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Lab 3 Particulate Absorption

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.

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Lab 3 Particulate Absorption

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  1. Lab 3 Particulate Absorption Collin Roesler 5 July 2007

  2. Particulate absorption • Spectrophotometer • Quantitative Filter Technique • Corrections • Kishino method • Ac9 • Calibration dependent method • Calibration independent method

  3. Absorption: Filtration method • Separates particles from dissolved • Concentrates particles from dilute medium

  4. Measure in Spectrophotometer • Reference • Match optical density of filter pad • No variability • Baseline • Blank filter pad in sample compartment (what is ODblankfilter:ODsample?)

  5. A h Compute absorption a (m-1) = 2.303 OD . L (m) What is L? Vfiltered = Aeff h L = h = V (m3) A (m2)

  6. What about the scattering by the filter?Path length amplification a (m-1) = 2.303 OD . V(m3) A(m2) • Filter pad • Creates nearly isotropic light field • Increases optical path length • Increases absorption signal • How to correct for it?

  7. b correction: path length amplification • Approach • Thick cultures • Measure absorption in cuvette • Measure absorption on filter pad • Determine ratio, b = ODfilt = optical . ODcuv geometric

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

  9. What about the scattering by the filter?Path length amplification, empirical approach a (m-1) = 2.303 ODf corrected V(m3) A(m2) • Filter pad creates multiple scattering environment • The relationship between ODf and ODc is non linear of the form: • ODf corrected = C1 * ODf + C2 * ODf2 C1 = 0.29 to 0.48 C2 = 0.05 to 0.75 Mitchell 1990, Cleveland and Weidemann 1993; Moore et al. 1995; Arbones et al. 1996 among others

  10. What about the scattering by the filter?Path length amplification, theoretical approach a (m-1) = 2.303 OD . V(m3) A(m2) • The geometric path length is t the thickness of the pad • The optical path length is r , • t /r = cos q where cos q is the average cosine of the light field • If the filter pad creates an isotropic light field, cos q = 0.5 • The optical path length is 2 * geometric path length a (m-1) = 2.303 ODfilt . 2V(m3) A(m2) Roesler 1998

  11. Partitioning of particulate absorption • First scan is total particles, ap • Extract with methanol and scan again, anap • aphyt = ap – anap • Other issues • Phytoplankton “parts” • Detrital pigments • Phycobilipigments • Inorganics

  12. Kishino et al. 1985

  13. 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, ap and anap • Phytoplankton calculation, aphyt = ap - anap

  14. Unfiltered water samples Pure water calibration T/S correction Scattering correction Particulate absorption method apart = atotal – afilt Calibration independent method ac9 approach

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

  16. 2. Temperature and salinity correction Absorption from AC9 c am • 500 600 700 • Wavelength

  17. Subtract am(715 nm) • “b not a function of l” • spectrophotometric approach Absorption from AC9 am • 500 600 700 • Wavelength 3. Scattering correction Stramski and Babin 2002

  18. b’(l) = c(l) – am(l) Absorption from AC9 c am • 500 600 700 • Wavelength 3. Scattering correction b. Subtract fraction of b(l) am(715) * b’(l) . b’(715) a(l) =am(l)-

  19. Pure water calibration Temperature/Salinity correction Scattering correction Calculate scattering spectrum Absorption from AC9 c b a • 500 600 700 • Wavelength

  20. Particulate Absorption AC9 method • Measure sample absorption, at • Apply water cal, T/S, scattering corrections • Measure sample filtrate absorpion, af • Apply water cal, T/S, (scattering corrections?) • Compute particulate absorption, ap = at – af • Configurations • Single profiling instrument, multiple casts • Automated filtering on single instrument • Dual profiling instruments, sensor intercalibration

  21. Particulate Absorption AC9 methodCalibration Independent • Measure sample absorption, at • T/S, scattering corrections • Measure sample filtrate absorpion, af • T/S, (scattering corrections?) • Compute particulate absorption, ap = at – af • Configurations • Single profiling instrument, multiple casts • Automated filtering on single instrument

  22. Automated shipboard flow-through method, calibration-independent atotal a<0.2mm Slade et al., 2006

  23. Automated shipboard flow-through method 0.04 0.03 0.02 ap(440) (m-1) 0.015 0.010 0.005 0.000 Slade et al., 2006

  24. ac9 Summary • Pure water calibrations • Whole sample measurement • Filtered sample measurement • Data analysis • Apply water cals • Apply Temperature/Salinity corrections • Compute ap = at – afilt, cp = ct - cfilt • Apply scattering correction to ap

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