Photochemical production of formaldehyde, acetaldehyde and acetone from chromophoric dissolved organic matter in coastal waters

Low molecular weight carbonyl compounds are produced photochemically from chromophoric dissolved organic matter (CDOM) in natural waters. Photoproduction rates for formaldehyde, acetaldehyde and acetone were measured in the laboratory using a pre-column 2,4-dinitrophenylhydrazine derivitization HPLC method as a function of optical properties (absorption coefficient, spectral slope) and irradiation time. Rates decreased linearly with decreasing absorption coefficient at 350 nm (a(350 nm)), a measure of CDOM levels, with substantial variability in low a(350 nm) beach waters. Apparent quantum yields (Θ) were unchanged for a(350 nm) = 2–16 m−1, but increased rapidly (×5) for beach waters with low a(350 nm) values. Θ increased linearly with increasing spectral slope for beach waters, consistent with enhanced production efficiency with photobleaching of CDOM in coastal waters. Θ trends with oxygen and molecular reaction probes (hydroxyl radical scavengers and producers) suggested a combination of direct photolysis and singlet oxygen quenching as primary production mechanisms.