Scattering Phenomenon: Key Concepts and Applications
Scattering:
What is it? Who does it?
A few demos to get us going
Why should you care about it?
*includes materials by C. Roesler and C. Mobley
Scattering Measurement Theory
t
a
b
Scattered Radiant Flux
o
b = fractional scatterance
per unit distance
b = (-1/x)
ln [
t
/
o
] –
(-1/x)
ln [
a
/
o
]
= c - a
Dimensionally, what should scattering dependent on?
When dealing with a single particles, presented as:
Ratio of optical cross section / Geometrical cross section (non-dimensional)
Size (cross-section, volume)
Index of refraction (difference with medium)
Wavelength (in medium)
To get back to scattering units [m
-1
]:
Optical cross section x concentration of particles
What physical properties determine the optical properties of
particles?
Size, composition (refractive index), shape, internal structure.
These properties interact…
Small Particle Scattering
follows Rayleigh Theory
0 30 60 90 120 150 180
VSF
400
500 600 700
Wavelength (nm)
b(
)
Example for water
~
-4
Similar results for viruses (Balch et al. 2000)
Scattering
by water
Density inhomogeneities:
Spectral dependence: Unlike
Rayleigh
~
-4.32
(e.g. Morel, 1974)
Salts: increase scattering (~30% for 37psu).
Weaker dependence on Temperature and Pressure.
Latest works X. Zhang and co., Optics Express 2009.
Phase function: symmetric and similar
to Rayleigh (
D<<
):
Scattering
by CDOM:
Scattering by molecules whose
D<<
Rayleigh scattering:
No evidence in the literature that scattering is significant (the only
place
I
have ever found significant dissolved scattering (
c
g
>a
g
) was
in pore water).
Large Particle Scattering
Three effects:
refraction
,
reflection
and
diffraction
refraction
Changes the speed of propagation leading to directional changes
and phase changes
Backscattering and scattering sensitivity to size:
Based on Mie theory (homogeneous spheres)
Boss et al., 2004, TOS
(
)
response to particle size distribution
First let
’
s talk about particle size distributions
(
)
and response to particle size distribution
/ V
p
Roesler and Boss, 2008
(
)
response to index of refraction
/ V
p
Roesler and Boss, 2008
Light within the ocean is scattered by:
H
2
O+salts
Colloids
Inorganic particles
Organic particles (bacteria, phytoplankton)
bubbles
What scatters in the oceans:
What particles scatter in the ocean?
Phytoplankton:
Variable in shape, size and pigment composition.
Variable in scattering and absorption properties
What particles scatter in the ocean?
Non-algal particles: Organic and inorganic.
Sand
http://www.aad.gov.au/default.asp
Aggregates:
Silt
Variable in scattering and absorption properties
clay
Scattering in the oceans (~60,000 1km
2
data):
The b
b
enigma:
Morel and Ahn, 1991: ‘Algal cells in
open ocean, and to lesser extent
small heterotrophs, dominate the
scattering coefficients; …On the
contrary, these organisms are
definitely insignificant contributors
to the backscattering coefficient.’
Stramski et al., 2001
Stramski et al., 2001: simulating
open-ocean (oligotrophic, 0.18mg
Chl/m
3
) 2-3% of the
backscattering coefficient is due
to plankton. 50% from particles
<0.2
m.
Phase functions:
Stramski et al., 2001
The b
b
enigma (or paradox):
Based on Mie theory, backscattering should be
dominated by inorganic particles and sub-micron
particles (the least known of the bunch).
Yet b
bp
correlates well with [chl] and POC (>0.7
m):
Huot et al., 2008
Stramski et al., 2008
An important aside about modeling (using homogeneous O):
From Clavano et al., 2007
Shape matters:
VSF of large particles depends on <G>.
Shape approximations
for light scattering calculations
Particle radius (
m)
Axis ratio
1
0.1
10
0.1
1
10
oblate
prolate
2
0.5
Mie-Theory
T-matrix
Axis ratios up to
convergence limit
T-matrix
Moderate Axis
ratios (0.5<AR<2)
Size limit
Slide From Volten
Meyer, 1979
Relative intensity
An other approach,
Coated spheres:
Backscattering dominated by membrane.
