Optical properties of oceanic particles

Wh t ph si l p p ti s d t mi th pti l p p ti s f p ti l s?What physical properties determine the optical properties of particles?

Size, composition (refractive index), shape, internal structure. These properties interact…

Based, in part, on a lecture prepared by C. RoeslerWe will not address fluorescence and polarization.

What particles absorb and scatter in the ocean?

Phytoplankton:

Variable in shape size and pigment compositionVariable in shape, size and pigment composition.

Variable in scattering and absorption properties

What particles absorb and scatter in the ocean?

Non-algal particles: Organic and inorganic.

Sand Silt clay

Aggregates:

http://www.aad.gov.au/default.aspVariable in scattering and absorption properties

IOP TheoryIOP Theory

L LLo

IncidentR d

TransmittedR d

Lt

Radiance Radiance

No attenuation (recap: radiance is constant along a ray)

IOP TheoryIOP Theory

LL LtLo

IncidentR d

TransmittedR dRadiance Radiance

Attenuation

Loss due to absorptionLoss due to absorption

La Absorbed Radiance

L LLo

IncidentR d

Lt

TransmittedR dRadiance Radiance

Loss due to scatteringLoss due to scattering

Lb Scattered Radiance

L LLo

IncidentR d

Lt

TransmittedR dRadiance Radiance

Loss due to absorption and scattering (attenuation)

Lb Scattered Radiance

La Absorbed Radiance

LL LtLo

IncidentR d

TransmittedR dRadiance Radiance

Loss due to absorption and scattering (attenuation)

Lb Scattered Radiance

La Absorbed Radiance

LL LtLo

IncidentR d

TransmittedR dRadiance Radiance

Lo = Lt + La + Lb

Beam Attenuation M t ThMeasurement Theory

Attenuance C = fraction of incident

radiance attenuated Lb

L

La

L

C (L L )/L

LtLo

C = (Lb + La)/Lo

C = (Lo – Lt)/Lo

Beam Attenuation M t ThMeasurement Theory

c = fractional attenuance it di t c = ΔC/Δx

Lb

per unit distance c Δx = - ΔL/L

x x

L

La

L

∫0 c dx = -∫0 dL/Lx x

c(x 0) [ ln(L ) ln(L )]LtLo c(x-0) = -[ ln(Lx)-ln(L0)]

c x = -[ ln(Lt)-ln(Lo)]

c x = - ln(Lt/Lo)

c (m-1) = (-1/x) ln(Lt/Lo)( ) ( ) ( t o)

Δx Lt=Loexp(-c x)

Lt=Loexp(-c x)τ=cx ≡ optical depth

Beer(1852), Lambert(1729), Bouguer (before 1729) p p

Important characteristics:

Absorption and scattering feature similarlyAbsorption and scattering feature similarly.

c is Linear with concentration or added substances.

Assumes no internal sources.

Class demo with food color/maalox

Sinks for photons – absorbersSinks for photons absorbers.What absorbs radiation in the oceans/lakes?

How do we measure absorption?

What is absorption:

Photons ‘disappear’ due to interaction with matter.

From: http://www haverford edu/chem/Scarrow/GenChem/quantum/BohrAtom gif

Following absorption energy could be reemitted (or not) at the same or different wavelength (e.g. scattering, fluorescence, thermal emission).

From: http://www.haverford.edu/chem/Scarrow/GenChem/quantum/BohrAtom.gif

Absorption due to molecular:Translational, rotational, vibrational and electronic modes (+combinations)and electronic modes (+combinations).

Observation include also spontaneous emission.

From: http://acept.la.asu.edu/PiN/rdg/irnuv/d7.gif

Explained by quantum mechanics.

b b /l kAbsorbers in ocean/lakes

• Water• Phytoplankton• Phytoplankton• Non-algal organic particles (CPOM)• Inorganic mineral particles (CPIM)Inorganic mineral particles (CPIM)• Dissolved organic matter (CDOM)

– not addressed today

Note: most available information is in the visible (400-700nm). Why?

