Short term temporal variability in the photochemically mediated alteration of chromophoric dissolved organic matter (CDOM) in rainwater

The goal of the research presented here was to determine the impact of photochemistry on the abundance and spectral qualities of chromophoric dissolved organic matter (CDOM) in precipitation. The relationship between sunlight and CDOM in rainwater was complex with both production and photobleaching of optical properties occurring simultaneously in different regions of the fluorescence excitation emission spectra (EEMs) over relatively short time scales. Spectral slope was inversely correlated with the observed changes in total integrated fluorescence suggesting that photo-induced modifications in the molecular weight of CDOM were occurring along with fluctuations in its optical properties. Atmospheric condensate collected near a high traffic roadway had a response to sunlight similar to authentic rainwater suggesting some fraction of the CDOM in atmospheric waters is derived from local anthropogenic gas phase sources. There was a dramatic increase in fluorescence in two samples photolyzed with photosynthetically active radiation only (PAR; 400–700 nm) compared to analogous samples exposed to full spectrum sunlight indicating that this less energetic light is capable of producing photochemically labile compounds in rainwater. The observed temporal variability in the molecular level response of CDOM to sunlight is important because it may alter the spectral attenuation and the amount of solar radiation reaching the earth’s surface.