Molecular Insights into Dissolved Organic Matter in Natural Dew Water: Biogrime Films on Leaf Surfaces

Dew water serves as a temporary reservoir of dissolved organic molecules that are released in the morning when the dew evaporates. Many locations allow for direct sunlight on the dew water in the morning, raising the possibility for photolysis-driven chemical reactions. The majority of prior works looking at dew water chemical composition has targeted freshly deposited dew on clean surfaces like Teflon. However, the dissolved organic material in dew is a mixture of compounds deposited during the night, as well as water-soluble compounds already present on the surface. Here, we analyzed six separate dew or frost water samples collected off grass and bush leaf surfaces in Southeast Virginia in the fall of 2020 and found evidence for a water-soluble surface grime layer that we term biogrime. The chemical composition and photoactivity of the mixtures were probed using Fourier transform ion cyclotron resonance mass spectrometry, offline-aerosol mass spectrometry, and UV/vis spectroscopy. Complex organic mixtures were found in all the samples with a total of ∼9600 identified molecular formulae containing C, H, N, O, S, and P. Many samples had strong absorption in the visible region, and all showed an initially rapid photodecay. The composition varied between samples with possible sources including plant guttation and microbial waste as well as deposition of atmospheric organic aerosol particles, soil particles, and fog droplets. The composition of organic molecules, combined with their photoactivity, suggests that dew water may be a complex source for water-soluble gases as it evaporates.