Formation of carbonyl compounds from UV‐induced photodegradation of humic substances in natural waters: Fate of riverine carbon in the sea

Abstract

Low‐molecular‐weight (LMW) carbonyl compounds, e.g. formaldehyde, acetaldehyde, and the α‐keto acid glyoxylate, were produced in a wide variety of natural waters upon irradiation with sunlight. Production rates were linearly related (r2 > 0.98) to initial absorbance at 300 nm and initial fluorescence (360/460 nm) in all waters tested. Photochemical production was also linearly related to loss (photobleaching) of absorbance and fluorescence during irradiation, irrespective of irradiation time and prior photobleaching history of the sample. These results were attributed to absorption of light by humic substances in the waters, as determined by experiments where purified humic and fulvic extracts were added to open‐ocean water. The wavelengths in the solar spectrum responsible for photoproduction of LMW carbonyl compounds and bleaching of dissolved organic matter light absorbance are in the UV‐B region (280–320 nm). These results, plus results from time‐course irradiations, suggest that photoproduction of carbonyl compounds from humic substances is closely related to photobleaching of absorbance. Based on our photoproduction rates of LMW carbonyl compounds, we estimate that the half‐life of humic‐rich riverine dissolved organic C in the oceanic mixed layer is 5–15 yr.