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

TLDR

Low‑molecular‑weight carbonyls such as formaldehyde, acetaldehyde, and glyoxylate are produced in natural waters by UV‑B sunlight, with the responsible wavelengths in the 280–320 nm range. The production arises from light absorption by humic substances, as shown by experiments adding purified humic and fulvic extracts to open‑ocean water. Photodegradation yields these carbonyls at rates linearly correlated with initial absorbance and fluorescence, linked to photobleaching, and implies a 5–15‑year half‑life for humic‑rich riverine dissolved organic carbon in the oceanic mixed layer.

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 ( r 2 > 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.