Thediffusionattenuationcoefficientfordownwellingirradiance,Kd(490),isanimportantopticalparameterofseawater.Theopticalproperty,Kd(490),aroundNanshaIslandsintheSouthChinaSeawasanalyzedbasedonopticaldataprofilesmeasuredwithSPMR(SeaWiFSProfilingMultichannelRadiometer)andSMSR(SeaWiFSMuitichannelSurfaceReference)instrumentinAprilandMay,1999.TheresultsshowthatKd(490)iscloselycorrelatedwithchlorophyll-aconcentration,Cchl.a,andthattheverticaldistributionofKd(490)showsthestrongstratificationofthewatercolumninthisarea.Kd(490)hasasimilarverticalco-variationasKL(490),thediffusiveattenuationcoefficientforupwellingradiance.Bothcoefficientsincreasewithdepthintheupperlayer,whereKL(490)isgreaterthanKd(490);andafteradepth,approximativelyatthechlorophyllmaximum,theydecreasewithdepth,withtheformerbeingsmallerthanthelatter.
