the 2008 north atlantic bloom experiment calibration...

NAB08 Transmissometer Intercalibration, V1.4 1

The 2008 North Atlantic Bloom Experiment

Calibration Report #2

Ship and Float Beam Transmissometer Intercalibration Report

Eric Rehm Applied Physics Laboratory, University of Washington

[email protected] Version 1.4 March 2011

Abstract Five WET Labs C-Star beam transmissometers were deployed during the 2008 North Atlantic Bloom experiment

(NAB008). Floats 47 & 48 (CST-1062, CST-1063), the R/V Knorr cruise 193 (CST-284, CST-282, CST-1090), and the

three cruises on the R/V Bjarni Saemundsson (CST-284, CST-1090) are compared and rectified to yield a consistent

measurement of transmittance (%) and attenuation coefficient due to particles cp at 652 nm. This will allow us to apply a

single correlation between beam cp and particulate organic carbon based on linear regression of cruise CTD measurements

and bottle measurements of POC.

Introduction

The measurement of the cycling of particulate organic carbon is a key goal of the 2008 North Atlantic Bloom experiment

(NAB08). Robust linear correlations for a given area and time has been previously shown between particulate organic

carbon (POC) from bottle samples and in situ measurements of attenuation coefficient due to particles beam cp (Gardner

et al. 1993, 2003. 2006). Therefore, the goal of the intercalibration of the various beam transmissometers presented here

is to place all the NAB08 transmissometers on a single absolute scale of beam cp. Then we can develop a single

correlation between beam cp and POC that can be applied to >21,000 beam transmittance measurements made from the

four NAB08 cruises and the two NAB08 Lagrangian floats.

NAB08 cruise rosettes provided depth-resolved measurements of beam transmittance voltage. At some stations, water

samples were collected for subsequent laboratory analysis for POC concentration. At a subset of these stations, inter-

calibration of the cruise transmissometer with float transmissometers was carried out by performing simultaneous

“calibration casts” from the research vessels near the floats. The plan was to use a single (“primary”) beam

transmissometer on all the NAB08 cruises to establish the correlation between beam cp and POC, then inter-calibrate the

primary and float beam cp measurements , and finally to apply the reference cp-POC correlation to estimate POC from

float-based transmittance beam cp. However, transmissometer failures meant that three separate beam transmissometers

were used during the course of the four NAB08 cruises (deployment, process, rescue, and recovery; see Figure 1and Table


3 below). Calibration casts were limited or missing for two of the three cruise transmissometers, resulting in the need

create a secondary calibration for these transmissometers.

The timeline below provides an overview of timing the four NAB08 cruises and associated beam transmissometer

deployment history, float deployment and recovery operations, and the calibration casts. Class A calibration casts were

performed by holding a float at the surface via satellite command until the ship was within sight of the float (< 0.5 km), at

which point a the float was commanded to profile down at the same time a CTD cast was performed from the ship. Less

precise Class B calibration casts were performed by maneuvering the ship within 1 km of the float(s) and performing a

CTD cast within approximately one hour of a scheduled down profile by the float.

Figure 1. Timeline (year day 2008) for cruise C-Star deployments. Color on the timeline indicates the cruise vessel and

duration. NAB08 float deployments are shown above the timeline, while NAB08 cruise transmissometer deployments are

shown below the timeline. Calibration casts are shown as vertical arrows in colors to match the corresponding cruise vessel.

Due to the software failures and subsequent mission interruption of Float 47, the largest number of float calibration casts

were carried out on the process cruise (R/V Knorr) between a single rosette transmissometer (CST-1090) and the

transmissometer on Float 48 (CST-1063). Two calibration casts each were performed on the deployment cruise between

CST-284 and Floats 47 and 48, and two more were performed between CST-284 and Float 48 on the process cruise. No

calibration casts were performed for CST-282, which failed on its only deployment cruise cast at approx. 400 m. See

Table 2 for a summary of calibration cast data.

Recovery cruise

CST-284

92 95 97 100 105 110 115 120 122 125 130 135 137 140 143 145 150 154 155 158 160…177 … 183

R/V Bjarni Sæmundsson R/V Knorr

Class A float calibration casts (100 – 500 m, 15 min separation)

Class B float calibration casts (up to 1 Km , 1 hr separation)

CST-xxxx : WET Labs C-Star serial number on ship’s rosette

Floats 47 & 48 deployed

Float 48 reboot (104)

Float 47 in recovery mode (107)

Float 47 recovered (139)

Float 47 redeployed Float 47 recovered

Process cruise

Float 48 recovered

Float 48 in recovery mode

Seagliders recovered

CST-284

CST-282

CST-1090

CST-1090 CST-1090

Deployment cruise Rescue cruise

Floats 47 reboot (103)


Calibration of a transmissometer To calibrate a single transmissometer, i.e., convert the recorded voltages or digital counts to transmittance (%) or beam cp

(m-1), the output signal (voltage or counts) must be measured a) with the receiver blocked (Vdark , Countsdark) and b) fitted

with a flow tube filled with pure fresh water (Vref , Countsref). From these measurements, transmissivity (%) and beam cp

can be computed. (See Appendices A & C for additional details.) Of the three beam transmissometers used on the

NAB08 cruises, only CST-1090 was factory calibrated in this manner within a short time before and after the NAB08

experiment. The float beam transmissometers (CST-1062, CST-1063) were calibrated at the factory and in a U.W. lab

using a slightly different approach (immersion in pure water to determine Countsref ) shortly before and after the NAB08

deployments.

The approach here was to select a single primary cruise transmissometer and intercalibrate in one or two steps all of the

other four beam transmissometers. The result of this intercalibration exercise expresses all beam transmissometer output

in the same primary transmissometer voltage, to which a single pure water transmissometer calibration value (Vref) can be

used to compute transmittance (%) and beam cp (m-1).

Basic Intercalibration Process 1. Select primary transmissometer and establish stability

2. Rectify Float 48 transmissometer measurements to primary transmissometer voltage response Vk at calibration cast

stations.

3. Rectify additional process cruise transmissometers to float 48 response and apply step 2.

4. Rectify Float 47 transmissometer measurement to float 48 response and apply step 2.

5. Apply primary transmissometer calibration coefficient Vref to all intercalibrated results, all of which are on the voltage

scale of the primary transmissometer, to compute transmittance (%) and beam cp (m-1).

