Inherent Optical Properties (IOP)
Sensor Intercomparison and Merger for Biological
and Interdisciplinary Oceanic Studies
This page contains notes from the IOP subgroup
at the August 1997 SIMBIOS conference at
Solomons Island, Maryland.
Contents:
Filter issues:
Questions raised concerning a possible species
specify absorption. Others asserted it is
rather a size distribution (synechococcus
below 1 micron may less than the wavelength
of light)
- On filter issue: Errors may be spectrally
dependent -or- different cells may act differently
on different filters -or- both may occur.
- GF/C filters may have a smaller porosity
than GF/F.
- G. Mitchell: "Increased detritus may
lead to increased absorption at 750 (Italians
reported)
- Report on TR method: measure transmission
and reflectance, solve for correction of
absorption from filter scattering (may not
be needed in case I waters)
- Report on FTF (freeze transfer technique)
of Bricaud et al., appears to agree well
w/ integrated sphere data, difficult procedure
CDOM issues:
The index of refraction is seen to exhibit
a larger value between fresh and saltwater
at larger wavelengths, this leads to need
to 'zero' scan data by the null wavelength
- Group suggests 850 nm as the null wavelength
for CDOM scans (and a 350-850 scanning range)
but not all instruments may go to 850
- Group suggest reporting the raw optical density
(OD) scans along with needed processing info
- Glenn Cota suggested the use of a power law
curve to pre-smooth low absorbing CDOM (as)
scans, re-attachment at a UV wavelength (325nm),
then null wavelength correction
- Heidi Sosik has performed extensive tests
to examine the effects of long term storage
of samples. Report status unknown.
Beta and bb:
AC-9 Issues:
- Protocols to follow at Wetlabs: www.wetlabs.com,
under news or instruments
- Use Pope & Fry pure water std.
- To submit, first apply the temp and sal correction
to both 'a' and 'c' and provide raw and fully
processed data
- Submitters should document scattering correction
method used. Three primary types:
- subtract null wavelength (at 715nm)
- assume Epsilon(Cb(lambda) - aTS(lambda))
<-- useful if not good temp & sal
correction available
- Epsilon = a(715)/c(715)-a(715)
must be: 0.08 < Epsilon < 0.5 should
be: 0.1 < Epsilon < .3 - small errors
in 'c' & 'a' strongly effect this value
** If scan has 670 > 650, assume phytoplankton
main component, if slowly decreasing w/ wavelength,
assume sediment
- Definitions:ar - absorption raw cr - beam
attenuation raw Submit: ar, cr, temp, sal
- Calibrate instrument on its frame due to
torquing problem w/ AC-9 (new meters have
better design and don't suffer from this
problem)
- Use clean DI water under pressurized flow
w/ a valve for back pressure when calibrating
- Filters on AC-9 for 'as' determination may
need null wavelength correction applied
- Scott Pegau uses tygon tubing (saw no difference
w/ teflon coated, except possibly in bluest
waters)
- Outstanding IOP issues:
- aph & ad techniques vary by investigator
- length of storage of samples may or may not
be a problem
San Diego Absorption Workshop
Information on the absorption workshop held
at Scripps during December 9-13, 1996 can
found at http://spg.ucsd.edu/~lab/absorption.html
- All San Diego absorption workshop instruments
compared well except the HP spectrophotometer.
- Workshop resulted in some convergence in Beta and null wavelength, more work needed
- Dave Karl suggested GF/F absorbs CDOM (per
G. Mitchell)
- "For SIMBIOS purposes, people should
specify their measurement methods" -G.
Mitchell