in dioxane solution was investigated under xenon lamp irradiation. The quantum yields (b2) for direct photodegradation of these TCDDs were 1.009x10-3, 2.798 X103, and 1.990x10-2mol einstein-1, respectively, at 313 nm, which is a wavelength usually used in determining the quantum yield of a test compound. However, the quantum yields of these TCDDs at wavelengths of the maximal photodegradation peak at more than 295nm were 2.133X103, 3.020x10-3, and 3.283X10-2, respectively, at their maximal photodegradation peaks, and were higher than those at 313 nm. Based on these quantum yields and the measured absorption spectra together with the solar intensity data available in the literature, we estimated the direct sunlight first-order photodegradation rate constants of these TCDDs under conditions of variable sunlight intensities on the surface of water bodies during various seasons at 40N latitude. Also determined were the corresponding half-life times.

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