One- and Two-Band Sensors and Algorithms to Derive aCDOM(440) from Global Above- and In-Water Optical Observations

Abstract

The colored (or chromophoric, depending on the literature) dissolved organic matter (CDOM) spectral absorption coefficient, aCDOM(λ), is a variable of global interest that has broad application in the study of biogeochemical processes. Within the funding for scientific research, there is an overarching trend towards increasing the scale of observations both temporally and spatially, while simultaneously reducing the cost per sample, driving a systemic shift towards autonomous sensors and observations. Legacy aCDOM(λ) measurement techniques can be cost-prohibitive and do not lend themselves toward autonomous systems. Spectrally rich datasets carefully collected with advanced optical systems in diverse locations that span a global range of water bodies, in conjunction with appropriate quality assurance and processing, allow for the analysis of methods and algorithms to estimate aCDOM(440) from spectrally constrained one- and two-band subsets of the data. The resulting algorithms were evaluated with respect to established fit-for-purpose criteria as well as quality assured archival data. Existing and proposed optical sensors capable of exploiting the algorithms and intended for autonomous platforms are identified and discussed. One-band in-water algorithms and two-band above-water algorithms showed the most promise for practical use (accuracy of 3.0% and 6.5%, respectively), with the latter demonstrated for an airborne dataset.

Verification and Validation of Hybridspectral Radiometry Obtained from an Unmanned Surface Vessel (USV) in the Open and Coastal Oceans

“…Improvements to the in-water approach were documented by [9,[15][16][17] with the resulting improvements to higher-order applications, e.g., algorithms, demonstrated by [10,27]. Similar accomplishments in above-water applications of microradiometers were presented by [30,[32][33][34].…”

“…As such, an increased breadth of in situ measurements is required, which will likewise require either increased funding for manual sampling activities or a community effort to advance automated sampling capabilities. Presently, activities to support autonomous ocean observing capabilities are increasingly common, demonstrated for example by expanded radiometric capabilities for Argo floats [14], algorithmic improvements to reduce costs in drone and float platforms [30], and autonomous airborne systems for opportunistic monitoring of inland bays and lakes [10,30,32,33].…”

“…All the absolute (φ) differences in Table 4 are within 5%, with the smallest differences associated with the K d (λ) input variables. In addition to being the variable with the best agreement in Table 3, K d (λ) was also shown by [30] to provide superior wavelength-bywavelength a CDOM (440) estimation than L W (λ) N for single-channel sensors. The largest differences in Table 4 are associated with the hyperspectral bandpass PBH and multispectral PMS data for the R rs (λ) and L W (λ) N input variables.…”

“…Verification is used herein to expressly acknowledge when a desired comparison cannot satisfy all the requirements for validation. Additional aspects of these two distinctions, the situations governing the usage of each comparison type, and possible criteria for assessing the utility of measurements are provided in pertinent part by the Protocols and more recently by [10,30,31].…”

“…In both algorithms, band ratios of optical parameters are used as inputs, but the a CDOM (440) algorithm can use both K d (λ) and L W (λ) N , whereas OC4 uses R rs (λ). The single-band versions of the EMA a CDOM (440) algorithm [30] are not considered. The combination of band ratios for the selected algorithms provides an analysis with a diverse set of input variables and spectral attributes.…”