Each step of the process is presented in the sections below. Details of the algorithm used in step 2 and used with some

modifications for steps 3 and 4 is presented in Appendix A. A schematic representation of the states in this process is

provided in Appendix B.


Summary of Transmissometer Intercalibration Factors

Table 1. Transmissometer Intercalibration Factors (* Applicable to all transmissometer data recorded by the float 48.)

C-Star Platform Applicable

Casts

Calibration

Casts

Slope Offset RMS error

cp (m-1)

N r2

1090 All All 1.0 0.0 0.008 (abs)

284 Knorr 001-028, 030 015 0.92957 0.69666 0.017 57 0.98

284 Bjarni S. B147 – B159 B157, B159 1.00373 0.32343 0.004 17 0.82

282 Knorr 029 030 0.91487 -0.07500 0.027 550 0.83

1062 Float 47 4-16 April B157, B158 4.3992e-4 2.80005 0.040 44 0.31

Float 47 16-18 May Knorr 091 4.8702e-4 2.66039 0.017 72 0.87

1063 Float 48 * Knorr 036, 045,

047, 064, 088,

091, 106, 122

0.0011355 0.034426 0.016 503 0.99


Table 2. Float calibration casts

Index Station Cast Class Ship Cstar

Yearday Cast date Cast Time (Z)

Station lat Station lon Float Float Down Time (Z)

Float lat Float lon Distance ship-float

BS1 NABE7 157 B 284 95 2008-04-04 16:40 59 05.8200 N 20 27.8700 W 47 16:54:33 59 06.0284 N 20 27.5434 W 496 m

48 17:09:41 59 05.6157 N 20 28.8130 W 974 m

BS2 NABE9 159 B 284 96 2008-04-05 4:18 59 11.8900 N 20 25.2400 W 47 04:33:30 59 12.7329 N 20 25.2317 W 1.56 km

48 04:39:27 59 13.0252 N 20 26.1426 W 2.27 km

Knorr1 10a 009 B 284 126 2008-05-05 16:45 60 55.3600 N 26 59.7700 W 48 15:36:59 60 55.5582 N 27 00.0593 W 450 m

Knorr2 21a 015 A 284 128 2008-05-07 15:40 61 04.1800 N 26 39.7900 W 48 15:48:54 61 04.0702 N 26 39.7738 W 204 m

Knorr3 35a 036 B 1090 131 2008-05-10 16:22 61 23.9970 N 26 13.1772 W 48 15:17:44 61 23.7041 N 26 13.4060 W 282 m

Knorr4 44a 045 A 1090 132 2008-05-11 16:15 61 26.3510 N 25 58.4875 W 48 16:19:07 61 26.2937 N 25 58.7844 W 248 m

Knorr5 51a 047 A 1090 133 2008-05-12 16:06 61 19.9560 N 25 58.3516 W 48 16:15:58 61 19.8235 N 25 58.5746 W 314 m

Knorr6 64b 064 A 1090 134 2008-05-13 16:00 61 11.9198 N 26 07.4573 W 48 15:57:18 61 11.9243 N 26 06.9140 W 344 m

Knorr7 94a 088 A 1090 136 2008-05-15 16:15 61 29.9299 N 26 20.9000 W 48 16:30:00 61 30.0138 N 26 20.3281 W 529 m

Knorr8 101b 091 A 1090 137 2008-05-16 16:55 61 27.6300 N 25 57.0600 W 47 16:39:18 61 27.6544 N 25 56.6096W 401 m

48 17:10:19 61 27.3500 N 25 56.8756 W 544 m

Knorr9 117a 106 A 1090 139 2008-05-18 16:00 61 14.0400 N 25 32.0200 W 48 16:04:07 61 13.9359 N 25 32.1232 W 214 m

Knorr10 142a 122 A 1090 141 2008-05-20 15:20 61 28.1898 N 25 43.7861 W 48 17:23:55 61 28.1898 N 25 43.7861 W 308 m

Table 3. History of C-Star Deployments on NAB08 Cruises

Cruise Vessel

Cast / CStar

CTD

Notes

Deploy Bjarni S. b0146 –

b0159

V2 CNV files have the beam transmissometer and CDOM voltages reversed. The correct assignments are: - Name 8 / Sensor 4 / External Volt 2 is the transmissometer - Name 9 / Sensor 5 / External Volt 3 is the CDOM fluorometer.

Process Knorr 029 V3 CST-282PR failed during cast 029.

Process Knorr 001-028 V3 CST-284PR is incorrectly labeled as CST-248PR in Seabird metadata and CTD error log. Possible failure of CST-284PR during casts 025-028.

Process Knorr 030 V2 CST-284PR is incorrectly labeled as CST-248PR in Seabird metadata and CTD error log. Possible failure of CST-284PR during cast 030.

Process Knorr 031-134 V6

Rescue Bjarni S. b0282 – b0290 V2

Recovery Bjarni S. b0290a – b0304 V2


1. Select Primary Transmissometer We select CST-1090 as the primary transmissometer because of a) the stability of the transmissometer between pre- and

post-cruise factory calibrations, b) the large number of float 48 calibration casts performed with CST-1090 gives us a

single-step correlation to float 48 using a majority of the process cruise calibration cast data points, and c) CST-1090 was

used for more profiles than any other cruise transmissometer, being used for 104 out of 134 casts on the process cruise

and on all CTD casts on the rescue and recovery cruises.

We investigate the stability of the CST-1090 transmissometer by examining pre- and post-cruise pure water and air

calibration values, as well as the stability of beam transmittance measurements at 600 m, the deepest point of the Knorr

CTD casts. The relative changes Vdark and Vref between pre- and post-cruise calibrations are less than 2%, indicating good

stability of CST-1090 over time. Comparing pre- and post-cruise WET Labs calibrations for CST-1090 (Table 4), the

change in Vdark is ~1 mV, which is roughly the resolution of the C-Star. The pure water reference voltage Vref increased by

16 mV or 0.3%. Acting alone, this would create a slight trend of increasing deep water transmissometer voltages with

time, if the bio-optical properties at 600 m were unchanging. However, examining the pre- and post-cruise changes in Vref

for the two Biofloat C-Stars (e.g., see Table 5), we see larger positive changes (+1.5%). Rather than the transmittance

actually increasing in water for all three C-Stars, we will assign this variability to the calibration process itself. Therefore,

we will not correct for changes in Vref, using the average of pre- and post-cruise values for Vref = 4.653 V.