Measurements across the
equatorial Pacific (Dall’olmo et
al., 2009):
b
bp
(D<0.2mm)
b
bp
(D>0.2mm)
<0.1
No filter effect visible
Uncertainty dominated by uncertainty in b
b
(H
2
O)
b
bp
well correlated with c
p
Angular scattering:
L
o
Incident
Radiance
d
x
L
s
Volume scattering function [m
-1
sr
-1
]:
Most often assume azymuthal isotropy (only
dependence).
Scattered
Radiance
Volume Scattering Function (
)
source
detector
o
b
/
a
= (-1/x d
)
ln[
b
(
)/
o
]
Back-Scattering Measurements
•
Detected flux
measurement must
correct for
attenuated
flux along pathlength
inner-filter effect
x
•
Define shape of
detection area
–
Calibration with known
substance
–
mathematically
= (-1/x d
)
ln[
b
(
)/
o
]
Scattering
by phytoplankton
Whitmire et al., 2010
In cultures (watch out for NAP)
Comparison with Mie theory of Stramski et al., 2001
b
b
+
F
chl
Sullivan and Twardowski (2009):
Consistency from 90->150degrees (except for one study…).
Using one angle to infer backscattering
Another commercial design: Eco-VSF
Nominal angles: 104, 131, 151degrees
Fit a 3
rd
order polynomial of
sin
(
)
including at
.
Integrate from
to
.
New designs to measure backscattering:
Independent of VSF !!!
Gainusa-Bogdan and Boss, 2011
Haubrich et al., 2011,
Applied Optics
Whitmire et al. (2010):
Phytoplankton cultures (5
):
Clean
With surfactant
Theory (clean)
Zhang et al., 2002, L&O
Phase function of a population of bubbles:
Scattering
by bubbles:
Scattering
by aggregates (and what happen with handling)
For particles with
D>>
:
When scattering centers are far enough, IOPs are additive.
Optical properties
cross-sectional area, additive
Depends on aggregate packaging (
‘
fractal
’
dimension).
Spectral dependence of scattering
Aggregates:
Boss et al., 2009
Summary:
1.
Scattering measurements are useful but are
not trivial.
2.
Beware of models…
3.
There still is no consensus about what
dominates backscattering -> ocean color.
Scattering is a fundamental process involving the interaction of light with particles, impacting various fields such as physics and environmental science. Learn about its measurement theory, dependence on physical properties, and examples like Rayleigh theory for small particle scattering and impacts on water density inhomogeneities. Explore the significance of optical properties and implications for different types of particles.
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Presentation Transcript
Scattering: What is it? Who does it? A few demos to get us going Why should you care about it? *includes materials by C. Roesler and C. Mobley
Scattering Measurement Theory b = fractional scatterance per unit distance b Scattered Radiant Flux a o t b = (-1/x)ln [ t / o] (-1/x)ln [ a / o] = c - a
Dimensionally, what should scattering dependent on? When dealing with a single particles, presented as: Ratio of optical cross section / Geometrical cross section (non-dimensional) Size (cross-section, volume) Index of refraction (difference with medium) Wavelength (in medium) To get back to scattering units [m-1]: Optical cross section x concentration of particles
What physical properties determine the optical properties of particles? Size, composition (refractive index), shape, internal structure. These properties interact
Small Particle Scattering follows Rayleigh Theory Example for water ~ -4 ( ) b( ) VSF 0 30 60 90 120 150 180 400 500 600 700 Wavelength (nm) Similar results for viruses (Balch et al. 2000)
Scattering by water Density inhomogeneities: Water clusters Water clusters with salt Phase function: symmetric and similar to Rayleigh (D<< ): 1 ( ) ( ) + 2 1 cos + 1 Spectral dependence: Unlike Rayleigh ~ -4.32(e.g. Morel, 1974) Salts: increase scattering (~30% for 37psu). Weaker dependence on Temperature and Pressure. Latest works X. Zhang and co., Optics Express 2009.
Scattering by CDOM: Scattering by molecules whose D<< Rayleigh scattering: ( ) ( 1+ ) ( ) ( ) 4 b 2 cos No evidence in the literature that scattering is significant (the only place I have ever found significant dissolved scattering (cg>ag) was in pore water).
