers, p is the standard pressure and the departure 4p of the actual pressurewith respect to the standard is set equal to土15 hPa.After getting Tg， τR and To, we can get the total中国煤化工heric attenuation inthe visible spectrum, which is marked as r,YHCNMHGτ,=Ta +Tp+ Toz(6)No.2ZHOU Liangming et al: A New Way to Study Water-Vapor Absorption CoefficientAccording to Gordon and Voss (1999)4+, the direct transmittance T isT = exp(- M)(7)where τ is the total optical thickness in both visible and near-infrared spectra, that ist=T, +Iw(8)and Tw and T, are presented in( 1 ) and( 6 ) respectively.In(7 ), M is the air mass, which was taken from Fargion G S and Mueller JL ( 2000) [5M = sec0 - 0.0018167 (sec0 -1)- 0.002875(sec0 -1)2 - 0.008083(sec0-1)'(9)where θ is the solar zenith angle.The relation between the direct incident irradiance above sea surface E and the extraterrestrialdolar irradiance Fo is followed as (Gordon and Voss, 1999)E.a= F。exp( -M )( 10)According to(1 ), (8 ) and ( 10 ), the absorption coefficient of water-vapor is confined as followsln(E。u 1F)/M-τ,kw=(11)Uw2 Data Processing and ResultsFrom June to August, 2003, we made in-situ observations at 136 points in the Bohai Sea. These datainclude ozone content, sea surface pressure, water-vapor content and aerosol optical thickness at 440 nm,500 nm, 675 nm, 870 nm and 1 020 nm. Fig.1 is the curve of Rayleigh scattering optical thickness. Fig.2shows the aerosol optical thickness with different wavelengths. Fig.3 is the ozone absorption thicknesscurve. Then we get the water-vapor absorption coefficient kw (presented as in Fig.4). The main peakvalues of water- vapor absorption coefficients are 0.025 cm"', 0.073 cm" , 0.124 cm*', 0.090 cm"', 0.141:m' and 0.417 cm', which respectively lie at 692 nm, 725 nm, 761 nm, 818 nm, 912 nm and 937 nm.00一0舶-2020中国煤化工00 -a0400Wav elength/nmTYHCNMHGFig. 1 Rayleigh scattering optical thickness varies with wavelength, where the pressure is 1 016 hPa70Marine Science BulletinVol.7050.50 -0.30 -20 -30400的Wavelengt/nmFig.2 Aerosol optical thickness varies with wavelengthae -口3 -r。。-则-4wavel ength/nmFig. 3 Ozone optical thickness, where the ozone content is 318.9DU040.35-0.15中国煤化工駒700 130 800.MYHCNMHGwavolongth/nmFig.4 The curve of water-vapor absorption coefficientNo.2ZHOU Liangming et al.: A New Way to Study Water-Vapor Absorption Coefficient3 Concluding remarksIn this paper, we present a new way to study water-vapor absorption cofficient, and the resultcorresponds with the conclusions given by predecessors. This way and work will facilitate studyingatmospheric attenuation and atmospheric correction of ocean color remote sensing.AcknowlegementsFinancial support was provided by The National High Technology Research and DevelopmentProgram of China (863 Program): No 2001AA633030 and 2001AA633080.References[1]Gordon H R. Remote Sensing of Ocean Color: A Methodology for Dealing with Broad Spectral Bands andSignifcant out-of-band Response. Applied Optics, 1995, 34(36): 8 363-8 374.[2]Nicolet M. The Solar Spectral Iradiance and Its Action in the Atmospheric Photodissociation Processes,Planet Space Sci. 1981, 29: 951 -974.[3]Stum B. Ocean Color Remote Sensing Using Nimbus CzCS Data. In oceanography from space edited byJ.FR.Gower, 1981, 267-288.[4] Gordon H R and Voss K J. MODIS Normalized Water leaving Radiance Algorithm Theoretical BasisDocument, Version 4, Under Contract Number NASS-3I363, 1999, 1-25.[5] Fargion G s, andMueller J L. Ocean Optics Protocols for Satellite Ocean Color Sensor Validation, Revision 2.NASA/TM-2000 209960, 2000, 4 189.中国煤化工MYHCNMHG