The maximum and average C-Star voltage between 590-600 m for each R/V Knorr cruise cast using CST-1090 (Figure

2a) shows decreasing transmittance over the course of the cruise, on the order of -16 mV. We hypothesize that this

decrease represents a real change in the transmittance of the water between 590-600 m as bloom production sinks. To test

this hypothesis, we examine backscattering at 700 nm. Averaging backscattering voltage between 590-600 m for each

cruise cast, we see increased backscatter with time (Figure 2b), consistent with measurements of the gliders at 600 m and

cruise sediment trap deployments. Further, this -16 mV change in transmittance is too large to be accounted for by

changes in Vdark (~ 1 mV). So, we conclude that the particle scattering in deep water is increasing, causing both

transmittance to decrease and particle backscattering to increase, and that we cannot use 600 m values alone as a measure

of transmissometer stability. Therefore, we will assign an uncertainty to the Vref of ±8 mV. Note: If increased scattering

was the only effect, and if we were to correct for the entire +16 mV change in pure water transmittance (Vref) at the end,

then we might have seen a change in deep water of as much as -32 mV = transmittance voltage of -16 mV (undo reference

value change) - 16 mV (increased scattering). Thus, we may be underestimating the decrease in transmittance and beam-

c, hence underestimating particle absorption or scattering and ultimately underestimating POC.


In summary, for the primary transmissometer (CST-1090), we will use Vdark = 0.063 V and Vref = 4.653 V ± 0.008 V. For

the range of voltages seen during calibration casts (3.8 – 4.6 V), this translates into an calibration uncertainty of ±0.17 %

(transmissivity) or ±0.008 m-1 (beam cp(652)).

130 132 134 136 138 140 142 1444.58

4.585

4.59

4.595

4.6

4.605

4.61

4.615

4.62

4.625

Yearday 2008

Vol

ts

Average and Maximum DeepBeam Transmissometer Voltage (CST-1090PR)

y = -0.0015575*x + 4.8211

y = -0.0016786*x + 4.8306

Vavg

(590-600 m)

Vmax

(590-600 m)

130 132 134 136 138 140 142 1440.12

0.125

0.13

0.135

0.14

0.145

0.15

Yearday 2008

Vol

ts

Average Deep Backscattering (β) Voltage (FLNTU-873)

y = 0.00096268*x + 0.0025162

Vavg

(590-600 m)

Figure 2. Change in (a) transmittance and (b) backscattering at (140 degrees) at ~600 m versus time.

The spectral nature of the beam attenuation coefficient has be related to the shape of the particle size distribution (Voss

1992, Boss, Pegau et al. 2001, Boss, Twardowski et al. 2001). Since it is likely that beam attenuation measurements may

be used in wavelength-resolved bio-optical models, we measured the actual center wavelength of the NAB08 beam

transmissometers using a TriOS RAMES ARC hyperspectral radiometer (serial no. 81DC, 320 – 950 nm, ~3.3 nm

spectral sampling, 0.3 nm accuracy). Collected radiance spectra were fit using a nonlinear least squares technique to a

Gaussian spectral model to estimate the center wavelength. In all cases, the spectral fit achieved an r2 of 0.99 or better.

The results are show in Table 6.


Table 4. WET Labs calibration values for C-Star CST-1090 used on the R/V Knorr process cruise.

Pre-Cruise

3/17/2008

17.4 C, 23.7 °C amb.

(volts)

Post-Cruise

9/29/2008

22.4 °C, 21.5 °C amb.

(volts)

Change

(volts.)

Uncertainty

(%)

(m-1)

CST-1090 Vdark 0.064 0.063 -0.001 0 0

Vref 4.644 4.660 +0.016 0.17%, 0.008

Vair 4.764 4.770 +0.006

Table 5. U.W. calibration values for C-Star`s CST-1062 and CST-1063 used on Biofloats 47 and 48, respectively

Pre-Cruise

3/12/2008

22 °C, 21.5 °C amb.

(counts)

Post-Cruise

8/28/2008

20.7 °C, 20 °C amb.

(counts)

Change

(counts, rel. % diff.)

Uncertainty

(%)

(m-1)

CST-1062 Vdark 65 65 0, 0.00% 0 0

(Float 47) Vref 3924 3983 59, 1.50% 0.76 0.032

Vair 4014 4044 30, 0.75%

CST-1063 Vdark 65 65 0, 0.00% 0 0

(Float 48) Vref 3971 4030 59, 1.49% 0.75 0.032

Vair 4053 4095 42, 1.04%

Offset 1063-1062

Vref 47 47 0, 0.0%

Table 6. Specified and measured center emission wavelength of C-Star Beam Transmissometers

C-Star WET Labs Nominal

Wavelength

Center Wavelength (95% confidence bounds)

CST-1062 660 nm 651.4 nm (651, 651.8)

CST-1063 660 nm 652.4 nm (652, 652.8)

CST-1090 660 nm 652.2 nm (652, 652.3)

CST-282 660 nm Not measured

CST-284 660 nm Not measured


2. Rectify Float 48 transmissometer measurements to primary transmissometer The largest number of calibration casts occurred on the R/V Knorr process cruise using the primary transmissometer CST-

1090 at stations where Float 48 (CST-1063) was held at the surface. Therefore, we proceed to establish, and examine the

quality of, a correlation between CST-1090 and CST-1063. The primary transmissometer was calibrated and data

acquired in volts. The Biofloat transmissometers were calibrated and data acquisition occurred in digital counts. A digital

output calibration was not performed in the pre-cruise calibration of CST-1090, nor was a voltage output calibration

performed in the pre-cruise calibration of CST-1062 or CST-1063. Therefore a direct comparison between the two

instruments in like units (volts or digital counts) is not possible. Therefore, a relationship will be established between

CST-1090 volts Vk (CST-1090 voltage after Vdark is subtracted) and CST-1063 counts, Counts1063, (CST-1063 output after

Countsdark is subtracted). In short, we will be able to take any Float 48 CST-1063 counts measurement, express it in CST-

1090 volts, and then apply the primary transmissometer Vref and compute estimated values of transmittance and beam c

relative to pure water.

