Absorption properties Scattering properties

ocean (water) color

light within water medium

(Voss et al 2007)

(Kirk 1994)

Lu(ϴ,ϕ)

They are modulated by water constituents!

Ed Lw

[Chl]

[CDOM]

[SPM]

IOPs

Inherent Optical Properties (IOPs)

a: absorption coefficient = aw + axi

bb: backscattering coefficient = bbw + bbxi

c: beam attenuation coefficient (a+b)

Sensor measures

color IOPs Water constituents

Boundary conditions

IOPs (Inherent Optical Properties):

The optical capability regardless of the ambient light environment.

Absorption properties; Scattering properties

Definition of absorption and scattering coefficients

r

P

PIOP

processprocess

1

r

P

Pa

absorption

1

r

P

Pb

scattering

1

Δr: infinitesimal (m) a&b: m-1

P P

P’

Δr

a = 1.2 m-1

b = 3.5 m-1

Units:

absorption

scattering

backscattering

Scattering has angular dependence

Transfer of energy

Redistribution of energy

Energy transfer processes:

photons

Rayleigh scattering Mie scattering

Scattering

“d” << λ “d” >> λ

Raman scattering

Elastic scattering In-elastic scattering

Scattering

Scattering

(google) (no wavelength change)

(wavelength change)

absorption coefficient: a (m-1)

Volume Scattering Function (VSF): β (m-1 sr-1)

Scattering coefficient: b (m-1)

db 0

)sin(2

dbb

2/)sin(2

db f 2/

0)sin(2

beam attenuation coefficient: c = a + b (m-1)

forward-scattering coefficient: bf (m-1)

backward-scattering coefficient: bb (m-1)

a = aw + axi

IOPs are additive.

b = bw + bxi

1. absorption properties

a = aw + axi

Very detailed:

(Stramski et al 2001)

a = aw + ap + ag

Practical (and common) division:

a = aw + aph + ad + ag

(google) (google)

Pure water (seawater): aw

Particulate: ap = aph+ad

Pigments of living phytoplankton: aph

Detritus: ad

Gelbstoff (yellow substance; colored dissolved

organic matter): ag

a = aw + aph + ad + ag

aw spectrum

(Mobley 1994)

aw spectrum

aw

(Mobley 1994)

(Morel et al 2007) (Pope and Fry, 1997)

Uncertainties of aw:

(Pegau et al 1997; Sullivan et al 2006)

aw is temperature and salinity dependent

ap spectrum

Bricaud and Stramski (1990) (google)

ad spectrum

)(

00)(

dS

dd eaa

Sd: ~0.005 – 0.015 nm-1

Bricaud and Stramski (1990)

aph = ap – ad

spectrum

Bricaud and Stramski (1990)

(Ciotti et al 2002)

Separated by size

“fatness”

By species or groups

(Dierssen et al 2006)

diatoms

(Balch et al 1991)

Coccolithophore

(Dupouy et al 2008)

(cyanobacteria)

(Bricaud et al 2004)

Separated by pigments

)(1)()(

phB

phph ChlAa

Example of one parameter hyperspectral aph(λ) model:

Bricaud et al (1995):

Lee (1994); Lee et al (1998):

PPaph )ln()(a)(a)( 10

P = aph(440)

Modeling aph spectrum

Example of two parameter model: (Ciotti et al 2002)

(Hoepffner and Sathyendranath, 1993)

Multiple parameter model:

Increase of absorption is NOT linearly proportionally to Chl concentration!

Package effect

Specific absorption/scattering coefficient = Concentration normalized absorption/scattering coefficient

Chl

aa

ph

ph*

Chl specific optical property (Bricaud et al 1995)

Simplified case:

VW

Sa

dV

S

W

aa

ph

1*

Size matters on efficiency! S: cross section V: volume W: weight

Absorption spectra of yellow substance (gelbstoff)

(Bricaud et al 1981)

ag spectrum

ag spectrum )(

00)(

gS

gg eaa

(Kirk 1994)

Sg: ~0.01 – 0.03 nm-1

(Carder et al 1989)

(Twardowski et al 2004)

(Twardowski et al 2004)

Slope changes with wavelength range

(Twardowski et al 2004)

Power-law model for ag spectrum:

