Point-source Integrating-cavity Absorption Meter Research Articles

Hyperspectral optical absorption closure experiment in complex coastal waters

AbstractAccurate measurements of absorption data are required for the development and validation of inversion algorithms for upcoming hyperspectral ocean color imaging sensors, such as the NASA Phytoplankton, Aerosol, Cloud, and ocean Ecosystem mission. This study aims to provide uncertainty estimates associated with leading approaches to measure hyperspectral absorption coefficients in complex coastal waters. Absorption spectra were collected at 12 different stations, all located in the Indian River Lagoon, Florida, USA, between 09 January 2017 and 13 January 2017. Measurements included spectral absorption coefficients in the visible range (400–700 nm) associated with dissolved, aCDOM, total particulate, ap, and total nonwater, anw, fractions, and were made both in situ and from discrete samples. Discrete sample approaches included dual‐beam spectrophotometer, liquid waveguide capillary cell, point‐source integrating cavity absorption meter (PSICAM) for dissolved matter absorption samples, and quantitative filter technique ICAM measurements and the dual‐beam spectrophotometer with center‐mounted integrating sphere filter pad technique, while the Turner Designs ICAM, and WET Labs AC‐s, and AC‐9 instruments were used to determine absorption coefficients in situ. The Gershun approach, determining absorption from measurement of the irradiance quartet with respect to depth was also assessed in situ. Measurement uncertainties and relative accuracies were quantified for each of these approaches. Results showed generally strong agreements between different discrete sample methods, with average percent absolute error %δabs < 7% for aCDOM and < 9% for ap. In situ approaches showed higher variability and reduced accuracy. For anw, %δabs deviation relative to PSICAM data was on average 12% to 20%. Results help identify remaining technological gaps and need for improvements in the different absorption measurement approaches.

Stick or Dye: Evaluating a Solid Standard Calibration Approach for Point-Source Integrating Cavity Absorption Meters (PSICAM)

Together with scattering, the process of absorption determines the propagation of light within the water column. It is influenced by the concentration and composition of optically active substances (phytoplankton, chromophoric-dissolved organic matter, and non-living particles). For this reason, the absorption coefficients of a water sample can provide corresponding information. However, the accurate determination of absorption coefficients in natural waters is often difficult due to the usually low concentration of absorbing material and the errors that scattering on particles introduce in the measurements. These problems can be overcome by instruments based on integrating cavities like the point-source integrating-cavity absorption meter (PSICAM). The accuracy of PSICAM measurements is to a large degree related to the accuracy of the measurement of the reflectivity inside its cavity, as this determines its mean optical path length. A reflectivity measurement (‘calibration’) is usually carried out by measuring a liquid dye (nigrosin) with known absorption coefficients, followed by bleaching and rinsing of the cavity. The procedure requires additional equipment like spectrophotometers and handling of a liquid standard. Therefore, it might be difficult or at least non-convenient under field conditions and is additionally a major obstacle for a potential automation of these systems. In the present study, an alternative calibration approach for a PSICAM is evaluated, taking advantage of a solid standard. The standard is characterized and its suitability for calibration is compared to that of a conventional, nigrosin-based reflectivity measurement. Furthermore, the application in an automated flow-through PSICAM system (HyAbS) used in the field is tested. The results show that the performance of the solid standard calibration is comparable to that of the nigrosine-based calibration. Furthermore, it improves the measurements of the automated system. Thus, due to its simplicity, the solid standard calibration might foster the use of PSICAM systems, which allow a more accurate determination of absorption coefficients in natural water samples compared to conventional spectrophotometric techniques. Furthermore, it will potentially facilitate further approaches to automate of these instruments.

Improved determination of particulate absorption from combined filter pad and PSICAM measurements.

