Lab 3 Particulate Absorption Collin Roesler 5 July 2007

Particulate absorption Spectrophotometer –Quantitative Filter Technique –Corrections –Kishino method Ac9 –Calibration dependent method –Calibration independent method

Absorption: Filtration method Separates particles from dissolved Concentrates particles from dilute medium

Measure in Spectrophotometer Reference –Match optical density of filter pad –No variability Baseline –Blank filter pad in sample compartment (what is OD blankfilter :OD sample ?)

Compute absorption a (m -1 ) = OD. L (m) What is L? V filtered = A eff h A h L = h = V (m 3 ) A (m 2 )

What about the scattering by the filter? Path length amplification Filter pad –Creates nearly isotropic light field –Increases optical path length –Increases absorption signal –How to correct for it? a (m -1 ) = OD. V(m 3 ) A(m 2 )

 correction: path length amplification Approach –Thick cultures –Measure absorption in cuvette –Measure absorption on filter pad –Determine ratio,  = OD filt = optical. OD cuv geometric

Determining  Kiefer and SooHoo ’82, Mitchell and Kiefer ’85, ‘88 For low loadings –Very variable For high loadings –Approaches 2  = ODf ODc OD filter pad Issue: scattering in cuvette? cuvette Filter pad

What about the scattering by the filter? Path length amplification, empirical approach a (m -1 ) = ODf corrected V(m 3 ) A(m 2 ) Filter pad creates multiple scattering environment The relationship between OD f and OD c is non linear of the form: OD f corrected = C 1 * OD f + C 2 * OD f 2 C 1 = 0.29 to 0.48 C 2 = 0.05 to 0.75 Mitchell 1990, Cleveland and Weidemann 1993; Moore et al. 1995; Arbones et al among others

What about the scattering by the filter? Path length amplification, theoretical approach a (m -1 ) = OD. V(m 3 ) A(m 2 ) a (m -1 ) = ODfilt. 2 V(m 3 ) A(m 2 ) The geometric path length is t the thickness of the pad The optical path length is r, t /r = cos  where cos  is the average cosine of the light field If the filter pad creates an isotropic light field, cos  = 0.5 The optical path length is 2 * geometric path length Roesler 1998

Partitioning of particulate absorption First scan is total particles, a p Extract with methanol and scan again, a nap a phyt = a p – a nap Other issues –Phytoplankton “parts” –Detrital pigments –Phycobilipigments –Inorganics

Kishino et al. 1985

Summary Filter pad technique Filter sample, want high loading to overcome the variability in the blank filter pad absorption itself, but not muddy Reference? Extraction to separate particulates, nap Computation –Offset correction, Stramski and Babin 2002 –Beta correction, try all models –Absorption calculation, a p and a nap –Phytoplankton calculation, a phyt = a p - a nap

ac9 approach Unfiltered water samples –Pure water calibration –T/S correction –Scattering correction Particulate absorption method –a part = a total – a filt –Calibration independent method

Absorption from AC9 But spectra are problematic water calibration applied 1. Pure water calibration

Absorption from AC Wavelength amam c 2. Temperature and salinity correction

Absorption from AC Wavelength amam 3. Scattering correction a.Subtract a m (715 nm) “b not a function of ” spectrophotometric approach Stramski and Babin 2002

Absorption from AC Wavelength amam 3. Scattering correction b. Subtract fraction of b( ) c a m (715) * b’( ). b’(715) b’( ) = c( ) – a m ( ) a( ) = a m ( ) -

Absorption from AC9 Pure water calibration Temperature/Salinity correction Scattering correction Calculate scattering spectrum Wavelength a c b

Particulate Absorption AC9 method Measure sample absorption, a t –Apply water cal, T/S, scattering corrections Measure sample filtrate absorpion, a f –Apply water cal, T/S, (scattering corrections?) Compute particulate absorption, a p = a t – a f Configurations –Single profiling instrument, multiple casts –Automated filtering on single instrument –Dual profiling instruments, sensor intercalibration

Particulate Absorption AC9 method Calibration Independent Measure sample absorption, a t –T/S, scattering corrections Measure sample filtrate absorpion, a f –T/S, (scattering corrections?) Compute particulate absorption, a p = a t – a f Configurations –Single profiling instrument, multiple casts –Automated filtering on single instrument

Automated shipboard flow-through method, calibration-independent Slade et al., 2006 a total a <0.2  m

Automated shipboard flow-through method Slade et al., 2006 a p (440) (m -1 )

ac9 Summary Pure water calibrations Whole sample measurement Filtered sample measurement Data analysis –Apply water cals –Apply Temperature/Salinity corrections –Compute a p = a t – a filt, c p = c t - c filt –Apply scattering correction to a p