Surface Wetness as an Unexpected Control on Forest Exchange of Volatile Organic Acids

Abstract:

The biosphere acts as both a source and a sink of volatile organic compounds. Numerous biological and chemical processes can simultaneously compete to control an observed flux over a forest. Volatile acids, including isocyanic and alkanoic acids, present a particularly intriguing example of bidirectional surface exchange. We report fluxes of these acids over a pine forest across multiple seasons. The exchange velocity of these acids is well correlated with dew point depression, suggesting an equilibrium-driven continuum of flux. Wetness on forest surfaces impacts the vertical exchange of gases, and we suggest that water films and droplets drive equilibrium partitioning, with acids being solvated in surface wetness and released through evaporation. Despite their volatility, these acids partition into neutral-to-alkaline aqueous films, consistent with reported dew pH. This relationship between exchange velocity and dew point depression holds for a wetter mixed forest, but not a very dry orchard. Dew point depression is an excellent indicator of acid fluxes so long as the canopy is occasionally wetted. This work highlights the unexpected role of surface wetness in controlling biosphere-atmosphere exchange, and provides a call to further explore the surface water of terrestrial ecosystems.