Filter pad light absorption measurements are subject to two major sources of experimental uncertainty: the so-called pathlength amplification factor, β, and scattering offsets, o, for which previous null-correction approaches are limited by recent observations of non-zero absorption in the near infrared (NIR). A new filter pad absorption correction method is presented here which uses linear regression against point-source integrating cavity absorption meter (PSICAM) absorption data to simultaneously resolve both β and the scattering offset. The PSICAM has previously been shown to provide accurate absorption data, even in highly scattering waters. Comparisons of PSICAM and filter pad particulate absorption data reveal linear relationships that vary on a sample by sample basis. This regression approach provides significantly improved agreement with PSICAM data (3.2% RMS%E) than previously published filter pad absorption corrections. Results show that direct transmittance (T-method) filter pad absorption measurements perform effectively at the same level as more complex geometrical configurations based on integrating cavity measurements (IS-method and QFT-ICAM) because the linear regression correction compensates for the sensitivity to scattering errors in the T-method. This approach produces accurate filter pad particulate absorption data for wavelengths in the blue/UV and in the NIR where sensitivity issues with PSICAM measurements limit performance. The combination of the filter pad absorption and PSICAM is therefore recommended for generating full spectral, best quality particulate absorption data as it enables correction of multiple errors sources across both measurements.

Evolution of dissolved and particulate chromophoric materials during the VAHINE mesocosm experiment in the New Caledonian coral lagoon (south-west Pacific)

Abstract. In the framework of the VAHINE project, we investigated the spectral characteristics and the variability of dissolved and particulate chromophoric materials throughout a 23-day mesocosm experiment conducted in the south-west Pacific at the mouth of the New Caledonian coral lagoon (22°29.073 S–166°26.905 E) from 13 January to 4 February 2013. Samples were collected in a mesocosm fertilized with phosphate at depths of 1, 6 and 12 m and in the surrounding waters. Light absorption coefficients of chromophoric dissolved organic matter (CDOM) [ag(λ)] and particulate matter [ap(λ)] were determined using a point-source integrating-cavity absorption meter (PSICAM), while fluorescent DOM (FDOM) components were determined from excitation-emission matrices (EEMs) combined with parallel factor analysis (PARAFAC). The evolutions of ag(λ) and ap(λ) in the mesocosm were similar to those of total chlorophyll a concentration, Synechococcus spp. and picoeukaryote abundances, bacterial production, particulate organic nitrogen and total organic carbon concentrations, with roughly a decrease from the beginning of the experiment to days 9–10, and an increase from days 9–10 to the end of the experiment. In the surrounding waters, the same trend was observed but the increase was much less pronounced, emphasizing the effect of the phosphate fertilization on the mesocosm's plankton community. Correlations suggested that both Synechococcus cyanobacteria and heterotrophic bacteria were strongly involved in the production of CDOM and absorption of particulate matter. The increase in phytoplankton biomass during the second part of the experiment led to a higher contribution of particulate material in the absorption budget at 442 nm. The three FDOM components identified (tryptophan-, tyrosine- and ultraviolet C (UVC) humic-like fluorophores) did not follow the evolution of CDOM and particulate matter, suggesting they were driven by different production/degradation processes. Finally, the results of this work support the idea there is indirect coupling between the dynamics of N2 fixation and that of chromophoric material via the stimulation of Synechococcus bloom.

Temperature and salinity correction coefficients for light absorption by water in the visible to infrared spectral region.

The light absorption coefficient of water is dependent on temperature and concentration of ions, i.e. the salinity in seawater. Accurate knowledge of the water absorption coefficient, a, and/or its temperature and salinity correction coefficients, Ψ(T) and Ψ(S), respectively, is essential for a wide range of optical applications. Values are available from published data only at specific narrow wavelength ranges or at single wavelengths in the visible and infrared regions. Ψ(T) and Ψ(S) were therefore spectrophotometrically measured throughout the visible, near, and short wavelength infrared spectral region (400 to ~2700 nm). Additionally, they were derived from more precise measurements with a point-source integrating-cavity absorption meter (PSICAM) for 400 to 700 nm. When combined with earlier measurements from the literature in the range of 2600 - 14000 nm (wavenumber: 3800 - 700 cm(-1)), the coefficients are provided for 400 to 14000 nm (wavenumber: 25000 to 700 cm(-1)).