This process involves computing CST-1090 voltages from R/V Knorr rosette at the corresponding depths at which the

float measurements were taken. Given the separation of the ship from the float of 100 to 300 m and possible influence of

internal waves on the vertical structure of the two water columns, we elect to interpolate ship measurements to float

measurements in density coordinate space. This approach automatically filters the influence of internal waves, comparing

transmittance data at depths corresponding to the mean isopycnal depth of the ship relative to a float measurement. Plots

of CST-1090 voltage vs. density (Figure 4) and CST-1090 voltage vs. depth (Figure 5) show that matchups with

correlated float measurements are quite good. These plots also show down cast (light color) can be quite different from

the up cast (isopycnal displacement, rosette dragging higher density water as it ascends), so only the down cast data was

used in the subsequent regression analysis.

The corresponding least squares regression1 Figure 3 uses N=503 comparisons, yielding a solid correlation (r2 = 0.99, see )

and an RMS error between ship and float measurements of εrms, volts = 17 mV or εrms, beam-cp = 0.016 m-1 (see Figure 6 and

Figure 8). Other than a slight bias is seen at Station 117a (cast 19303106), the residuals are evenly displaced about zero.

The residuals show increased error near the surface. As a result, the regression is based only on data at or below 10 m.

1 A Model I ordinary least squares regression of transmissometer volts (Y, dependent variable) vs. float counts (X, dependent variable)

is computed here. Often a Model II regression (geometric mean regression, GMR) is used when correlating two measurements where

it is unclear which of X or Y is the independent variable and X and Y data are mutually independent pairs from a bivariate normal

distribution that has been sampled at random (Ricker 1973, Schnute 1984). However, since there is no information (yet) on the

underlying sample statistics of each variable, and the use of Model II regression can be controversial (“GMR is not recommended

when there are errors in both variables”, Emery & Thomson, 2004), a standard Model I Y vs. X regression was used here.


The calibration uncertainty in CST-1063 from Table 5 is ±29.5 counts or 33.5 mV in CST-1090 units. On its own, that

would translate to an uncertainty of 0.73% or 0.032 m-1. However, after applying the linear regression and adding in the

reference transmissometer uncertainty of 8 mV and the RMS regression error of εrms, volts = 17 mV, the combined

uncertainty is 1.3% or 0.056 m-1.

3300 3400 3500 3600 3700 3800 3900 40003.8

3.9

4

4.1

4.2

4.3

4.4

4.5

4.6

4.7

Float 48 (counts)

Kno

rr C

TD (v

olts

)

Beam Transmission: Float 48 counts vs. Knorr CTD voltsat calibration stations

y = 0.0011355*x + 0.034426, r2 = 0.99

036045047064088091106122

Figure 3. Regression of Float 48 transmissometer counts vs. Knorr rosette transmissometer voltage interpolated on density interface


Figure 4. Ship rosette and Float 48 beam transmissometer voltage minus Vdark vs. density for float calibration stations


Figure 5. Ship rosette and Float 48 beam transmissometer voltage minus Vdark vs. depth for float calibration stations


-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.35a, CTD 19303036

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.44a, CTD 19303045

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.51a, CTD 19303047

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.64b, CTD 19303064

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.94a, CTD 19303088

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.101b, CTD 19303091

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.117a, CTD 19303106

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.142a, CTD 19303122

Transmissometer Conversion Residuals: Knorr CTD - Converted Float 48] (volts) vs. depth (m)

Residual error in beam cp (m-1) Residual error in beam cp (m-1)

Residual error in beam cp (m-1)

Dep

th (m

)D

epth

(m)

Dep

th (m

)

Figure 6. Residual error (volts) versus depth for Knorr and converted Float 48 transmissometer measurements.


Figure 7. Knorr rosette and Float 48 beam attenuation at float calibration stations

C TD Down cas t : color

C onverted F loat 48:


-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.35a, CTD 19303036

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.44a, CTD 19303045

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.51a, CTD 19303047

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.64b, CTD 19303064

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.94a, CTD 19303088

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.101b, CTD 19303091

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.117a, CTD 19303106

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.142a, CTD 19303122

Residual error in beam cp (m-1) Residual error in beam cp (m-1)

Residual error in beam cp (m-1)

Dep

th (m

)D

epth

(m)

Dep

th (m

)

Figure 8. Residual error in beam cp (m-1) versus depth for Knorr and converted Float 48 transmissometer measurements.


3. Rectify additional process cruise transmissometers to float 48

3.1 Calibrate CST-284 on Knorr Process Cruise using Float 48 Calibration Casts

CST-284 used on all but one of the first 30 Knorr process cruise stations (Table 2). Two calibration casts with Float 48

(10a / 009 and 21a / 015) were performed during this time. Only station 21a (cast 015) was a “type A” calibration cast

where the float was visually sited from the ship.

CST-284 is indirectly tied to the primary transmissometer (CST-1090) using only the class A cast from station 21a. (See

Figure 10 and Figure 11 for details on why only Station 21a was used.) Specifically, CST-284 measurements are

correlated to CST-1063 measurements expressed on the scale CST-1090 volts (Figure 9) using the strong relation

established in the section 2.

The corresponding least squares regression uses N=57 comparisons from Knorr station 21a (cast 015), yielding a strong

correlation (r2 = 0.98) and an RMS error of εrms, volts = 18 mV or εrms, beam-cp = 0.017 m-1 (Figure 10). The regression is

based only on data at or below 10 m.

