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 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.
Factory vs. Lab Calibration
6000
Factory 100
Factory 125
Factory 150
Lab 100
Lab 125
Lab 150
5000
Raw EcoVSF Counts
4000
3000
2000
1000
0
DI
Water
0
1
2
3
4
Cp(650) from AC-9
Beads
5
6
7
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 b
Corrected b at 100,120 and 150 from EcoVSF
-4
20
x 10
fsw (di)
culture (fsw)
tsw (di)
tsw (fsw)
15
b
10
5
0
-5
100
110
120
130
140

150
160
170
180
Effect of Absorption Correction
b Correction (%)
1.6
1.4
Effect on bbp
after
integration:
30 beads (a=0.055)
120 beads (a=.19)
Dytilum (a=.14)
Unfiltered Sea Water (a=.04)
100*(b
corrected
)
/b
measured
1.2
1
Sample
Diff
Dytilum
.67 %
0.8
Unfiltered .19 %
sea water
0.6
0.4
0.2
0
100
105
110
115
120
125

130
135
140
145
150
VSF for Beads
-4
5
x 10
Same particle

Same shape
of VSF
b
30 beads
4
3
100
-4
x 10
9
105
110
115
120
125
130
135
140
145
150

60 beads
b
8
7
6
5
100
105
110
115
120
1.6
125
130
135
140
145
150

-3
x 10
120 beads
b
1.4
1.2
1
100
105
110
115
120
125

130
135
140
145
150
VSF for Samples
-5
2
x 10
Filtered Sea Water
b
1
0
-1
100 -4
x 10
105
110
115
120
125
130
135
130
135
130
135

b
4
2
100 -3
x 10
2
105
110
115
120
125

b
1.8
1.6
1.4
100 -3
x 10
2
105
110
115
120
125
• Filtered sea
water scatters at
angles larger
140
145
150
than other
Dytilium
samples
• Mean size of
Dytilum ~ mean
140
145
150
size of total sea
Total Sea Water (DI)
water from
LISST
measurements
140
145
150

Total Sea Water (fsw)
b
1.8
1.6
1.4
100
105
110
115
120
125

130
135
140
145
150
bbpvia two methods
bbp estimated from 3 measurements vs. 1 measurement
.012
mean
b(100)
b(120)
b(150)
.01
• bbp from
b(100) best
matches bbp
estimate from
all three
angles
• Overall, very
good
correlation
between
methods
Total Sea Water
120 drops
b
bp
from b at three angles
.008
.006
60 drops
.004
30 drops
Dytilum
.002
0
Filtered Sea Water
0
.002
.004
.006
.008
bbp from b at one angle * (theta)
.01
.012
Backscattering Ratio bbp:bp
bp (ac-9)
~
bbp
x=g+3
Sample
bbp
Filtered sea
water
Dytilum
0.00002 -0.0435
-0.0007
0.0019
0.3828
0.0050
3.7
Unfiltered sea 0.0111
water
1.4490
0.0077
4.0
30 beads
0.0024
0.2523
0.0117
60 beads
0.0042
0.4254
0.0115
120 beads
0.0084
0.9161
0.0115
Discussion
• 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< x<4)
• As we move from less scattering (Dytilum)
through scattering
(Sea water) to highly scattering
~
(beads), bbp increases from .5% to 1.1%
What can we say about Dytilum
brightwellii?
• Backscattering ratio
– Lower than for unfiltered seawater and homogenous
x 10
2 concentrations of 10 µm non-absorbing beads
-5
Filtered Sea Water
• Highly absorbing and large: D=25-100 µm
• Shape of b:
b
1
0
-1
100 -4
x 10
105
110
115
120
125
130
135
140
145
150

– Monotonically decreases between 100 andDytilium
150
Magnitude of b:
b
•
4
–2100x ~1
order
of magnitude
(.0004
105
110
115
120
125
130 - .0002)
135
140less
145than
150that for unfiltered
10

2
sea water (.002 - .0014)
Total Sea Water (DI)
-3
1.8
b
• PSD inferred from cp :
1.6
1.4 Larger fraction of large particles than sea water. (x = 3.7 vs. 4.0)
–
100
105
110
115
120
125
130
135
140
145
150
x 10
-3
2

Total Sea Water (fsw)
b
1.8
1.6
What can we say about Dytilum
brightwellii?
Particle size distribution measured with the LISST
1.4
~20-60 mm
1.2
volume concentration [ul/l]
1
0.8
0.6
0.4
0.2
0
0
10
1
2
10
10
Mean particle diamteter [microns]
3
10
Unfiltered Sea Water
Comparison with LISST
Particle size distribution measured with the LISST
0.5
~6-70 mm
0.45
0.4
volume concentration [ul/l]
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
10
1
2
10
10
Mean particle diamteter [microns]
3
10
What can we say about Dytilum
brightwellii?
Volume Scattering Function at small angles measured with the LISST
4
10
3
10
2
10
1
VSF [m-1 sr-1]
10
0
10
-1
10
-2
10
-3
10
-4
10
-1
10
0
1
10
10
angle [degrees]
2
10
Filtered Seawater
PG (Unfiltered Seawater)
0.4
3
0.3
2.5
g=1.29
Aw
Bw
Cw
2
0.2
(m-1)
(m-1)
g=11.31
Af
Bf
Cf
0.1
1.5
1
0
-0.1
400
0.5
500
600
700
Wavelength (nm)
0
400
800
Dytilum Culture
800
2.5
g=0.69
ADyt
BDyt
CDyt
g=0.97
2
(m-1)
0.6
(m-1)
600
700
Wavelength (nm)
Particulate (PG - FSW)
0.8
0.4
0.2
0
400
500
Ap
Bp
Cp
1.5
1
0.5
500
600
700
Wavelength (nm)
800
0
400
500
600
700
Wavelength (nm)
800
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 b
~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 g
• 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 domain
2. 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}
Cruise 2, 25 m cast with 5mm filter, purge valve open
chlraw *10
bb*10e5
.5*chl:bb
Pressure (dbar)
-5
-10
Peak in chl,
bb and
chl:bb
-15
Raw ECOVSF counts
-20
-25
0
50
100
Pressure (db)
150
200