Data Reduction Scheme

by Wilford Gardner
A 25 cm Sea Tech Transmissometer was interfaced with the University of Washington's SeaBird CTD for all EQPAC cruises. For TT008, beam transmission values were extracted from raw CTD files and passed through SeaBird's SeaSoft data processing package with no data averaging and the spikes were removed. The data were compared with the output using the SeaSoft spike removal and 1 db bin averaging routine. There was no substantial difference in the data using the two methods, so for TT007, TT011, and TT012 the data were taken from the SeaSoft output directly. Beam transmission values were selected at 1db intervals and de-spiked. The data were corrected for factory and field air calibrations and adjusted for changing Temperature, Salinity, Index of Refraction, and Pressure. Beam transmission was converted to beam attenuation coefficients using c=-(1/z)*ln(%Tr/100) where c=beam attenuation coefficient (m^-1), z=beam path length (m), ln=the natural log, and Tr=% beam transmission.

The two spring cruises and the two fall cruises were treated as paired data sets and processed further as follows. Most profiles only went to 400 m, so for TT008 the minimum beam c was determined over that depth range for each profile, the minimum beam c for the cruise was determined and all profiles were shifted to the cruise minimum beam c. The average depth of the minimum was 250 m (200 m for TT012), so stations shallower than this amount were adjusted by the same amount as an adjacent station rather than the actual minimum of the shallow station. The above procedure automatically corrects for any drift in the LED light source and uses the reasonable assumption that there was no change in the minimum beam c during the three weeks at the equator. It also eliminates errors in air calibrations.

To calibrate beam c with particulate matter concentrations (PMC), beam c was picked from the adjusted data at each depth where samples were filtered on the equator. Regressions of beam c verses PMC for TT008 and TT012 yielded the equations:

     TT008      c=0.00221821*(PMC)+0.3103.   r^2=0.87
     TT012      c=0.00154626*(PMC)+0.3700.   r^2=0.90
The r^2 values for these two cruises are the best correlations between beam c and PMC that we have seen for any data from surface waters.

Sea Tech transmissometers were factory calibrated to have a beam c of 0.364 m^-1 in particle-free water, so the difference between the regression intercept (e.g. 0.3103 for TT008) and the factory calibrated intercept (0.364) was added to all profiles to obtain calibrated profiles for analysis. Beam cp, the attenuation due to particles in the water, was obtained by subtracting the attenuation due to water (0.364 m^-1). Inverting the above equations yielded the relationship between beam cp and PMC (ug/Kg):

     PMC=451*cp         Spring Time Series
     PMC=647*cp         Fall Time Series
By comparison, the relationship for data in surface waters of the North Atlantic Bloom Experiment was:

     PMC=1022*cp        North Atlantic Bloom Exp.
For cruise TT007 beam c was calculated as above and detrended for decay of the transmissometer LED. The minimum beam c was determined in the upper 400 m for each latitudinal Station (1-11 profiles) and all profiles at a particular Station were adjusted to the minimum beam c for the Station to preserve latitudinal variations. Cruises TT007 and TT008 used different transmissometers, so data from the two cruises were tied together with the assumption that the minimum beam c at the equator did not change between the two cruises. The difference between the minimum beam c at the Equatorial stations was calculated for TT007 and TT008 and all TT007 profiles were adjusted by that amount. Thus the filtration calibration made during TT008 could be reasonably applied to TT007 data. Values at 12 deg. N during TT007 had incorrect air calibrations, so the profiles were adjusted to have the same minimum in the upper 400 m as stations at 9 deg. N (this required a decrease in beam c of only 0.015 m^-1).

Data from cruises TT011 and TT012 were processed and tied together in the same way. The spring and fall cruises are also comparable because of the filtered water calibration. The minimum beam c in the upper 400 m at the equator for TT008 was 0.3828 m^-1 and 0.3830 m^-1 for TT012.