3.6 3.7 3.8 3.9 4 4.1 4.24

4.1

4.2

4.3

4.4

4.5

4.6

Floa

t 48

(in C

ST-

1090

vol

ts)

Knorr CST-284 (volts)

Beam Transmission: Float 48 (VoltsK) vs. Knorr CST-284 Volts

at calibration stations

y = 0.9295712*x + 0.696661, r2 = 0.98

015

Figure 9. Regression of CST-284 converted to CST-1090 volts vs. Float 48 (Knorr Station 21a / 015 only.)


a)

b)-0.1 -0.05 0 0.05 0.1

-250

-200

-150

-100

-50

0Sta.10a, CTD 19303009

Residual (m-1)

Pre

ssur

e (d

bar)

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.21a, CTD 19303015

Residual (m-1)

Pre

ssur

e (d

bar)

Figure 10. a) Beam cp (CST-1090 volts) for CST-284 and corrected Float 48 (Knorr) b) Residuals for corrected Float 48- CST-284.

Station 10a was not included in the regression due to large residuals between 50-150 m.


Figure 11. Transmittance (CST-1090 volts) for CST-284 and corrected Float 48 (Knorr) versus density. Station 10a values shows a

substantial difference from Float 48 between σθ 27.36 and 27.38.


-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.10a, CTD 19303009

Residual (VoltsK)

Pre

ssur

e (d

bar)

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.21a, CTD 19303015

Residual (VoltsK)

Pre

ssur

e (d

bar)

Figure 12. a) Transmittance (CST-1090 volts) for CST-284 and corrected Float 48 (Knorr) versus depth. b) Residuals for corrected

Float 48- CST-284.


3.2 Calibrate CST-284 on Bjarni S. Deployment Cruise using Float 48 Calibration Casts

CST-284 used also used on all Bjarni Saemundsson deployment cruise stations (Table 2). Two calibration casts with

Floats 47 and 48 (B157 and B159) were performed during this time.

CST-284 is again indirectly tied to the primary transmissometer (CST-1090). Specifically, CST-284 measurements are

correlated to Float 48 (CST-1063) measurements expressed on the scale CST-1090 volts (Figure 9) using the strong

relation established in section 2.

The corresponding least squares regression uses N=17 comparisons from Knorr station 21a (cast 015), yielding a

correlation r2 = 0.82 and an RMS error of εrms, volts = 4 mV or εrms, beam-cp = 0.004 m-1 (Figure 10). The error is smaller than

other correlations due to a deep mixed layer and little dynamic range in the transmittance measurements. The regression is

based only on data at or below 10 m.

4.11 4.12 4.13 4.14 4.15 4.164.45

4.455

4.46

4.465

4.47

4.475

4.48

4.485

4.49

4.495

4.5

Floa

t 48

(Vol

ts k)

Knorr CST-284 (Volts)

Beam Transmission: Float 48 (Volts_k} vs. Knorr CST-284 Voltsat Bjarni S. calibration stations

y = 1.0037295*x + 0.323430, r2 = 0.82

157159

Figure 13. Regression of CST-284 converted to CST-1090 volts vs. Float 48 (Bjarni S. Stations)


a)

b)-0.1 -0.05 0 0.05 0.1

-250

-200

-150

-100

-50

0Sta.157, CTD 19303157

Residual (m-1)

Pre

ssur

e (d

bar)

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.159, CTD 19303159

Residual (m-1) Figure 14. a) Beam cp (CST-1090 volts) for CST-284 and corrected Float 48 (Bjarni S.) b) Residuals for corrected Float 48- CST-

284.


4.45 4.5 4.55-27.32

-27.3195

-27.319

-27.3185

-27.318

-27.3175

-27.317

-27.3165

-27.316

-27.3155

-27.315Sta.157, CTD 19303157

Transmittance (VoltsK)

Den

sity

( σth

eta)

CST-284CST-284 binnedCorrected Float 48

4.45 4.5 4.55-27.32

-27.3195

-27.319

-27.3185

-27.318

-27.3175

-27.317

-27.3165

-27.316

-27.3155

-27.315Sta.159, CTD 19303159



Figure 15. Transmittance (CST-1090 volts) for CST-284 and corrected Float 48 (Bjarni S.) versus density


4.45 4.5 4.55-250

-200

-150

-100

-50

0Sta.157, CTD 19303157


Pre

ssur

e (d

bar)


4.45 4.5 4.55-250

-200

-150

-100

-50

0Sta.159, CTD 19303159


Pre

ssur

e (d

bar)


-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.157, CTD 19303157

Residual (VoltsK)

Pre

ssur

e (d

bar)

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.159, CTD 19303159

Residual (VoltsK)

Pre

ssur

e (d

bar)

Figure 16. a) Transmittance (CST-1090 volts) for CST-284 and corrected Float 48 (Bjarni S.) versus depth b) Residuals

for corrected Float 48- CST-284.


3.3 Calibrate CST-282 on Knorr Process Cruise using Float 48 Calibration Casts

CST-282 used at a single Knorr Process Cruise stations 33a (cast 029, see Table 3). Only the data for the down cast will

be examined, as this transmissometer failed on or before the up cast at this station. Station 33a was not a float calibration

station, therefore we cannot perform the indirect approach of intercalibration via float calibration casts used in sections 3.1

and 3.2.

Instead, we examine the Knorr casts before and after cast 029 for similar water properties. In particular, the following

cast, 030, was performed at nominally the same station (33d) as cast 029 (station 33a). The distance between the two

casts was 0.86 Km. Cast 030 uses transmissometer CST-284, which was intercalibrated with the primary transmissometer

in Section 3.1 above. Figure 17 shows that cast 030 (green) has similar density, chlorophyll fluorescence and

backscattering profiles to cast 029 (blue) and implies that we measured water with similar properties on both casts 029

and 030. Comparison of transmissometer voltages between casts 029 and 030 show similar decreased transmittance

between 50 – 125 m corresponding to maxima in chlorophyll fluorescence (Figure 17, highlighted in blue and green

circles for casts 029 and 030, respectively). Optical backscattering at 700 nm for both casts show similar profiles, but do

not show distinct corresponding features between 50-125. The density profiles show a downward shift in the pycnocline

in this same vertical region; interpolating transmittance in density space should correct for this (see Appendix A, Float 48

Transmissometer Conversion Algorithm, for details.)

