backscattering lab

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Backscattering Lab Julia Uitz Pauline Stephen Wayne Slade Eric Rehm

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Backscattering Lab. Julia Uitz Pauline Stephen Wayne Slade Eric Rehm. Wetlabs EcoVSF. Samples the Volume Scattering Function (VSF) at three angles 100°, 125 °, and 150° One wavelength: 660 nm for our model is safely in absorbing part of H 2 O spectrum - PowerPoint PPT Presentation

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Page 1: Backscattering Lab

Backscattering Lab

Julia UitzPauline Stephen

Wayne SladeEric Rehm

Page 2: Backscattering Lab

Wetlabs EcoVSF

• Samples the Volume Scattering Function (VSF) at three angles– 100°, 125 °, and 150°– One wavelength: 660 nm for our model is

safely in absorbing part of H2O spectrum

• Integrate curve fit of VSF samples from 90 to 180 degrees to compute backscattering coefficient bb.

• Employs three transmitters coupled to a single receiver

Page 3: Backscattering Lab

Backscattering Coefficient bb

• bb carries useful information about seawater constituents • Potential to derive information about

– Abundance and types of suspended marine particles– Such particles play different roles in ocean ecosystems and

biogeochemical cycling• A proxy for particle abundance

– Also depends significantly on particle size distribution and particle composition: size, index of refraction, absorption

• Smaller particles scatter more• Particles with index of refraction higher than water scatter more• Particles that are highly absorbing scatter (e.g., water filled phytoplankton)

scatter less, but in absence of inorganic scatters, can be seen in backscatter.

• bb is proportional to spectral reflectance of the ocean (aka “ocean color”). – Understanding bb is required to interpret ocean color

Page 4: Backscattering Lab

ECO-VSF Calibration• Dark Counts

– Factory: 31.047, 30.488, 158.093– Lab: 29 29 140– At 150, we have a lower count value– Was our room darker than Wetlabs’?

• DI Water– Factory: 39.212, 43.364, 196.515– Lab: 40 41 144– At 150, we have lower count value.

• Discussion– 150 light source or detector could have changed since factory

calibration. Note that blue and red reference values were not output by this EcoVSF.

– Our water could be cleaner than Wetlabs’– Or, since small particles scatter a larger angles, may suggest that the

fraction of small particles in their DI water is greater than ours.

Page 5: Backscattering Lab

0 1 2 3 4 5 6 70

1000

2000

3000

4000

5000

6000

Cp(650) from AC-9

Raw

Eco

VS

F C

ount

s

Factory vs. Lab Calibration

Factory 100Factory 125Factory 150Lab 100Lab 125Lab 150

DI Water

Beads

Page 6: Backscattering Lab

Why use the factory calibration instead of trusting our own?

• Good question…– We should have trusted our calibration and

used those dark counts and slopes

• In the original presentation subsequent plots used factory calibration

• Updated slides will use our calibration

• We did as good a job as Wetlab at calibration…

Page 7: Backscattering Lab

Corrected

100 110 120 130 140 150 160 170 180-5

0

5

10

15

20x 10

-4

Corrected at 100,120 and 150 from EcoVSF

fsw (di)culture (fsw)tsw (di)tsw (fsw)

Page 8: Backscattering Lab

Effect of Absorption Correction

100 105 110 115 120 125 130 135 140 145 1500

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

100*

(co

rrec

ted /

m

easu

red)

Correction (%)

30 beads (a=0.055)120 beads (a=.19)Dytilum (a=.14)Unfiltered Sea Water (a=.04)

Sample Diff

Dytilum .67 %

Unfiltered sea water

.19 %

Effect on bbp after integration:

Page 9: Backscattering Lab

VSF for Beads

100 105 110 115 120 125 130 135 140 145 1503

4

5x 10

-4

30 beads

100 105 110 115 120 125 130 135 140 145 1505

6

7

8

9x 10

-4

60 beads

100 105 110 115 120 125 130 135 140 145 1501

1.2

1.4

1.6x 10

-3

120 beads

Same particle

Same shape of VSF

Page 10: Backscattering Lab

VSF for Samples

100 105 110 115 120 125 130 135 140 145 150-1

0

1

2x 10

-5

Filtered Sea Water

100 105 110 115 120 125 130 135 140 145 1502

4

x 10-4

Dytilium

100 105 110 115 120 125 130 135 140 145 1501.4

1.6

1.8

2x 10

-3

Total Sea Water (DI)

100 105 110 115 120 125 130 135 140 145 1501.4

1.6

1.8

2x 10

-3

Total Sea Water (fsw)

• Filtered sea water scatters at angles larger than other samples

• Mean size of Dytilum ~ mean size of total sea water from LISST measurements

Page 11: Backscattering Lab

bbpvia two methods

0 .002 .004 .006 .008 .01 .012

0

.002

.004

.006

.008

.01

.012

bbp

from at one angle * (theta)

b bp f

rom

a

t th

ree

angl

es

Filtered Sea Water

Dytilum

Total Sea Water

bbp

estimated from 3 measurements vs. 1 measurement

30 drops

60 drops

120 drops

mean(100)(120)(150)