Values of aph and ag of natural waters

(Kirk 1994)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

400 450 500 550 600 650 700

aw

aph

adg

a_tot

0

0.05

0.1

0.15

0.2

0.25

400 450 500 550 600

aw aph

adg a_tot

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

400 450 500 550 600 650 700

aw

aph

adg

a_tot

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

400 450 500 550 600

aw aph

adg a_tot

Contrast of absorption spectra of optically active components

Wavelength [nm]

Wavelength [nm]

2. Scattering properties

Size distribution

(Stramski and Kiefer 1991)

(Stramski et al 2001)

Very detailed:

b = bw + bxi bb = bw + bbxi

Commonly separated groups for scattering:

Molecules Suspended ‘particles’ Bubbles Turbulence

pw bbb

POMPIMw bbbb

Or,

VSF of pure water (βw)

0

0.4

0.8

1.2

1.6

2

0 50 100 150 200

Scattering angle (o)

βw

(Ω)/β

w(9

0o)

Scattering of water molecules

bw = 2 bbw

32.4

0

450

w

17.4

0

450

w

Morel 1974:

Shifrin: 1988

Spectral dependence

βw is also found salinity dependent; its value could be ~30% higher for marine waters.

32.4450

0023.0)(

bwb

Value and spectrum of seawater bbw:

(Morel 1974)

3.4450

0020.0)(

bwb

(Zhang et al 2009)

Raman scattering of water molecules: 3.5

4 490107.2)(

exex

Rwb

(Bartlett et al 1998)

Contribution of Raman scattering to water color is quite complicated though, as it also depends on the light color of the incident radiation; therefore, Raman scattering coefficient is not exactly inherent optical property. Fluorescence is also a result of in-elastic scattering. However, because of fluorescence efficiency is light-environment dependent, fluorescence cannot be grouped into “inherent” optical property.

emex

cm

1)(3400

1 1

Volume Scattering Function with particles

(Petzold 1972)

(Lee and Lewis, 2003)

MVSM measurements

MASCOT measurements

(Sullivan and Twardowski, 2009)

Particles are strongly forward scatters!

05.0005.0~~

;5.0~

bp

bw

b

b

90 deg β shape changes in a narrow range in the backward domain

(Mobley 1994)

Backscattering ratio: b

bb b

b ~

Twardowski et al (2001)

bpb~

and refractive index

44 )cos1()cos1(

1~

bf

5.12

2

)cos21(

1

4

1

gg

g

Henyey-Greenstein (1941)

Beardsley and Zaneveld (1969)

Examples of β model

Fournier and Forand (1994)

Spectrum of scattering coefficient

(Bricaud et al 1988)

625.100113.0

625.100113.0)()(

r

rbb

(Gould et al 1999)

weakly wavelength dependent

0.0002

0.002

0.02

400 450 500 550 600 650 700

pure seawater

particles

particles

Wavelength [nm] B

acks

catt

eri

ng

coe

ffic

ien

t [

m-1

]

bbw: ~0.0001 – 0.004 m-1

0

0)( bbbp

η: ~0-2.0

bb spectrum contrast

bubbles

(Zhang et al 2002)

Not known the spectral characteristics of bubble scattering, considered spectrally flat

(Stavn and Richter 2008)

Organic vs inorganic separation

(Stramski et al 2001)

(Prog. Oceanog. 28, 343-383, 1991)

Key points:

1. In addition to boundary conditions, IOPs play the key role in forming ocean/water color.

2. Primary IOPs include absorption and scattering coefficients; the latter is direction dependent.

3. Bulk IOPs are lump sum contributions of the many individual, dissolved and suspended, molecules and particles.

4. Absorption and scattering coefficients of pure (sea)water are considered constant (change with temperature/salinity), but uncertainties still exist, especially for absorption in the UV range.

5. In addition to water molecules, practically and generally, for absorption: there are three major optically active components: phytoplankton pigments, detritus and gelbstoff (CDOM); for scattering: there are organic and inorganic particulates, bubbles, and many times lumped into one term.

6. Spectrally, water molecules are strong absorber in the longer wavelengths; phytoplankton absorption generally has two distinct peaks with a stronger peak centered around 440 nm and weaker peak centered around 675 nm; have varying spectral shapes detritus and gelbstoff are strong absorbers in the shorter wavelengths, and gelbstoff has steeper spectral slope; Water molecules are strong scatter in the shorter wavelengths; ‘particle’ scattering is weakly wavelength dependent. It is strongly dependent on size, refractive index, and abundance.