Optical Measurements Optical Measurements & &

KKd d valuesvalues

Elizabeth Cox29 November 2010

How is Light Measured?How is Light Measured?Optical measurements are based on light

penetration through the water column◦ Scalar irradiance sensor- PAR region of spectrum ◦ Vector irradiance sensor (cosine collector)

Measure scalar irradiance◦ 2π- surface (on deck of boat)◦ 4π- water column at varying depths

Determine the diffuse attenuation coefficient (Kd)

Instrumentation: Instrumentation: 22ππ and 4 and 4ππ scalar irradiance scalar irradiance sensorssensors

4π

2π

Instrumentation:Instrumentation:22ππ scalar irradiance sensor scalar irradiance sensor

Instrumentation: Instrumentation: 44ππ scalar irradiance sensor scalar irradiance sensor

IrradianceIrradianceAmount of light

◦ Exponential relationship with depth

Units: µEinm-2s-1

Measurements taken in the field:◦ Surface- deck of ship◦ Deep- water column

0.5, 1, 2, 3, 4, 4, 3, 2, 1, 0.5 (meters)

Example: Irradiance vs Example: Irradiance vs DepthDepth

Light AttenuationLight AttenuationDecrease in light intensity with depth in

the water column

Dependent on amount of light absorbed or scattered◦ Absorbs: water, CDOM, suspended sediments,

phytoplankton◦ Scatters: water, suspended sediments, small

phytoplankton and bacteria

Diffuse attenuation coefficient Diffuse attenuation coefficient (K(Kdd))Attenuation coefficient- quantifies

the rate at which light is attenuated

Apparent optical property◦ Magnitude is a function of optical

properties of water and light direction◦ Not always constant with depth◦ Varies by location

Units: m-1

Diffuse attenuation coefficient (KDiffuse attenuation coefficient (Kdd) ) continuedcontinuedDepends on solar angle of light

reflection◦Dependence is small so we neglect it in

our measurements◦Simultaneous air and water

measurement taken to reduce dependence

Larger Kd = more light attenuated◦Phytoplankton production is low in

estuary of LCFRhttp://uncw.edu/cms/aquaticecology/laboratory/lcfrp/WQ%20Reports/LCFRP%202005%20report/LCFRP%202005%20Completed%20Report.pdf

Errors in KErrors in Kdd values values

Fluctuations in incoming light◦Correct by:

1. Two system to normalize (air and water data)

Perturbations by ship (shading)◦Correct by:

1. Leaning over side of ship to take measurements

2. Take measurements from sunny side of ship

Steps to Determine KSteps to Determine Kdd

1. Take light measurements in the field

2. Calculate ln(Ed(0)/Ed(z))

3. Calculate Kd using the ln(Ed(0)/Ed(z)) data from any two depths

Example: Determination of Example: Determination of KKdd

Station 61 Date: 9/13/10 Time: 11:05 am Cruise 1

Depth (m) Deck (PAR) Ed(0) Watercolumn (PAR) Ed(z) ln Ed(0)/Ed(z)

0.5 2444 766.3 1.1598

1.0 2449 288.1 2.1401

2.0 2449 41.85 4.0693

3.0 2448 7.193 5.8299

4.0 2443 1.0589 7.7438

4.0 2445 1.05 7.7530

3.0 2448 5.594 6.0813

2.0 2446 39.26 4.1320

1.0 2443 231.5 2.3564

0.5 2441 697.7 1.2524

Kd = (2.1401- 1.1598)/(1.0-0.5) = 1.96 m-1

KKdd values for 2010 values for 2010

Station Cruise 1 (September) Cruise 2 (November)

M18 0.733 1.311M23 1.070 1.303M35 1.790 2.200M42 1.650 2.099M54 2.200 2.090M61 1.860 2.230HB 2.280 3.429

Example: KExample: Kdd vs Depth vs Depth

Year HB M61 M54 M42 M35 M23 M18

LCFR data Sept Nov Sept Nov Sept Nov Sept Nov Sept Nov Sept Nov Sept Nov

2005 2.82 4.76 2.31 4.34 2.37 3.72 1.61 3.98 1.71 3.53 1.16 2.23 1.02 2.14

2007 2.56 3.01 1.44 1.64 1.74 1.62 1.23 1.4 0.98 0.97 0.51 0.57 0.83 0.69

2008 4.02 3.54 3.03 2.91 2.07 2.44 2.48 2.28 1.91 2.14 1.01 1.71 1.07 1.68avera

ge3.13

33.77

02.26

02.96

32.06

02.59

31.77

32.55

31.53

32.21

30.89

31.50

30.97

31.50

3

                             Class data

Cruise 1

Cruise 2

Cruise 1

Cruise 2

Cruise 1

Cruise 2

Cruise 1

Cruise 2

Cruise 1

Cruise 2

Cruise 1

Cruise 2

Cruise 1

Cruise 2

2008 3.77 2.81 2.37 2.13 2.19 1.92 2.2 1.69 2.11 2.82 1.58 2.43 2.05 2.56

2009 2.6 2.46 2.59 2.46 2.11 2.4 2.15 1.95 2.05 1.79 1.37 1.43 1.36 1.55avera

ge3.18

52.63

5 2.482.29

5 2.15 2.162.17

5 1.82 2.082.30

51.47

5 1.931.70

52.05

5

                             

2010 2.283.42

9 1.86 2.23 2.2 2.09 1.652.09

9 1.79 2.2 1.071.30

30.73

31.31

1

Historical Kd values

Relationship: KRelationship: Kdd & Salinity & SalinityKd decreases with increasing

salinity

Salinity and DOC are often inversely related◦Relationship shows that

chromophoric carbon (CDOM) is a major factor in absorption of light

Relationship: KRelationship: Kdd & Salinity & Salinity

Cruise 1 Cruise 2

Station Salinity Kd Salinity Kd

m18 35.7 0.733 32.9 1.311

m23 35.5 1.07 33.5 1.303

m35 30.3 1.79 25.8 2.2

m42 27.1 1.65 22.3 2.099

m54 21.8 2.2 15.9 2.09

m61 18.4 1.86 9.5 2.23

HB 7.9 2.28 9.8 3.429

M18

Relationship: KRelationship: Kdd & DOC & DOC

Kd increases with increasing DOC concentration due to chromophoric carbon increase◦Intensity of light in water column

decreases

Relationship: KRelationship: Kdd & & turbidityturbidityKd increases with increasing

turbidity

Intensity of light decreases in more turbid waters

• Suspended sediments scatter light in all directions

M18

M23

M54

M23

Conclusions:Conclusions:Intensity of light in water column

decreases as a function of depth due to:◦High CDOM concentrations◦Turbid water

Station HB- more turbid, high CDOM concentration more light is attenuated (large Kd value)

Thanks for Thanks for your your

attention!attention!