Large Particle Scattering Three effects: refraction, reflection and diffraction
refraction Changes the speed of propagation leading to directional changes and phase changes
Backscattering and scattering sensitivity to size: Boss et al., 2004, TOS Based on Mie theory (homogeneous spheres)
() response to particle size distribution First let s talk about particle size distributions r-3 r-5 Stramski and Kiefer 1989
() and response to particle size distribution ( ) / Vp Roesler and Boss, 2008
() response to index of refraction ( ) / Vp Roesler and Boss, 2008
What scatters in the oceans: Light within the ocean is scattered by: H2O+salts Colloids Inorganic particles Organic particles (bacteria, phytoplankton) bubbles
What particles scatter in the ocean? Phytoplankton: Variable in shape, size and pigment composition. Variable in scattering and absorption properties
What particles scatter in the ocean? Non-algal particles: Organic and inorganic. Sand Silt clay Aggregates: http://www.aad.gov.au/default.asp Variable in scattering and absorption properties
The bb enigma: Morel and Ahn, 1991: Algal cells in open ocean, and to lesser extent small heterotrophs, dominate the scattering coefficients; On the contrary, these organisms are definitely insignificant contributors to the backscattering coefficient. Stramski et al., 2001: simulating open-ocean (oligotrophic, 0.18mg Chl/m3) 2-3% of the backscattering coefficient is due to plankton. 50% from particles <0.2 m. Stramski et al., 2001
Phase functions: Stramski et al., 2001
The bb enigma (or paradox): Based on Mie theory, backscattering should be dominated by inorganic particles and sub-micron particles (the least known of the bunch). Yet bbp correlates well with [chl] and POC (>0.7 m): Stramski et al., 2008 Huot et al., 2008
Shape matters: VSF of large particles depends on <G>. From Clavano et al., 2007
Shape approximations for light scattering calculations Mie-Theory T-matrix Moderate Axis ratios (0.5<AR<2) 10 oblate 2 1 0.5 T-matrix Axis ratios up to convergence limit prolate 0.1 0.1 1 10 Particle radius ( m) Slide From Volten
An other approach, Coated spheres: Relative intensity Backscattering dominated by membrane. Meyer, 1979
Measurements across the equatorial Pacific (Dall olmo et al., 2009): bbp well correlated with cp bbp(D<0.2mm) bbp(D>0.2mm) No filter effect visible <0.1 Uncertainty dominated by uncertainty in bb(H2O)
Angular scattering: d Scattered Radiance Ls Lo Lt Incident Radiance Transmitted Radiance Volume scattering function [m-1sr-1]: x ( ( ) ) 1 L ( ) lim lim s s 0 0 i s x x L Most often assume azymuthal isotropy (only dependence). 0 i
Volume Scattering Function () ( ) = (-1/x d )ln[ b( )/ o] detector b/ source a o
Back-Scattering Measurements ( ) = (-1/x d )ln[ b( )/ o] Detected flux measurement must correct for attenuated flux along pathlength inner-filter effect x Define shape of detection area Calibration with known substance mathematically b ( ) o source detector
Scattering by phytoplankton In cultures (watch out for NAP) bb+Fchl Whitmire et al., 2010 Comparison with Mie theory of Stramski et al., 2001
Using one angle to infer backscattering 2 bp b = ( ) 0, 0 p p Consistency from 90->150degrees (except for one study ). Sullivan and Twardowski (2009):
Another commercial design: Eco-VSF Fit a 3rd order polynomial of sin( ) including at = . Integrate from to . Nominal angles: 104, 131, 151degrees
New designs to measure backscattering: Independent of VSF !!! Haubrich et al., 2011, Applied Optics Gainusa-Bogdan and Boss, 2011
Whitmire et al. (2010): Phytoplankton cultures (5 ): ( ) b = 2 0, 0 bp p p
Scattering by bubbles: Phase function of a population of bubbles: Clean With surfactant Theory (clean) Zhang et al., 2002, L&O
Scattering by aggregates (and what happen with handling) Aggregates: Boss et al., 2009 For particles with D>> : When scattering centers are far enough, IOPs are additive. Optical properties cross-sectional area, additive Depends on aggregate packaging ( fractal dimension). Spectral dependence of scattering
Summary: 1. Scattering measurements are useful but are not trivial. 2. Beware of models 3. There still is no consensus about what dominates backscattering -> ocean color.