Proceeding, we indirectly tie CST-282 to the primary transmissometer (CST-1090). Specifically, CST-282 measurements

are correlated to CST-284 measurements expressed on the scale CST-1090 volts (Figure 18). The corresponding least

squares regression uses N=550 comparisons between CST-282 at station 33a (cast 029) and station 33d (cast 030),

yielding a correlation r2 = 0.83 and an RMS error of εrms, volts = 26 mV or εrms, beam-cp = 0.027 m-1 (Figure 10).

Plotting the intercalibrated voltages versus density (Figure 20) and depth (Figure 21) shows good correspondence in two

ranges (50-150 m, 300-375 m), with the upper range corresponding to the previously mentioned chlorophyll fluorescence

maximum. Data from CST-282 below 400 m may be suspect as transmittance shows no change with depth, indicating

possible failure of CST-282 at 400 m.

While POC samples were taken from the Knorr rosette at 3, 10, 20, 50, and 300 m at station 33a (cast 029), limited

intercalibration data is available. Therefore this cast 029 was not the part of the BCO-DMO dataset and is not used for the

final derivation of POC vs. cp relationship.


0.05 0.1 0.15 0.2 0.25-600

-500

-400

-300

-200

-100

0

Chlorophyll Fluorescence (Volts)0.1 0.15 0.2 0.25 0.3

-600

-500

-400

-300

-200

-100

0

Backscattering (Volts)27.3 27.4 27.5 27.6

-600

-500

-400

-300

-200

-100

0

Density (σθ)

Pre

ssur

e (d

bar)

028029030

Figure 17. Comparison of R/V Knorr casts near Station 33a (cast 029)

4.15 4.2 4.25 4.3 4.35 4.4 4.453.7

3.75

3.8

3.85

3.9

3.95

4

CST-282 (volts)

CS

T-28

4 (C

ST-

1090

vol

ts)

Beam Transmission

y = 0.9148739*x + -0.075001, r2 = 0.83

030

Figure 18. Regression of CST-284 converted to CST-1090 volts (cast 30) vs. CST-282 volts (cast 29)

3.6 3.8 4 4.2 4.4 4.6-600

-500

-400

-300

-200

-100

0

Transmittance (volts)

CST-284 CST-282


Figure 19. Transmittance (CST-1090 volts) for CST-284 (cast 030) and corrected CST-282 (cast 029) versus density.


a) b) -0.1 -0.05 0 0.05 0.1

-600

-500

-400

-300

-200

-100

0Sta.33d, CTD 19303029

Residual (m-1)

Pre

ssur

e (d

bar)

Figure 20. a) Beam cp (m-1) for CST-284 (cast 30) and corrected CST-282 (cast 29) b) Residuals for corrected CST-282 - CST-284

a) b) -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08

-600

-500

-400

-300

-200

-100

0Sta.33d, CTD 19303029

Residual (CST-1090 volts)

Pre

ssur

e (d

bar)

Figure 21. a) Transmittance (CST-1090 Volts) for CST-284 (cast 30) and corrected CST-282 (cast 29) b) Residuals for corrected

CST-282 - CST-284


4. Rectify Float 47 transmissometer measurement to float 48 response.

Float 47 (CST-1062) was deployed at nearly the same time and location as Float 48 (CST-1063) on 4-Apr-2008. Two

“class B” calibration casts are performed after float deployment at a point between the two floats. Float 47 entered

recovery mode 15 days after deployment, retrieved and brought onboard the R/V Knorr during the process cruise. After

cleanup and checkout, Float 47 was redeployed near Float 48 on 16-Apr-2008, and calibration cast performed. Float 47

was retrieved again on 18-Apr-2008 due to a depleted battery pack. (See Figure 1.)

The calibration casts offer the opportunity to calibrate Float 47’s transmissometer against Float 48’s transmissometer, and

then apply the conversion of section 2 to express both float measurements in terms of the primary transmissometer (CST-

1090) voltage and calibration factors. Other indirect calibration routes are possible, as well as direct calibration against

the primary transmissometer on the 16-April-2008 calibration cast.

First, we compare CST-1062 (Float 47) and CST-1063 (Float 48) output counts at the Bjarni and Knorr calibration

stations. It appears that there is shift in deep water values (upper right of Figure 22). Thus, a single intercalibration

between the two float transmissometers is not possible. Instead, we shall compute a separate regression for each cruise,

and apply the first regression to the first (Bjarni) deployment and the second regression to the second (Knorr)

redeployment.

3400 3450 3500 3550 3600 3650 3700 3750 3800 3850 39003700

3750

3800

3850

3900

3950

4000

Float 47 (CST-1062) Transmissomter (Counts)

Floa

t 48

(CS

T-10

63)

Tran

smis

som

ter (

Cou

nts)

Bjarni stations 157, 159Knorr Station 101b / 19303091

Figure 22. Float 48 vs. Float 47 Transmissometer measurements at Bjarni and Knorr

float calibration stations along with least-squares regression lines.


Note that the regressions discussed in sections 4.1 and 4.2 below (Float 47 counts Float 48 counts) are combined with

the regression of section 2 (Float 48 counts CST-1090 volts) to produce the final regression coefficients shown in

Table 1,

4.1 Calibrate Float 47 (CST-1062) at Bjarni Calibration Stations

The corresponding least squares regression uses N=44 comparisons between Float 47 (CST-1062) and Float 48 (CST-

1063) at Bjarni deployment cruise stations B157 and B158 yield a weak correlation r2 = 0.31 ~2 km separation of the

floats (Figure 23). The water is well mixed here and there is little dynamic range in the measurements. For this

regression, there is a RMS error of εrms, volts = 40 mV or εrms, beam-cp = 0.04 m-1 relative to the Bjarni transmissometer (CST-

284) measurements, corrected to the primary transmissometer (CST-1090) voltage scale.

Plotting the intercalibrated voltages versus density (Figure 25) and depth (Figure 26) shows good correspondence

throughout the water column. The residuals between float measurements are similar to those between Float 47 and the

calibration stations. Thus, most of the error corresponds to different optical properties of the water column measured by

Float 47 and that measured by both Float 47 and Bjarni transmissometers.