• bbp from (100) best matches bbp estimate from all three angles

• Overall, very good correlation between methods

Page 12: Backscattering Lab

Backscattering Ratio bbp:bp

Sample bbp bp (ac-9)

Filtered sea water

0.00002 -0.0435 -0.0007

Dytilum 0.0019 0.3828 0.0050 3.7Unfiltered sea

water0.0111 1.4490 0.0077 4.0

30 beads 0.0024 0.2523 0.011760 beads 0.0042 0.4254 0.0115120 beads 0.0084 0.9161 0.0115

bpb~

Page 13: Backscattering Lab

• Backscattering ratio for dock sample (.0077) is in published range for Case I and Case II waters (Twardowski, et al., JGR, 2001)

– Case I: .006 – .020– Case II: .005 – .013

• Particle Size distribution for dock sample (calculated from AC-9 cp) is in “typical” published range (3.5<

• As we move from less scattering (Dytilum) through scattering (Sea water) to highly scattering (beads), increases from .5% to 1.1%

Discussion

bpb~

Page 14: Backscattering Lab

What can we say about Dytilum brightwellii?

• Backscattering ratio– Lower than for unfiltered seawater and homogenous

concentrations of 10 µm non-absorbing beads

• Highly absorbing and large: D=25-100 µm• Shape of :

– Monotonically decreases between 100 and 150

• Magnitude of – ~1 order of magnitude (.0004 - .0002) less than that for unfiltered

sea water (.002 - .0014)

• PSD inferred from cp :– Larger fraction of large particles than sea water. (vs.

100 105 110 115 120 125 130 135 140 145 150-1

0

1

2x 10

-5

Filtered Sea Water

100 105 110 115 120 125 130 135 140 145 1502

4

x 10-4

Dytilium

100 105 110 115 120 125 130 135 140 145 1501.4

1.6

1.8

2x 10

-3

Total Sea Water (DI)

100 105 110 115 120 125 130 135 140 145 1501.4

1.6

1.8

2x 10

-3

Total Sea Water (fsw)

Page 15: Backscattering Lab

What can we say about Dytilum brightwellii?

100

101

102

103

0

0.2

0.4

0.6

0.8

1

1.2

1.4Particle size distribution measured with the LISST

Mean particle diamteter [microns]

volu

me

conc

entr

atio

n [u

l/l]

~20-60 m

Page 16: Backscattering Lab

Unfiltered Sea WaterComparison with LISST

100

101

102

103

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5Particle size distribution measured with the LISST

Mean particle diamteter [microns]

volu

me

conc

entr

atio

n [u

l/l]

~6-70 m

Page 17: Backscattering Lab

What can we say about Dytilum brightwellii?

10-1

100

101

102

10-4

10-3

10-2

10-1

100

101

102

103

104

Volume Scattering Function at small angles measured with the LISST

angle [degrees]

VS

F [

m-1

sr-1

]

Page 18: Backscattering Lab

400 500 600 700 800-0.1

0

0.1

0.2

0.3

0.4

=11.31

Wavelength (nm)

(m-1

)

Filtered Seawater

AfBfCf

400 500 600 700 8000

0.5

1

1.5

2

2.5

3

=1.29

Wavelength (nm)

(m-1

)

PG (Unfiltered Seawater)

AwBwCw

400 500 600 700 8000

0.2

0.4

0.6

0.8

=0.69

Wavelength (nm)

(m-1

)

Dytilum Culture

ADytBDytCDyt

400 500 600 700 8000

0.5

1

1.5

2

2.5

=0.97

Wavelength (nm)

(m-1

)

Particulate (PG - FSW)

ApBpCp

Page 19: Backscattering Lab

Eric was confused about EcoVSF

• What do you do with it if you don’t own an AC-9 and your measurements are in-situ?– No a No absorption correction for

~O(1%) error– No b No backscattering ratio– No ap, bp No ap:bp proxy for pigmented

material (Twardowski et al., 2001)– No other data on PSD

• No cp No • No Coulter counter

Page 20: Backscattering Lab

There is some hope…Case I waters, Global Scale

(Behrenfeld, 2004) Note: I cut out 8 of Behrenfeld’s 14 steps….

1. cp is dominated by particles in the phytoplankton domain2. cp covaries with POC (7 references)3. cp :chl should track phytoplankton Carbon:chl

– (cp:chl tracks changes in phytoplankton physiology like photosynthetic rate)

4. “Mie calculations indicate that bbp is dominated by submicron particles, but in field populations bbp likely has a significant tail in the phytoplankton size domain.”

5. Satellite bbp covaries with POC (2 references)– (Should be true in-situ too…)

6 chl:bbp should track chl:Carbon and thus phytoplankton growth rates {once a correction for bacterial background is accounted for}

Page 21: Backscattering Lab

0 50 100 150 200

-25

-20

-15

-10

-5

Cruise 2, 25 m cast with 5m filter, purge valve open

Pressure (db)

chlraw

*10

bb*10e5.5*chl:bb

Pre

ssur

e (d

bar)

Raw ECO-VSF counts

Peak in chl, bb and chl:bb