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ABSORBANCE, ABSORPTION COEFFICIENT, AND APPARENT QUANTUM YIELD: A COMMENT ON AMBIGUITY IN THE USE OF THESE OPTICAL CONCEPTS
Citation:
Hu, C., F. E. MullerKarger, AND R G. Zepp. ABSORBANCE, ABSORPTION COEFFICIENT, AND APPARENT QUANTUM YIELD: A COMMENT ON AMBIGUITY IN THE USE OF THESE OPTICAL CONCEPTS. LIMNOLOGY AND OCEANOGRAPHY 47(4):1261-1267, (2002).
Impact/Purpose:
The overall objective of this task is to develop quantitative relationships for assessing the vulnerability of aquatic resources to global change. The task will contribute experimental and modeling tools for assessments of the interactions of global climate and UV changes with coral reefs and selected watersheds and estuaries in the U.S. These activities are contributing primarily to two APGs in the ecosystems component of the Global Change Research Multiyear Plan: the 2006 APG (APG 3) on building the capacity to assess global change impacts on coastal aquatic ecosystems, including coral reefs and estuaries and the 2004 APG (APG 2) on building capacity to assess and respond to global change impacts on selected watersheds. One major task objective is to assess interactions of global warming and UV exposure that are contributing to the observed coral bleaching and disease. Our lab is working with scientists at the NHEERL Gulf Ecology Lab to characterize UV exposure and effects at several coral reef sites. Other research in this task is examining the interactions between UV-induced breakdown of refractory organic matter in estuaries and coastal areas that enhance UV penetration into the water and concurrently form biologically-labile nitrogen-, phosphorus- and carbon-containing substances that stimulate productivity and microbial activity. This task also involves research in central Brazil that is part of the Large Scale Biosphere Atmosphere Experiment (LBA). The objectives of this project are to assess the impacts of land use and climatic changes on soil nutrient cycles and microbiota, trace gas exchange and water quality in the Brazilian cerrado. This work involves a close collaboration between EPA and a group of scientists from the Department of Ecology, University of Brasilia, Brazil. Other objectives of this task are to assess the interactions of land use and climate changes with the ecological functioning of streams in watersheds of the Piedmont region of the southestern U.S.
Description:
Several important optical terms such as "absorbance" and "absorption coefficient" are frequently used ambiguously in the current peer-reviewed literature. Since they are important terms that are required to derive other quantities such as the "apparent quantum yield" of photoproduction, ambiguity in the application of these concepts leads to results that are difficult or impossible to interpret correctly. Such ambiguity also hinders comparison of results between studies, and ultimately harms proper parameterization of numerical models of oceanic processes as well as refinement of remote sensing algorithms. We review these concepts and the implications of such ambiguities. A few simple recommendations that follow conventions developed by optical oceanographers are provided to authors dealing with these concepts. In particular, the symbol "a" is recommended for the absorption coefficient (in Napierian form, m-1), which is also preferred over absorbance (dimensionless) when data are presented; the symbol "a" is not recommended for absorbance; the symbol "A" should be used with caution because although it has been widely used for absorbance in photochemistry and photobiology, it has also been used for absorptance in physics and optical oceanography; the term "absorptivity" is not recommended due to conflicting definitions in the current literature; the "pathlength" value should always be given whenever absorbance data are presented; and "normalization" of photoproduction rates to absorbance or absorption coefficient should be performed only on optically thin samples unless the inner filter effects are accounted for and corrected.