3700 3750 3800 3850 39003850

3900

3950

4000

Float 47 (counts)

Floa

t 48

(cou

nts)

Beam Transmission: Float 48 vs. Float 47 (counts)at deployment calibration stations

y = 0.3874251*x + 2435.594534, r2 = 0.31

157159

Figure 23. Regression of Float 48 vs. Float 47 Counts at Bjarni calibration stations


a)

b)

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.157, CTD 19303157

Residual (m-1)

Pre

ssur

e (d

bar)

F48 - F47Bjarni - F47

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.159, CTD 19303159

Residual (m-1)

Pre

ssur

e (d

bar)

Figure 24. a) Beam cp (CST-1090 volts) for CST-284 and corrected Floats 47 and 48 at Bjarni calibration stations. b) Residuals for

corrected Float 48- corrected Float 47 and corrected Float 48- CST-284.


Figure 25. Transmittance (CST-1090 volts) for Float 47, Float 48 and Bjarni (CST-284) versus density.


a)

b)-0.1 -0.05 0 0.05 0.1

-250

-200

-150

-100

-50

0Sta.157, CTD 19303157

Residual (VoltsK)

Pre

ssur

e (d

bar)

F48 - F47Bjarni - F47

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.159, CTD 19303159

Residual (VoltsK)

Pre

ssur

e (d

bar)

Figure 26. a) Transmissometer voltage for CST-1090 and corrected Floats 47 and 48 at Bjarni calibration stations. b) Residuals for



4.2 Calibrate Float 47 (CST-1062) at Knorr Calibration Station

The corresponding least squares regression uses N=72 comparisons between Float 47 (CST-1062) and Float 48 (CST-

1063) at Knorr process cruise station 101b (cast 091) yields a significant correlation r2 = 0.87 and an RMS error of εrms, volts

= 19 mV or εrms, beam-cp = 0.017 m-1 (Figure 27).

Plotting the intercalibrated voltages versus density (Figure 28) and depth (Figure 30) shows good correspondence below

75 meters. Both Float 48 and Float 48 values between 25-75 m correspond more closely with the Knorr up cast,

indicating the possibility of internal wave activity.

Figure 27. Regression of Float 48 vs. Float 47

Counts at Knorr calibration station

Figure 28. Transmittance (CST-1090 volts) for Float 47, Float 48 and Knorr

(CST-1090) versus density.

3100 3200 3300 3400 3500 3600 3700 3800 39003600

3650

3700

3750

3800

3850

3900

3950

4000

Float 47 (counts)

Floa

t 48

(cou

nts)

Beam Transmission: Float 48 vs. Float 47 (counts)at calibration stations

y = 0.4289032*x + 2312.600896, r2 = 0.87

091

3.8 4 4.2 4.4 4.6-27.55

-27.5

-27.45

-27.4

-27.35

-27.3

-27.25

-27.2Sta.101b, CTD 19303091


Den

sity

(σth

eta)

CST-1090 downCST-1090 upCST-1090 binnedCorrected F47Corrected F48


a) b)

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.101b, CTD 19303091

Residual (m-1)P

ress

ure

(dba

r)

F48 - F47Knorr - F47

Figure 29. a) Beam cp for CST-1090 and corrected Floats 47 and 48 at Knorr calibration station. b) Residuals for corrected Float 48-

corrected Float 47 and corrected Float 48- CST-1090.

a) b)

-0.1 -0.05 0 0.05 0.1-250

-200

-150

-100

-50

0Sta.101b, CTD 19303091

Residual (VoltsK)

Pre

ssur

e (d

bar)

F48 - F47Knorr - F47

Figure 30. a) Transmissometer voltage for CST-1090 and corrected Floats 47 and 48 at Knorr calibration station. b) Residuals for



Appendix A. Float 48 Transmissometer Conversion Algorithm

1. Identify Knorr and Float calibration cast data for 10-250 m, down cast only.

2. Subtract post-cruise dark offset Vdark = 0.063 V from Knorr CST-1090 transmissometer measurements:

darkmeasK VVV −=

3. Subtract post-cruise dark offset Countsdark = 65 from Float 48 CST-1063 transmissometer measurements.

darkmeasf CountsCountsCounts −=

4. Bin-average Knorr CTD density Kθσ and transmissometer voltage KV to identical 1 m increments, finer than the

(average) 3.5 m float 48 profiling vertical sample spacing. This allows us to express Knorr transmissometer

measurements in density coordinates iKK V ),( θσ . It also implicitly low-pass filters the data, as we want an

average beam transmission without spikes to drive the subsequent interpolation (Step 5) and linear regression

(Step 6). (It is unlikely that float and ship transmissometers will see the same spikes.)

5. Interpolate Knorr rosette transmissometer voltage transmissometer voltage to find voltages KV at Float 48 density

values fθσ , creating jKf V ),( ′θσ , where j signifies the jth calibration cast. Now we have Knorr transmissometer

voltages and Float 48 transmissometer counts at the same density values, thus correcting for moving isopycnals

due to internal waves.

6. Estimate slope m and intercept b via linear regression of interpolated ship rosette transmissometer values KV ′ vs.

Float 48 transmissometer fCounts using data from all calibration casts:

ε++=′ bCountsmV fK *

where ε is the residual error in the linear regression. (Figure 3 and Figure 6.)

7. For each float calibration down cast, apply regression to convert Float 48 counts to Knorr-based transmissometer

volts. (Figure 4, Figure 5, and Figure 6.)

bCountsmV ff += *ˆ

The final equation to use to convert Float 48 transmissometer counts to Knorr-compatible transmissometer volts

is: 0.034426*0.0011355ˆ += ff CountsV


8. Convert Knorr and Float transmissometer voltages to transmittance and beam-c values using post-cruise L = 0.25

m, Vdark = 0.063 V and Vref = 4.652 V: (Figure 7, Figure 8)

%)662(

−=

darkref

KK VV

Vtr , ( ) 1mln1)662( −−= KK trL

c

%ˆ

)662(ˆ

−=

darkref

ff VV

Vrt , ( ) 1mln1)662(ˆ −−= ff tr

Lc

9. Compute beam attenuation due to particles. Total beam attenuation ct(λ) can be written as the sum of beam

attenuation due to particules, attenuation due to CDOM and attenuation due water:

)(c )(c + )(c = )(c wcdompt λλλλ +

The beam attenuation computation in step 8 computes beam attenuation relative to water:

)(c + )(c = )(c- )(c )c( cdompwt λλλλλ =

If aborpstion due to chromophoric dissolved organic matter acdom(λ) is very small relative to absorption plus

scattering due to particles, i.e.,[acdom(λ) = ccdom(λ)] << [cp(λ) = ap(λ)+ bp(λ)], then

)(c)c( p λλ =

and the computation of step 8 is a valid estimate of beam cp(652). Several spectrophotometric measurements of

acdom(λ) were carried out during the R/V Knorr process cruise to verify that ccdom(λ) << cp(λ).

