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Lab 2 CDOM Absorption

4 June 2019

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Intro

  • CDOM (colored dissolved organic matter) is a complex substance with specific optical properties
  • Filter through 0.2 µm pore size to remove particulates.

Filter 0.2 µm

http://www.dpi.inpe.br/labisa/static/doc/Workshop_LabISA_alinedmv.pdf

Collin Roesler

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Methods

  • Measuring absorption with WETlabs ac9, acs, and Cary Spectrophotometer

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  • Analyzed filtered samples of “pure” DI water, DI + salt water, DRE water, Biscay Pond (BP) water at some different temperatures.

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  • Measured absorption with lux meter, dyed water & box setup

WetLabs AC-S Cary Spectrophotometer

Lux meter

Cardboard Box

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Beer-Lambert Law Introduction

Amount of Beer (or light)

Time (or depth or distance)

Thirst effects dominate

1/IQ effects dominate

0

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Pink dye

Similar slopes for groups 2A/B

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Green dye

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Absorption and Dye Concentration

Absorbance increases with dye concentration.

A linear relationship is expected for a given wavelength according to Beer’s Law.

We may not be effectively filtering light in a narrow bandwidth, and hence some (lower) wavelengths might be reaching the collector when there is low dye concentration, but they are “lost” (completely absorbed) when the concentration increases.

An important highlight is the error on the measurements. Propagating the instrument’s oscillation we get errors between 10% to 50% of A (decreasing for higher concentrations).

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Absorption and Temperature

How does the absorption spectrum of purified water vary with temperature? Be specific about the wavelength ranges, compute for the specific wavelengths, or the spectrum.

How does temperature-dependent absorption impact the absorption measurement for discrete samples in the lab?

How does temperature-dependent absorption impact the absorption measurement in the ocean?

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Absorption and Temperature

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→ In near infrared (NIR), you can expect higher absorption differences due to temperature

→ Our “pure” water was not completely pure due to some absorption in the blue wavelengths

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Comparison between empirically derived ∆a/∆T and theoretical 𝚿T spectra (Sullivan et al., 2006)

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a (m-1) = 2.303 * A / L

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Absorption and Salinity

With T correction Without T correction

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Absorption and Salinity

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Pathlength vs. a(440)

Effects of Geometric Pathlength

  • Absorbance is linearly related to pathlength
  • Absorption coefficient at 440nm is inversely proportional to pathlength because of shadowing effects

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Optical Depth vs. Pathlength

  • Optical depth = a*pathlength
  • Dimensionless coefficient that gives a measure of opaqueness of a material
  • It’s recommended that the optical depth be kept between 0.1 and 0.4
  • That’s because it’s important to keep the equivalent of a thin layer so no event is shaded
  • Optimal pathlength depends on concentration of sample

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CDOM Spectral Slopes

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CDOM Spectral Slopes

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CDOM Spectral Slopes