Photolysis of Chloral under Atmospheric Conditions

The photolysis of chloral under atmospheric conditions was studied at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. The photodissociation rate coefficient, J(chloral), was measured directly under different sunlight conditions during April 1999. Values in the range of J(chloral) = (4.61-6.11) × 10^-5 s^-1 were obtained, yielding an average value of J(chloral)/J(NO₂) = (6.15 ± 0.62) × 10^-3. This corresponds to a photolysis lifetime of 4.5-6 h under conditions appropriate to the solar flux during summer months and confirms that atmospheric photolysis is the major degradation pathway for chloral. The overall quantum efficiency of photolysis under atmospheric conditions was determined to be 1.00 ± 0.05. The atmospheric photolysis of chloral produced phosgene, CO, and Cl atoms with molar yields of 0.83 ± 0.04, 1.01 ± 0.05, and 1.18 ± 0.06, respectively. The product yield data are consistent with a mechanism in which the primary photolysis channel produces a Cl atom and a CCl₂CHO radical. The latter species is converted to the oxy radical OCCl₂CHO, which decomposes by both C-C and C-Cl bond fission. A chemical mechanism for the photolysis of chloral by sunlight is proposed, and the atmospheric implications are discussed.