Evaluation of scatter corrections for ac-9 absorption measurements in coastal waters

Determinations of inherent optical properties of natural waters are fundamental in marine optical research. In situ measurements of light absorption are mostly obtained with an instrument that uses a reflective tube design to reduce concomitant errors induced by light scattering (ac-9, WETLabs Inc.). The remaining, generally still substantial, error is commonly corrected using one of a number of different approaches, each of which is based on a set of assumptions. Until now, the errors in these measurements have only been theoretically examined using Monte Carlo modeling Leymarie et al. (2010). The study presented here used a lab-based point source integrating cavity absorption meter (PSICAM) which avoids scattering errors. The PSICAM data were used to evaluate the absorption determination with an ac-9 in coastal waters for each of the scattering correction approaches. The results showed that the assumption of negligible absorption at wavelengths >700 nm is not valid in coastal waters and that, as a result, ac-9 measurements strongly underestimate absorption at longer wavelengths (>600 nm). An empirical relationship between uncorrected (for scattering) ac-9 measurements and the true absorption at 715 nm was included in the correction scheme; this improved the quality of ac-9 data at longer wavelengths but showed overestimation at shorter wavelengths. However, additional inclusion of a scatter correction for the ac-9 attenuation measurement resulted in a significant improvement of the proportional scatter error correction across the spectrum. Despite these innovations, variations in scattering properties can, combined with low absorption at specific wavelengths, result in relatively large percentage errors for individual measurements.

Evaluation and Improvement of an Iterative Scattering Correction Scheme for in situ Absorption and Attenuation Measurements

Abstract The performance of several scattering correction schemes for reflecting-tube absorption and beam attenuation measurements is evaluated with data collected in European shelf seas. Standard scattering correction procedures for absorption measurements perform poorly because of nonzero absorption in the near infrared and wavelength-dependent scattering phase functions. A previously described iterative correction procedure based on Monte Carlo simulations of the Western Environmental Technologies Laboratories (WET Labs) ac-9 and independent estimates of particle backscattering initially performs poorly, but is greatly improved when realistic losses at flow-tube walls are incorporated into the model. The updated Monte Carlo scattering correction provides excellent agreement with independent absorption and attenuation measurements made with a point-source integrating-cavity absorption meter (PSICAM) and a Laser In Situ Scattering and Transmissometer (LISST, Sequoia Scientific), respectively. Implications for historic datasets and requirements for application to future datasets are discussed.

Flow-through PSICAM: a new approach for determining water constituents absorption continuously

Determination of spectral absorption coefficients in seawater is of interest for biologic oceanographers for various reasons, but faces also several problems, especially if continuous measurements are required. We introduce the flow-through point-source integrating cavity absorption meter (ft-PSICAM) as a new tool for the continuous measurement of spectral absorption coefficients in a range of 400–710 nm. A description of the system is given and its performance in comparison with a conventional PSICAM has been evaluated on two cruises in 2011 in the southern part of the North Sea (German Bight). Furthermore, factors influencing the measurement are discussed. When comparing the data of both systems, a good linear correlation has been found for all wavelengths (r 2 > 0.91). Deviations between systems were different with respect to the wavelength examined with slopes of linear fits between 1.1 and 1.65 and offsets between −0.1 and 0.01, with the higher values at shorter wavelengths. They were caused mainly due to contamination of the flow-through system during operation by phytoplankton particles. Focus was also laid on the measurement of chlorophyll-a concentrations ([chl-a]) and total suspended matter concentrations ([TSM]) on the basis of absorption coefficient determination. For this, appropriate relationships were established and [chl-a] and [TSM] values were calculated from the relevant ft-PSICAM absorption coefficients. Their progression matches well with the progression of fluorescence and turbidity measurements made in parallel. In conclusion, the ft-PSICAM is successful in measuring spectral absorption coefficients continuously and resolving relative changes in seawater optical properties.

