Simplified model of spectral absorption by non-algal particles and dissolved organic materials in aquatic environments.

B B Cael,Emmanuel Boss

doi:10.1364/oe.25.025486

Abstract

Absorption by non-algal particles (NAP, ad) and colored dissolved organic matter (CDOM, ag) are frequently modeled by exponential functions of wavelength, either separately or as a sum. We present a new representation of NAP-plus-CDOM absorption adg based on the stretched exponential function adg(λ) = A exp{-[s(λ - λo)]β}, whose parameter β can be considered a measure of optical heterogeneity. A double exponential representation of adg can be fit extremely well by a stretched exponential for all plausible parameter combinations, despite having one fewer free parameter than a double exponential. Fitting two published compilations of in situ adg data - one at low spectral resolution (n = 5, λ = 412-555 nm) and one at high spectral resolution (n = 201, λ = 300-700 nm) - the stretched exponential outperforms the single exponential, double exponential, and a power law. We thereby conclude that the stretched exponential is the preferred model for adg absorption in circumstances when NAP and CDOM cannot be separated, such as in remote sensing inversions.

Highlights

The total absorption spectrum a(λ) in aquatic environments is frequently decomposed as the sum of four components (e.g. [1])a(λ) = aw (λ) + aφ (λ) + ad (λ) + ag (λ) (1)where aw (λ) is the absorption by water, aφ (λ) is the absorption by phytoplankton pigments, ad (λ) is the absorption by non-algal particles (NAP), and ag (λ) is the absorption by colored dissolved organic materials (CDOM)
We present a new representation of NAP-plus-CDOM absorption adg based on the stretched exponential function adg (λ) = A exp{−[s(λ − λo )]β }, whose parameter β can be considered a measure of optical heterogeneity
We thereby conclude that the stretched exponential is the preferred model for adg absorption in circumstances when NAP and CDOM cannot be separated, such as in remote sensing inversions

Summary

Introduction

CDOM is defined as material which passes through a given filter (most often with pore size 0.2 μm) and NAP is defined as that which does not (other than soluble pigments, which are separated into aφ), forming a continuum of material in terms of size [8]. This suggests it would be useful to model the absorption by NAP and CDOM as a the absorption of a continuum of heterogeneous material.

Objectives

Discussion

Conclusion