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How Are Changing Solar Ultraviolet Radiation and Climate Affecting Light-induced Chemical Processes in Aquatic Environments?
Citation:
ZEPP, R. G. How Are Changing Solar Ultraviolet Radiation and Climate Affecting Light-induced Chemical Processes in Aquatic Environments? . Presented at Northeast Regional Meeting of the American Chemical Society, Burlington, VT, June 29 - July 02, 2008.
Impact/Purpose:
Presentation on how changing solar ultraviolet radiation and climate are affecting light-induced chemical processes in aquatic environments.
Description:
Changes in the ozone layer over the past three decades have resulted in increases in solar UV-B radiation (280-315 nm) that reach the surface of aquatic environments. These changes have been accompanied by unprecedented changes in temperature and precipitation patterns around the globe. Here the interactions of these environmental changes with light-induced chemistry are highlighted. The chromophoric (or colored) component of dissolved organic matter (DOM) in aquatic environments, often referred to as CDOM, strongly absorbs solar ultraviolet (UV) radiation and thus controls the penetration of biologically harmful solar UV into many freshwater and marine ecosystems. Climatic conditions can strongly influence CDOM concentrations and thus UV penetration. For example, long periods of dry, warm weather reduce CDOM inputs, thus permitting the removal processes to significantly reduce CDOM levels and enhance UV penetration. This effect can be amplified during the summer when stratification greatly slows inputs of CDOM into the upper layers of a water body from deeper waters. Photodegradation of CDOM results in loss of UV and visible absorbance, a process referred to as "photobleaching". Action spectra (plots of the ratio, molecules reacted: incident photons, versus wavelength) indicate that the UV-B component of sunlight is most effective at inducing CDOM photoreactions. In addition to spectral changes, photoreactions result in changes in the biological availability of CDOM carbon- and nitrogen-containing constituents and of metal-DOM complexes. For example, the organonitrogen components are photo degraded to produce ammonium. Photoreactions of organic complexes with metals (e.g. iron, copper) involve either direct photoreactions of the complexes or reactions of the complexes with reactive oxygen species that are produced photochemically. Methods that are being used to study these interesting reactions in the laboratory and field will be discussed as will approaches to modeling the effects of these photoreactions on aquatic biogeochemical cycles.