The MODIS Ocean Product for Particulate Inorganic Carbon

The MODIS Ocean Product for Particulate Inorganic Carbon (MOD
25): Refinement of calcium carbonate estimates in the global ocean
William M. Balch, Bigelow Laboratory for Ocean Science, W. Boothbay Harbor, ME, 04575;([email protected])
3-band PIC algorithm

Abstract
This research focuses on three areas of refinement of the MODIS particulate inorganic carbon algorithm (MOD 25): 1)
validation of particulate inorganic carbon (PIC; otherwise known as calcium carbonate) concentrations from a wide variety of
environments, 2) elucidation of the magnitude of error in MODIS-derived PIC estimates that are caused by suspended
biogenic silica (associated principally with diatom frustules), and 3) comparison of the current MODIS two-band PIC
algorithm with a three-band PIC algorithm. An objective comparison of the two and three-band algorithms, using data from a
wide range of geographic locations, is critical for interpreting PIC algorithm results from different satellite platforms.
Data for this work will be principally derived from three Atlantic Meridional Transect (AMT) cruises which traverse between
the UK and Falkland Islands. Each AMT cruise covers a wide variety of environments, providing the most globallyrepresentative validation for the MODIS PIC algorithm. Moreover, the AMT cruises cross sub-polar mesoscale regions of
elevated PIC (already observed by MODIS-Terra and Aqua). If validated, the PIC in these regions would be of major
significance to the global carbonate budget. Given the possibility of suspended biogenic silica in these sub-polar regions (as
well as many other areas), it is important to confirm that these sub-polar regions of elevated PIC are not simply artifacts due
to intense scattering associated with biogenic silica in diatom blooms. We also have cruise efforts planned in the Gulf of
Maine and Equatorial Pacific. We would use these opportunities for further collection of PIC and biogenic silica data for
refinement of the PIC algorithm. The results of this work will significantly advance global-scale validation of the PIC
product (MOD 25) and provide a suite of potential improvements for more accurate retrievals.

Objectives

Project Status (contract start 22 April 04):

(Gordon et al. , 2001)
Interference from chlorophyll and cDOM
minimized in red and near-IR.
Absorption coefficient of water is so high in red and
near IR that added phytoplankton and cDOM
absorption is negligible.
Algorithm is based on 670, 765, and 865nm bands)
nLw much lower in red and near-IR, thus these
bands are less likely to saturate.
Must assume background bb term (i.e. all other
bb), which is subtracted from bb tot to estimate PIC bb.

1) Validate MODIS-derived PIC estimates during three Atlantic
Meridional Transect (AMT) cruises.
2) Parallel sampling for biogenic silica during AMT cruises to examine
potential errors due to this mineral.
3) Validation work associated with Gulf of Maine cruises (involving both
PIC and biogenic silica measurements and concurrent ship and satellite
measurements of normalized water-leaving radiance).
4) Partition variance in backscattering between POC, PIC and biogenic silica
5) Perform objective comparison to test the accuracy of 2-band and 3-band
PIC algorithms.

Two Band PIC algorithm
(Balch et al., 2004, JGR submitted)
This is a backscattering-based method that uses water-leaving
radiance in the blue and green plus the scattering cross-section of
calcite coccoliths and chlorophyll-containing cells to iteratively
solve for calcite and chlorophyll concentration.
Fig. 1- Look-up table for
two-band calcite algorithm
where inputs of waterleaving radiance provide
estimated chlorophyll (in
g pigment per liter) and
coccolith/calcite
concentrations (in units of
x106 coccoliths per liter or
g PIC per liter.

Fig. 2 MODIS-Terra image (version 4.2.2) of calcite concentration based on 2
band algorithm during April 2001. AMT cruise track shown for reference. Color
bar for PIC concentration shown at lower right. Note, high concentrations in
high latitude regions and in African upwelling regions.
Computerdriven
stepping
moter

Compass,
pitch/tilt
sensors

Fig. 2 A) Testing of bow mounted SAS radiometer system
aboard R/V Cape Hatteras. B) Close-up of new bow-mounted
radiometer system for AMT trips. This system is automatically
sun-steered using a computer-driven stepping motor coupled to
solar azimuth prediction software. The system contains a builtin compass, pitch and roll sensors which will allow post-cruise
data filtering for optimum nadir and azimuth angles (which
will allow better bi-directional reflectance correction). C)
Underway system being built for AMT #15 continuous
measurements of total backscattering, acid-labile
backscattering, attenuation, absorption, chlorophyll
fluorescence, temperature and salinity (ac-9 not shown). All
measurements will be made for alternating, total and 0.2mfiltered, in order to separate the role of particulate and
dissolved fractions to the inherent optical properties.

-Discrete underway samples for PIC, biogenic silica,
POC and chlorophyll collected from Spring 04 AMT
cruise #14 (Falklands to U.K.; samples collected every
2h). Samples being processed now.
-Bow mount built for Satlantic MicroSAS sensors.
Hardware and software have been completed which
will adjust sensor viewing angle for solar azimuth
viewing angle, and also incorporate ship pitch and roll
into the data stream.
-Underway system for AMT cruise #15 built which
will make semi-continuous measurements of
temperature and salinity, bb(total and acid-labile),
attenuation, absorption, and chlorophyll fluorescence
(all with alternating total and 0.2m-filtered water)
-Attended AMT #14 post cruise meeting and AMT#15
pre-cruise meeting in Southampton, U.K..
-Collected PIC and biogenic silica data in the Gulf of
Maine for validation activities(4 cruises so far in 2004)
-Preparing for Equatorial Pacific trip in DecemberJanuary in which more validation data will be
collected for this MODIS project.

Future activities:
- AMT cruise #15 aboard RRS Discovery (6.5 week)
will begin mid-September in Southampton, U.K.
-Eight more cruises planned in the Gulf of Maine for
PIC algorithm validation activities
-One month Equatorial Pacific cruise (December04January 05) in which PIC and coccolith count data
will be collected.
-AMT cruises #16 and 17.
References
Gordon, H.R., Boynton, G.C., Balch, W.M., Groom, S.B., Harbour, D.S., and Smyth, T.J.,
2001, Retrieval of coccolithophore calcite concentration from SeaWiFS imagery:
Geochemical Research Letters, v. 28, p. 1587-1590.
Balch, W. M., Howard Gordon, B. C. Bowler, D. T. Drapeau, E. S. Booth. 2004. Calcium
carbonate budgets in the surface global ocean based on MODIS Data. Submitted to the
Journal of Geophysical Research-Oceans.

Acknowledgements- This work has been generously supported by NASA
with ship time provided by the National Environmental
Research Council, U.K..

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