Preliminary results and uncertainties of scattering measurements for SORTIE Michael Twardowski 1, Scott Freeman, Jim Sullivan, Ron Zaneveld, Chuck Trees, and the SORTIE Team 1 WET Labs, Inc., Narragansett, RI

SORTIE IOP Objectives 1. Use IOPs in radiative transfer models to help constrain uncertainties for radiometric measurements 2. Map horizontal-vertical spatial variability in optical properties around station 3. Evaluate and refine IOP measurement protocols 4. Evaluate and refine IOP uncertainties

MASCOT package MASCOT: VSF  (10:10:170 deg; 650 nm) ECOVSF: VSF  (100, 125, 150 deg; 650 nm) ECOBB3: VSF  (117 deg; 470, 532, 650 nm) AUV-B: total scattering (650 nm) AC9: a and c (9 )

Dolphin package ACS, AC9, ECO BB3, ECO BB2C, SBE49 CTD, DH4 data handler, and Notus gearfinder pinger

Spatial Variability: c pg (532) at the MOBY site

Spatial Variability: c pg (532) trace during tow Estimated error

Spatial Variability: Hyperspectral a and c during tow MOBY site

Spatial Variability: Vertical profiles of cpg532 MOBY site Mamala Bay

VSF calibration protocol Relationship between raw VSF counts (  ) and   : angle DO: dark offset SF: scaling factor (relative gain) L: pathlength (0.2 m)  : fraction of b p not reaching detector b p : particulate scattering a t : total absorption 2 unknowns for each  : SF and   (  ) = [  (  ) – DO(  )] * SF(  ) * exp [ L * (b p *  + a t ) ] pathlength attenuation term

VSF calibration protocol [  – DO(  )] = b p * [P(  ) / SF(  )] * exp [ - L * (b p *  + a t ] Since there is no  “standard” for vicarious calibration, introduce particle standard with known phase function, P(  ). Solve for b p : Inverting the above to solve for [  (  ) – DO(  )], we obtain:  (  ) = [  (  ) – DO(  )] * SF(  ) * exp [ L * (b p *  + a t ) ] b p = [  (  ) – DO(  )] * [SF(  ) / P(  ) ] * exp [ L * (b p *  + a t ) ] Now there are 3 unknowns for each  : P, SF and 

Important Point #1 For VSF measurements, dark offsets should always be measured in-situ

MASCOT VSF calibrations 12/15/06 Arizona Road Dust If we assume apg650~0, we can solve for (P/SF) and  with a nonlinear fit to the empirical data for each channel *but in practice there are relatively large error bars with this method [  – DO(  )] = b p * [P(  )/SF(  )] * exp [ - L * (b p *  + a t ]

MASCOT VSF calibrations Arizona Road Dust But we know something else :  should be approximately constant for each detector ()() All channels normalized to area Detector field-of-view unimportant  = 0.56 [  – DO(  )] = b p * [P(  )/SF(  )] * exp [ - L * (b p *  + a t ]

MASCOT VSF calibrations Now we can solve for (P/SF) for each angle Microspherical beads (P/SF) [  – DO(  )] = b p * [P(  )/SF(  )] * exp [ - L * (b p *  + a t ]

MASCOT VSF calibrations Phase function for 1.992±0.025 um beads

MASCOT VSF calibrations Weighting functions for MASCOT angles

MASCOT VSF calibrations Phase function values for MASCOT angles P(  )

MASCOT VSF calibrations theoretical P empirical P/SF = SF(  ) Now all calibration parameters are solved

MASCOT VSF calibrations Calibrated VSFs from the AZRD exp’t Concurrent ECO-VSF measurements

10 m binned VSFs from Hawaii MASCOT ECO-VSF

 profiles from Hawaii MASCOT ECO-VSF

 profiles from Hawaii MASCOT with ECO-VSF overlay  sw (150°,650 nm)