Measurement of light absorption by aquatic particles: improvement of the quantitative filter technique by use of an integrating sphere approach

Determination of particulate absorption in natural waters is often made by measuring the transmittance of samples on glass-fiber filters with the so-called quantitative filter technique (QFT). The accuracy of this technique is limited due to variations in the optical properties of the sample/filter composite, and due to uncertainties in the path-length amplification induced by multiple scattering inside the filter. Some variations in the optical properties of the sample/filter composite can be compensated by additional measurements of the filter's reflectance (transmittance-reflectance method [T-R] [S. Tassan and G. M. Ferrari, Limnol. Oceanogr. 40, 1358 (1995)]). We propose a different, rarely used approach, namely to measure the filter's absorptance in the center of a large integrating sphere, to avoid problems with light losses due to scattering. A comparison with other QFTs includes a sensitivity study for different error sources and determination of path-length amplification factors for each measurement technique. Measurements with a point-source integrating-cavity absorption meter were therefore used to determine the true absorption. Filter to filter variability induced a much lower error in absorptance compared to a measured transmittance. This reduced error permits more accurate determination of the usually low absorption coefficient in the near IR spectral region. The error of the T-R method was lower than that of the transmittance measurement but slightly higher than that of an absorptance measurement. The mean path-length amplification was much higher for the absorptance measurement compared to the T-R method (4.50 versus 2.45) but was found to be largely independent of wavelength and optical density. With natural samples the path-length amplification was less variable for the absorptance measurement, reducing the overall error for absorption to less than ±14%, compared to ±25% for the T-R method.

Assessment of the scattering by sub-micron particles in inland waters

Current protocols prescribe 0.2 µm filters for absorption measurements of Coloured Dissolved Organic Matter (a(CDOM)). However, a large historical dataset of a(CDOM) was obtained over 0.7 µm filters. Since small particles may pass 0.7 µm filters, a(CDOM) was probably overestimated because of residual scattering. Furthermore, current protocols require suspended matter to be filtered over 0.7 µm. So the 0.2 µm - 0.7 µm fraction is missing from the particles and attributed to enhanced a(CDOM), potentially leading to poor optical closure. In this paper natural water samples from a eutrophic lake were analysed with both filter pore sizes and two different absorption determination methods. Besides a standard bench spectrometer with a cuvette a calibrated Point-Source integrating-Cavity Absorption Meter (PSICAM) was used to gauge the absorption measurements. After filtration over 0.7 µm we found a systematic 8% overestimation of a(CDOM) measured in the cuvette caused by residual scattering. By filtering over 0.2 µm the a(CDOM) cuvette values are 6% underestimated because also a significant fraction of absorption was removed from the sample. As a consequence we recommend testing for coloured residues on the 0.2 µm filter after prime filtration with a glass-fibre (GF/F) filter. In case the 0.2 µm filter shows colouration, the method presented in this paper is a fast and simple check to correct the CDOM measurements.

Measurements of optical absorption by chromophoric dissolved organic matter using a point‐source integrating‐cavity absorption meter

The light absorption of 0.22‐µm filtered seawater samples from the North Sea and the Atlantic Ocean was measured by a point‐source integrating‐cavity absorption meter (PSICAM), and the results were compared with measurement in a spectrophotometer. The absorption coefficient of this Gelbstoff or chromophoric dissolved organic matter (CDOM) ranges from high values of 0.103 to 0.519 m−1 at 442 nm for samples from the North Sea to very low values of 0.004 to 0.046 m−1 for samples from the Atlantic Ocean. At these low values, the influence of sample temperature and salinity on water absorption was significant. Hence, instrument‐specific temperature and salinity coefficients were determined and, subsequently, used for correcting absorption data. Spectrophotometrically determined absorption was significantly higher at shorter wavelengths (< 442 nm) than absorption measured by the PSICAM, probably showing a significant influence of scattering by small particles remaining in the sample after filtration. At wavelengths between 442 and 500 nm, the correlation of PSICAM and spectrophotometer followed the 1:1 correspondence, whereas at longer wavelengths, the absorption was below the detection limit of the spectrophotometer. A similar agreement was obtained for the exponential slope of the CDOM absorption. The PSICAM matches the spectrophotometer results but is more sensitive and not affected by scattering. It will improve our ability to determine CDOM absorption in oligotrophic waters for the visible light spectral region.

