IOP measurements
IOPs, such as absorption, scattering, and beam attenuation coefficients can be measured using a variety of sensors and methods (see table below). In-situ spectrophotometers or transmissometers are designed for use in various environments, including being submersed in the water column or used as benchtop instruments. Water is pumped through tubes or an integrating cavity and measurements of IOPs across one or many wavelengths are taken by internal sensor, and internal sensors take measurements of IOPs across one or many wavelengths. Measurement units can vary depending on the property, but typically, absorption and scattering coefficients are expressed in units of per meter (m⁻¹), or in units of per meter per steradian (m-1 sr-1).
IOP sensors require careful calibration, correction, and field deployment procedures. Absolute calibration standard reference materials, such as pure water, certified absorption standards, calibrated scattering standards, or stable absorbing and scattering materials, are needed for their calibration. Absorption and scattering are temperature and salinity dependent, and ideally corrected using simultaneous temperature and salinity measurements and well documented correction equations (Morel, 1974; Pegau and Zaneveld, 1994; Pegau et al., 1997).
Examples of IOPs and in-situ sensors used for measuring them, including pros and cons for each type of sensor. Note, this is not an exhaustive list and we do not endorse any particular sensors or companies. Specific sensors named here are simply to provide examples.
|
Sensor types |
Example Sensor |
Pros |
Cons |
|
Absorption (a) |
Absorption and attenuation sensors (e.g., Sea-Bird Scientific ac-s) or absorption sensors (e.g., TriOS OSCAR) |
Sensors can be used for laboratory and/or in-situ absorption and/or attenuation measuring including profiling Sensors can be calibrated for water quality attribute measuring ac-s is a spectrophotometer providing hyperspectral absorption and attenuation whereas OSCAR is a flow-through online hyperspectral integrating cavity absorption meter providing accurate absorption |
Sampling location specific calibrations can be needed for water quality attribute measuring Different instrument designs and approaches to account for scattering can lead to different measurement accuracies Pathlengths optimized for oceanic waters may be too long for turbid waters resulting in low signal. Degassing OSCAR in situ can be challenging |
|
Backscatter (Bb) |
Scattering sensors |
Sensors can be used for bbp measuring representative of particle abundance ECO Triplet sensors can be customized for bb, bb-based, and/or fluorescence-based water quality attribute measuring in up to three channels ECO Puck sensors are specifically designed for use in AUVs, profiling floats, and Slocum gliders Hyper-bb can be used for hyperspectral bb measuring and addresses the limited spectral coverage of existing bb sensors |
Default gains of ECO sensors can lead to channel saturation in some waters Sampling location specific range adjustments or calibrations can be needed for bb- and fluorescence-based water quality attribute measuring Some sensor calibrations are challenging and sensors used for bb measuring are typically serviced at the manufacturers LISST-VSF is single wavelength |
|
Beam attenuation (c) |
Absorption and attenuation sensors (e.g., Sea-Bird Scientific ac-s) or transmission and attenuation sensors (e.g., TriOS VIPER, Sea-Bird Scientific C-Star, and C-Rover, Sequoia Scientific LISST-Tau) |
Sensors can be used for laboratory and/or in-situ absorption and/or attenuation measuring including profiling ac-s is a spectrophotometer providing multispectral absorption and attenuation whereas VIPER is a VIS transmissometer providing hyperspectral attenuation Other sensors include transmissometers targeting single wavelength transmission and/or attenuation |
Different instrument designs including different fields of view (FOV) and approaches to account for scattering at angles smaller than the FOV can lead to different measurement accuracies |
References
Morel, A., 1974. Optical properties of pure water and pure seawater, in: Optical Aspects of Oceanography. Academic Press.
Pegau, W.S., Zaneveld, J.R.V., 1994. Temperature dependence of the absorption coefficient of pure water in the visible portion of the spectrum, in: Jaffe, J.S. (Ed.), Presented at the Ocean Optics XII, Bergen, Norway, pp. 597–604. https://doi.org/10.1117/12.190104
Pegau, W.S., Gray, D., Zaneveld, J.R.V., 1997. Absorption and attenuation of visible and near-infrared light in water: dependence on temperature and salinity. Appl. Opt. 36, 6035. https://doi.org/10.1364/AO.36.006035