Pure water scattering Twardowski et al. 2007

Important Point #2 In clear water, accurate pure seawater VSF values are critical for deriving particulate VSF values

Agreement with theoretical modeling Fournier-Forand phase functions

Intercalibration

ECO-BB3 comparisons 3 different devices

More VSF comparisons NY Bight: May 2007 MVSM (Ukrainian device at NRL) ECOVSF MASCOT

More VSF comparisons NY Bight: May 2007

Backscattering analysis: from Hawaii and NY Bight apparent underestimation of b b by ECOVSF by few percent

Backscattering analysis: from Hawaii and NY Bight ECOVSF 3 rd order polynomial (polyfit) method for obtaining b b MASCOT polyfit MASCOT fully integrated b b ECOVSF polyfit ~8% underestimation of b b by polyfit method ~4% difference now

Important Point #3 Currently recommended “polyfit” method appears to underestimate b bp by a few percent This is because the polyfit extrapolation to 90 degrees from the degree measurements is not quite steep enough.

Backscattering analysis So why do we use it? Ocean Optics 2000Monaco

Analysis of shape of VSF in backward direction Sullivan et al. 2005: 532 nm ECO-VSF 150° vs 125° for 9 different coastal US sites

Analysis of shape of VSF in backward direction Adding 657 nm ECO-VSF NY Bight data

Analysis of shape of VSF in backward direction Adding 650 nm MASCOT NY Bight data

Analysis of shape of VSF in backward direction 100° vs 125° for 9 different coastal US sites Sullivan et al. 2005: 532 nm ECO-VSF

Analysis of shape of VSF in backward direction Adding 657 nm ECO-VSF NY Bight data

Analysis of shape of VSF in backward direction Adding 650 nm MASCOT NY Bight data

MASCOT % variation in b bp normalized data (2  (  )/b bp ) [ Lowest prediction errors in estimating backscattering coefficient Consistent with Oishi 1990; Boss and Pegau 2001

Important Point #4 a)  p ( ) is best range to pick a single angle measurement for estimating b bp b) using 1 or a few angles to estimate b bp has merit c) No obvious spectral variability in VSF shape was observed

VSF Uncertainties Table 3. Parameters from scattering measurements in the South Pacific central gyre. All values expressed in parameter  (nm)  t (117°) (m -1 sr -1 ) raw uncertainty a  t (117°) (m -1 sr -1 ) estimated uncertainty b  swB (117°) c (m -1 sr -1 ) b bswB c (m -1 )  t (117°), mean ±  (m -1 sr -1 ) central gyre, m 3.2± ± ±0.15 b bp (m -1 ) estimated uncertainty d b bp, mean ±  (m -1 ) central gyre, m 2.7± ± ±0.81 b bp, mean ±  (m -1 ) central gyre, m 2.0± ± ±0.37 b bp, lowest measured (m -1 ) ~0 a i.e., random electronic error b computed over 1-m depth bins; see text c pure water scattering computed from Buiteveld et al. (1994) at 20°C; [ (35)/37] adjustment for dissolved salts applied after Morel (1974) Twardowski, M.S., H. Claustre, S.A. Freeman, D. Stramski, and Y. Hout Optical backscattering properties of the “clearest” natural waters. Biogeosciences, 4, 1041–1058. Detailed Methodology Detailed uncertainties analysis Dark offsets measured in-situ for the first time New values for pure seawater scattering recommended

Summary 1. For VSF measurements, dark offsets should be measured IN SITU 2. In clear ocean water, accurate pure seawater VSF values are critical for deriving particulate VSF values 3. More work may be needed to refine method of estimating b bp from 3-angle measurements 4. Shape of VSF in the backward direction is remarkably consistent i.  p (110°-120°) is the best range for picking a single angle measurement for estimating b bp ii. Using 1 or a few angles to estimate b bp has merit iii. No obvious spectral variability in VSF shape was observed