The laser-induced chlorophyll fluorescence has been normalized by the water Raman backscatter to correct for variations in the optical attenuation properties of the upper water column along the flight track. A discussion of the AOL instrumentation and the water Raman normalization procedure are given in the references listed below. The laser-induced chlorophyll fluorescence was acquired using 532 nm excitation from a frequency doubled Nd:YAG laser with an output power of ~15 mjoule/pulse. The laser was operated at 10 pulses/second. At the nominal 120 m/sec velocity of the NASA P-3A aircraft, this sampling rate yields an observation every 1.2 m along the flight track. The data contained in these files are 30 point simple averages providing an observation approximately every 0.4 km. The 532 nm laser excitation also stimulates fluorescence from the phycoerythrin pigment contained in some strains of marine phytoplankton. The amount of laser-induced phycoerythrin fluorescence found during the 21 May 1989 survey was found to be low (below the detection limitation of the AOL sensor) and accordingly is not included with the data set contained on this disk.
The relationship between water Raman normalized laser-induced chlorophyll fluorescence ratio and chlorophyll concentration varies somewhat due to changes in the fluorescence pigment quantum efficiency of the in situ chlorophyll molecules much as chlorophyll fluorescence from a continuous underway fluorometer does. Changes in the fluorescence quantum efficiency are affected by such factors as available light, nutrients, etc., however from previous experiments we have seldom seen variations in excess of 20% within a single mission over a period of 4 to 5 hours under daylight conditions. In comparing the data set with available chlorophyll data from the R/V Discovery (U.K.) over a contemporaneous span between 52.5 N and 53.5 N (over a period of 9 - 17 hours after the airborne sampling) we found that a scale factor 4.95 applied to the water Raman normalized laser-induced chlorophyll provided reasonable agreement. The data set from the R/V Discovery and the Raman normalized laser-induced chlorophyll fluorescence data from the AOL were resampled at 0.02 degree increments as a function of latitude and combined. A linear regression between the absolute chlorophyll values and the chlorophyll fluorescence yielded an R coefficient of 0.80 and the above 4.95 scale factor between the Raman normalized laser-induced chlorophyll fluorescence values and chlorophyll in ug/liter units. The 4.95 scale factor was applied to all data sets obtained during the May 21 mission. The SST observations were collected from an auxiliary Barnes PRT-5 infrared radiometer. A linear regression of the PRT-5 ocean surface temperature values along all 7 flight lines with the ocean surface temperature values from the Airborne Expendable Bathythermographs (AXBT's) yielded a r coefficient of 0.95. Note that Leg 3 was not included because of some instrument problems experienced at that point in the mission.
References:
Hoge, F. E., and R. N. Swift, Airborne dual laser excitation and mapping of phytoplankton photopigments in a Gulf Stream warm core ring, Appl. Opt. 22, 2272-2281, 1983.
Smith, R. C., O. B. Brown, F. E. Hoge, K. S. Baker, R. H. Evans, R. N. Swift, and W. E. Esaias, Multiplatform sampling (ship, aircraft, and satellite) of a Gulf Stream warm core ring, Appl. Opt. 26, 2068-2081, 1987a.