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.