Citation:

WANG, W., O. C. ZAFIRIOU, I. Y. CHAN, R. G. ZEPP, AND N. V. BLOUGH. PRODUCTION OF HYDRATED ELECTRONS FROM PHOTOIONIZATION OF DISSOLVED ORGANIC MATTER IN NATURAL WATERS. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 41(5):1601-1607, (2007).

Impact/Purpose:

The overall objective of this task is to develop quantitative relationships for assessing the vulnerability of aquatic ecosystems (freshwater and coastal) and their services 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. and Brazil These activities are contributing to two APGs in the ecosystems focus area of the Global Change Research Multiyear Plan: the 2008 APG (APG 2) on developing information and tools that managers will use in their decision-making about how to adapt to the effects of global change on aquatic ecosystems; and the 2010 APG (APG 3) on providing information and models that will support development of biocriteria for corals. 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 in the Florida Keys. This collaboration will contribute to one ERD APM in 2006 and three joint NERL-NHEERL APMs in the 2008 - 2010 period. Other research is examining the effects of changing climate and UV on microbial activity in waters close to beaches in the U.S. Work is being completed on the interactions of land use and climate changes with the ecological functioning of streams in watersheds of the southeastern U.S. The task also includes two sub-tasks that are funded mainly by funds-in IAGs. One sub-task funded by NASA involves research in central Brazil that is part of the Large Scale Biosphere Atmosphere Experiment (LBA). This work involves a close collaboration between EPA and a group of scientists from the Department of Ecology, University of Brasilia, Brazil. 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. Another sub-task funded by the Office of Naval Research is examining interactions between nitrogen and organic substances in aquatic ecosystems that produce the colored dissolved organic matter (CDOM) that controls penetration of solar UV radiation into coastal waters.

Description:

Under UV irradiation, an important primary photochemical reaction of colored dissolved organic matter (CDOM) is electron ejection, producing hydrated electrons (e-aq). The efficiency of this process has been studied in both fresh and seawater samples with both steady-state scavenger (SS-S) and time-resolved laser flash photolysis (LFP) methods. However, the apparent quantum yields (AQYs) of (e-aq) for the same samples using the two methods differ by as much as a factor of 100, calling for a closer re-examination of how the process is measured. We developed a highly sensitive multi-pass LFP apparatus that allows detection of transient species at very low and variable UV irradiation intensities. Under single-photon conditions, we measured the AQY from Laurentian fulvic acid as 1.3 x 10(-4), and set the upper limit for other CDOM samples at 6 x 10(-5), bringing the LFP results very close to those from SS-S methods. We also examined the ionization at elevated irradiation intensities and clearly demonstrated that multiphoton ionization occurs at intensities well below those usually used in LFP experiments. The multiphoton ionization is probably the cause of the high AQYs reported by earlier LFP work. In addition, we also observed in real time other photochemical reactions, such as triplet quenching and bleaching, in the single photon regime.

Record Details: