Optical backscattering in water: measurements and uncertainties

1. Optical backscattering in water: measurements and uncertainties Jim Sullivan & Mike Twardowski WET Labs, Inc. Contact: jims@wetlabs.com

2. Why do we measure bb? Ocean color remote sensing reflectance ~bb /a Radiative transfer modeling VSF phase functions, bb /b Particle characterization composition (np), abundance How do we measure bb?

3. Volume Scattering Function (VSF): β(θ) dI(θ): radiant flux in direction θ dV: volume E: incident irradiance π (180°) b = 2π∫ sin(θ ) β(θ) d θ 0 scattering coefficient (b):

4. Volume Scattering Function (VSF): β(θ) dI(θ): radiant flux in direction θ dV: volume E: incident irradiance π (180°) bb= 2π∫ sin(θ ) β(θ) d θ π/2 (90°) backscattering coefficient (bb):

5. How do we measure bb? MASCOT Broad angular range VSF devices: MVSM Petzold’s GASM

6. If we measure the full VSF: π (180°) bb= 2π∫ sin(θ ) β(θ) d θ π/2 (90°) However, if we assume a “constant” VSF shape or phase function (e.g. Oishi 1990): bb = 2πχ(θ) β(θ) single angle VSF measurement

7. q Single angle backscattering devices: Hydroscat ECO BB broad angular response

8. How accurate are single angle bb sensors? There is no calibration standard but, typical (good) comparisons ~ 10% How can we reduce bb measurement uncertainties? Sources: 1. χ factors, i.e. assuming VSF shape

9. How constant are VSF phase functions? (Sullivan & Twardowski 2009)

10. How constant are VSF phase functions? (Sullivan & Twardowski 2009) ~ 2 to 5% variability assumes no spectral dependence

11. How accurate are single angle bb sensors? Typical (good) comparisons ~ 10% How can we reduce bb measurement uncertainties? Sources: 1. χ factors ( few % ) 2. sensor electronic noise ( typically < 1 % ) 3. calibration and drift ( 10% or more )

12. q Calibration: angular weighting function w(θ)

13. Micro-spherical bead calibration overview: Compute β(θ)/bfor bead solution and convolve with W (θ): β(θ)/b Measure sensor counts and b in a bead solution concentration series and obtain slope, b /counts. Compute scaling factor, SF: See Sullivan et al., 2012 for complete review

14. Spectral -angular VSF shape of NIST beads 2.0 µm beads 0.2 µm beads 0.1 µm beads

15. Calibration drift: 4% per year drift (red LEDs) ~10% yr. blue ~ 3% yr. green ~4% yr. red Sullivan et al., 2012

16. How can we reduce bb measurement uncertainties? regular calibration (1 - 2 times/yr.) use 0.1 µm NIST beads for calibrations regularly confirm background noise < 5% uncertainty achievable

17. SCIENCE! Is there spectral dependency in bb/b ? NO! Mie theory, lab and field work reporting < 10% variability (Berthon, Boss, Whitmire, etc.) YES! Field work of McKee, Chami in plankton and mineral rich waters

18. Recent bb/b measurements using our best methods:

19. bb example vertical profile of chl b

20. 0.5 m bins of spectral bb/b from vertical profile bb / b

21. average of bb/b for all East Sound vertical profiles bb / b

22. Recent bb/b measurements using our best methods:

23. bb example vertical profile of chl b

24. bb / b

25. average of bb/b for all NE Atlantic vertical profiles

26. Comparison of spectral bb/b: NE Atlantic East Sound, WA

27. Questions? Recent article of interest: Sullivan, J. M., M. S. Twardowski, J. R. V. Zaneveld & C. Moore (2012) Measuring optical backscattering in water. Light Scattering Reviews 7: Radiative Transfer and Optical Properties of Atmosphere and Underlying Surface, A. A. Kokhanovsky [Ed.], S Praxis Books, pp. 189-224. See me for a PDF copy.