Read notes from the Inherent Optical Properties (IOP) subgroup workshop, which took place at the first Sensor Intercomparison for Marine Biological and Interdisciplinary Ocean Studies (SIMBIOS) conference.

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

• Balch addressed de-bubbling techniques

AC-9 Issues

• Protocols to follow at Wetlabs
• 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:
  1. subtract null wavelength (at 715nm)
  2. assume Epsilon(Cb(lambda) - aTS(lambda)) <-- useful if not good temp & sal correction available
  3. Epsilon = a(715)/c(715)-a(715)

Note: must be: 0.08 < Epsilon < 0.5 should be: 0.1 < Epsilon < .3 - small errors in 'c' & 'a' strongly effect this value

Note: 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:
  1. aph & ad techniques vary by investigator
  2. length of storage of samples may or may not be a problem

San Diego Absorption Workshop

The absorption workshop was held at Scripps from Dec. 9 to 13, 1996.

• 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