Appendix B. Schematic of Intercalibration States


Transmittance %

cp(652) m-1

Correlate to POC measuremens

POCvs

cp(652)

CST-284Voltage(Knorr)

CST-1062Counts1062Float 47

CST-1063Counts1063Float 48

CST-1090voltage(Knorr,Bjarni)

Apply CST-1090 pure water voltage Vref

CST-284Voltage(Bjarni)

CST-1062Estimated Counts1063

CST-1090VK =

V1090 – Vdark

CST-1063Est. VK

LinearRegression

ReferenceInstrument

3300 3400 3500 3600 3700 3800 3900 40003.8

3.9

4

4.1

4.2

4.3

4.4

4.5

4.6

4.7

Float 48 (counts)

Kno

rr C

ST-

1090

(Vol

ts10

90)

Beam Transmission: Knorr CST-1090 volts vs Float 48 Countsat calibration stations

y = 0.0011355*x + 0.034426, r2 = 0.99

036045047064088091106122

3.6 3.7 3.8 3.9 4 4.1 4.24

4.1

4.2

4.3

4.4

4.5

4.6

Floa

t 48

(in C

ST-

1090

vol

ts)


Beam Transmission: Float 48 Volts1090 vs. Knorr CST-284 Voltsat calibration stations

y = 0.9295712*x + 0.696661, r2 = 0.98

015

4.11 4.12 4.13 4.14 4.15 4.16

4.445

4.45

4.455

4.46

4.465

4.47

4.475

4.48

4.485

4.49

4.495

Floa

t 48

(in C

ST-

1090

vol

ts)


Beam Transmission: Float 48 Volts1090 vs. Knorr CST-284 Voltsat calibration stations

y = 1.0037295*x + 0.323430, r2 = 0.82

157159

Convert V284 to estimated Vk via CST-1063

Convert Counts1063 t o estimated Vk Convert Counts1063 t o Counts1063Convert V284 to estimated Vk via CST-1063

38

Appendix C. Principle of Operation A beam transmissometer measures the attenuation of light due to dissolved and particulate substances in the water, as well

as water itself. The instrument consists of a collimated light beam, with a source in one housing and a detector facing the

source.

Light that is absorbed in the path or scattered out of the path does not reach the detector. An aperture at the focal point

removes off-axis scattered light, and the transmitted light falls on the detector. A real transmissometer does not have a

perfectly collimated source or detector and has a finite field of view or “acceptance angle”. Thus, some forward scattered

light arrives at the detector causing an overestimate of transmitted light and a corresponding underestimate of the beam

attenuation coefficient. Current practice (Pegau et al., 2002) does not apply a scattering correction; rather the beam

attenuation coefficient is simply reported as measured along with the acceptance angle of the instrument. We used the

same instrument model (WET Labs C-Star, plastic housing) for all measurements, and it has an acceptance angle of 1.2°.

The transmissometer output value increases linearly with increasing transmittance over the instrument’s measurement

range. The C-Star output signal V is proportional to the amount of light received by the detector over a given pathlength L

= 0.25 m. With the instrument in water, the output V should vary from a minimum value equaling the dark value (Vdark ,

obtained by a blocked beam reading) to a maximum signal equal to the pure water reference (Vref),, measured in controlled

laboratory conditions). The ratio of the signal output to the reference output is known as transmittance tr and will vary

from 0 to 1 or 0 to 100 percent.

−−

=darkref

dark

VVVVtr

Transmittance is related to beam attenuation c by the Beer-Lambert-Bouguer Law: cLetr −=

Thus, beam attenuation (beam c) can be calculated from transmittance as:

( ) 1mln1 −−= trL

c

In Appendix A we show that if asborption due to colored dissolved organic material aCDOM is small, then beam attenuation

due to particles (beam cp) is:

c c p =

39

References Boss, E., W. S. Pegau, W. D. Gardner, J. R. V. Zaneveld, A. H. Barnard, M. S. Twardowski, G. C. Chang, and T. D.

Dickey, 2001. The spectral particulate attenuation and particle size distribution in the bottom boundary layer of a

continental shelf, J. Geophys. Res. 106, 9509–9516.

Boss, E., M. S. Twardowski and S. Herring 2001. Shape of the particulate beam attenuation spectrum

and its inversion to obtain the shape of the particulate size distribution, Applied Optics 40 (27). 4885-4893.

Gardner, W.D., I.D. Walsh, M.J. Richardson 1993. Biophysical forcing of particle production and distribution during a

spring bloom in the Atlantic, Deep-Sea Research II, Vol. 40, No 1/2, pp. 171-195.

Gardner W.D., M.J. Richardson, C.A. Carlson, D.A. Hansell, A.V.Mishonov, 2003. Determining True Particulate Organic

Carbon: Bottles, Pumps and Methodologies. Deep-Sea Research II, 50(3-4): 655-674.

Gardner, W.D. , Mishonov, A.V. , Richardson, M.J., 2006. Global POC concentrations from in-situ and satellite data,

Deep-Sea Research II, 50 (5-7), 718-740

Pegau, S., J. R. V. Zaneveld and J. L. Mueller. (2002). Beam Transmission and Attenuation Coefficients:Instruments,

Characterization, Field Measurements and Data Analysis Protocols, in Mueller, J.L, G. S. Fargion and C. R. McClain,

(eds.). Ocean Optics Protocols for Satellite Ocean Color Sensor Validation, Revision 4, Volume IV, National

Aeronautical and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland, 76 p.

Voss, K. J., 1992. A spectral model of the beam attenuation coefficient in the ocean and coastal areas, Limnol. Oceanogr.

37,501–509.

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