Absorption by (liquid) water:F t s:Features:

•Extremly complex

•Variety of vibrational modes

•Combination possible•Combination possible

Assumes no internal sources.

http://www.lsbu.ac.uk/water/vibrat.html

PhytoplanktonSpecies-specific pigment composition

Phytoplankton

3

2

3

a(67

6)

15101520

Cell size

0

1a()/a 30

4050

0400 500 600 700

Wavelength (nm)

(Morel and Bricaud 1981)

Phytoplankton vs Chlorophyll

Sosik & Mitchell 1991

http://chaitanya1.wordpress.com/2007/07/09/strawberries/

chloroplast chlorophyll

cell Packaging: a/[chl] is function of size and [chl]Duysens (1956)

a/V= QabsG/{1.33πr3}Theoretical results:

n’=0.01

n’=apureλ/4π

‘Packaging’

Cabs = areaa/V ∝ 1/D

Molecular absorption ∝ volume.

a/V 1/D

Non-algal particlesNon algal particlesHeterotrophic Ciliates and Flagellates, and bacteria

D t itDetritus

Ahn and Morel, 1983 Iturriaga and Siegel 1989Iturriaga and Siegel 1989

aCPOM(l) = aCPOM(400)*exp(-SCPOM(l-400)) SCPOM = 0.007 to 0.011Roesler et al., 1989

CDOM: chromophoric dissolved organic matter

aCDOM(l) = aCDOM(400)*exp(-SCDOM(l-400)) SCdOM = 0.011 to 0.022

Kirk 1983 Carder et al. 1989

Absorption MeasurementsAbsorption Measurementsa = (-1/x) ln[(L + L )/L ]a = (-1/x) ln[(Lt + Lb)/Lo]

Detected flux measurement must include scattered flux

La Lb

mu u f usource

Lo Lt

detector

Lo Ltx

d t t d tt d fl undetected scattered flux must be corrected for

Ex. Absorption meter: E b WETLabs ac-9 & ac-S

• Dual path, a and c• b inferred by y

difference.• Correct for photon that

are not collected

Ex Absorption meter: Ex. Absorption meter: WETLabs ac9/acS

• Detector FOV ~ 40o

• ~90% of scattered flux

Absorption Measurementp

Detected flux measurement includes all scattered flux

Integrating sphere:Detected flux measurement includes all scattered flux.

Reflectance of sphere needs to be known very accurately (problems with fouling)accurately (problems with fouling).

Absorption MeasurementAbsorption MeasurementQFT: quantitative filter-pad techniqueSeparate sample to dissolved and particulate. Concentrate particulate material on a filter.

M b f di l dMeasure beam-c for dissolved.Measure transmission through filter pad (correct for ‘filter effects’ path length etc’)filter effects , path-length, etc ).

Absorption MeasurementsAbsorption MeasurementsIn all measurements of transmission (for a or c) there is a need for a ‘reference’c) there is a need for a reference .

Need for a stable (temperature salinity) Need for a stable (temperature, salinity) predictable standard.

Currently we use the cleanest waters we can get…

/l kScatterers in ocean/lakes

• Water + salts – not today• Phytoplankton• Phytoplankton• Non-algal organic particles (CPOM)• Inorganic mineral particles (CPIM)Inorganic mineral particles (CPIM)• Dissolved organic matter (CDOM)

– mostly negligible

OPs are good predictors of PM.

Optics is a standard method to measure turbidity, a primary determinant of water quality ( ISO 7027)(e.g. ISO-7027).

|Model-PM|/PM c(660) bb(700) bs(880)

5% 2% 1% 2%

50% 16% 9% 21%50% 16% 9% 21%

95% 54% 36% 51%

Angular dependence of scattering on size:

•Near forward scattering: Strong dependence on size,

‘large’

‘small’g p ,less on n.

•b /b: Strong dependence on •bb/b: Strong dependence on n, less on size.

Roesler and Boss, 2008

c/V=Qext·G/{1.33πr3}

Resonant curve. Change of max with n (and λ).

c ∝ a and Volume

σext = 2·areac/V ∝ 1/DVo um c/V /D

Mie theory tells us that the relationship between optical properties and mass is size dependent:

bbp/Volume

bp/Volume

1/D

D3

bs /Mass

•All curves are ‘resonant’ curves

•Highest sensitivity for micron sized ti l bsp/Mass particles

•Size of max response varies

Scattering and absorptionObservations:

Mie theory:

A photon absorbed is A photon absorbed is not scattered (at any direction).

