Backscattering Lab Julia Uitz Pauline Stephen Wayne Slade Eric Rehm
Jan 04, 2016
Backscattering Lab
Julia UitzPauline Stephen
Wayne SladeEric 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 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
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
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.
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
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…
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)
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:
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
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
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
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~
• 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~
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)
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
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
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
]
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
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
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}
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