Determination of the particulate absorption of microalgae using a point‐source integrating‐cavity absorption meter: verification with a photometric technique, improvements for pigment bleaching, and correction for chlorophyll fluorescence

The determination of the spectral light absorption coefficient of planktonic algae in seawater is crucial for many applications, including optical remote sensing. Common techniques are adversely affected by light‐scattering effects, or by low particle concentrations in situ. Measurements with a point‐source integrating‐cavity absorption meter (PSICAM) can overcome both difficulties. In our study, we assessed the performance of a PSICAM for measuring the absorption coefficients of microalgae by a comparison with state‐of‐the‐art techniques. The PSICAM was evaluated by comparative measurements of diluted algal cultures with a photometric setup, where the sample cuvette is placed in the center of an integrating sphere (CIS). The accuracy of both the CIS and the PSICAM technique was calibrated against a commercial spectrophotometer by measurements of dense algal cultures. The remaining sources of error for the PSICAM technique were the bleaching procedure and chlorophyll fluorescence. We propose an improved bleaching technique and introduce a correction procedure for the fluorescence effects. Applying both, the overall differences between PSICAM and CIS measurements were less than ± 1%. We further demonstrate that the accuracy of the PSICAM is at least as good as that of a commercial spectrophotometer, but its precision should be two to three orders of magnitude higher. Due to its high sensitivity and accuracy, and to the fact that sources of error can be identified and accounted for, the PSICAM has a great potential to significantly improve the determination of absorption spectra of particulate matter in natural seawater.

Practical test of a point-source integrating cavity absorption meter: the performance of different collector assemblies

A prototype point-source integrating cavity absorption meter (PSICAM) is presented and compared with spectrophotometric absorption measurements. Different light collector assemblies of the PSICAM were tested regarding their capability to determine the absorption of water constituents accurately over a wide range of concentrations and scattering properties. The PSICAM setup with a radiance-type sensor showed the best performance. It was compared with a photometric absorption determination using nonscattering dye solutions. The mean difference between both methods was less than 2.4% in the spectral range of 400-700 nm. The absorption determination with the PSICAM, when equipped with a radiance sensor as a light collector, was only little affected by scattering and temperature. We conclude that the PSICAM can be used to determine the absorption of natural seawater samples at ambient temperatures.

Analysis of a point-source integrating-cavity absorption meter

We evaluate the theoretical performance of a point-source integrating-cavity absorption meter (PSICAM) with Monte Carlo simulations and a sensitivity analysis. We quantify the scattering errors, verifying that they are negligible for most ocean optics applications. Although the PSICAM detector response is highly sensitive to the value of the wall reflectivity, the absorption of an unknown fluid can be accurately determined with a PSICAM if appropriate reference solution(s) are chosen. We also quantify the error that results if the source is not perfectly isotropic, finding that moderate amounts of source anisotropy can be tolerated provided that the detector is properly located with respect to the source.

Point-source integrating-cavity absorption meter: theoretical principles and numerical modeling

A mathematical model has been developed for photon behavior within a spherical integrating-cavity absorption meter with a point light source at the center of the cavity. Explicit expressions for the average number of collisions with the wall per photon, the average path length per photon, and the transmittance of the cavity containing a water sample relative to that containing pure water are derived for an absorbing nonscattering medium. Monte Carlo modeling shows that the operation of the point-source integrating-cavity absorption meter is essentially unaffected by scattering. Calculations of the performance of the absorption meter as a function of the cavity diameter, absorption coefficient of the medium, and the reflectivity of cavity are presented.