Boss et al 2001

Validation:Beam attenuation:c=a+b

Spectral attenuation and PSD:

Boss et al., 2001

Mie Theory (homogenous spheres

c a b

Mie Theory (homogenous spheres, Volz, 1954):

For non-absorbing particles of the d h b l d b same n and an hyperbolic distribution

from Dmin=0 to Dmax=∞,

If:

⎞⎛γ

N(D)=No(D/Do)-ξIf:

then:

( ) ( ) 3 ,0

0 +γ=ξ⎟⎟⎠

⎞⎜⎜⎝

⎛λλ

λ=λγ−

pp cc0 ⎠⎝

expect a relation between attenuation spectrum and PSD.

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 contrary, these organisms are definitely insignificant contributors to the backscattering coefficient.’

Stramski et al., 2001: simulating open-ocean (oligotrophic 0 18mg open-ocean (oligotrophic, 0.18mg Chl/m3), 2-3% of the backscattering coefficient is due t l kt 50% f ti l

Stramski et al., 2001to plankton. 50% from particles <0.2μm.

Phase functions:

St mski t l 2001Stramski 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 b correlates well with [chl] and POC (>0 7 m):Yet bbp correlates well with [chl] and POC (>0.7μm):

Huot et al., 2008 Stramski et al., 2008

Possible explanation for the bb enigma:

1. Mie results are correct. However, all particles in the open ocean covary, hence the tight relationship.

2. Mie theory is not applicable. Organic particle actually backscatter more than we ascribe to themactually backscatter more than we ascribe to them.

How do we obtain the backscattering in the ocean:

bb=2πχβ(θ), 0.7<χ<1.3

Uncertainty ~10%

Boss and Pegau, 2001

Shape consideration

Clavano et al., 2007

Shape approximationsfor light scattering calculations for light scattering calculations

Mie-Theory

T-matrixModerate Axis

10 oblate

Moderate Axis ratios (0.5<AR<2)

12

0.5 T-matrix

0.1

0.1 1 10

prolate0.5

Axis ratios up to convergence limit

Particle radius (μm)0.1 1 10

Slide From Volten

Clavano et al., 2007

Internal structure:

sity

ve in

tens

Rel

ativ

Meyer, 1979Backscattering dominated by membrane.

Measurements across the equatorial Pacific (Dall’olmo et q (al., 2009):

bbp well correlated with cp

bbp(D<0.2mm)

bbp(D>0.2mm) <0.1

No filter effect visible

Uncertainty dominated by uncertainty in bb(H2O)

Aggregate modeling:

2

Latimer (1985)

2

For marine aggregates

4mm

For marine aggregates size and solid fraction correlate.

points having size F as in -points having size-F as in Maggi, 2007, or Khelifa and Hill, 2006.

Theoretical calculations: monodispersionAggregates

Single grain

Observed range{

Single grain

Mass normalized beam attenuation for aggregates assuming a relationship Mass normalized beam attenuation for aggregates assuming a relationship between solid fraction and size as in Khelifa and Hill, 2006 (blue lines) and solid particles (red lines). Solid lines denote particles with n=1.05+i0.0001, dashed lines n=1.05+0.005 and dotted lines n=1.15+0.0001. Each data point prepresent a population of particle all of a single size.

Laboratory experiment: aggregation

Start with <D>~7μm clay Start with <D>~7μm clay

Add salt

As aggregate size As aggregate size changes from 7 70μm cp/mass and bbp/mass stay constant (<size> stay constant (<size> confirmed by microscopy).

Remember:

The two fundamental IOPs:a & β:

b=∫βdΩ∫β

b ∫βdΩc=a+b=a+∫βdΩAd d t i l i l ti tt i (R CDM d Advanced materials: inelastic scattering (Raman, CDM, and phytoplankton pigments), polarization.