We report bidirectional exchange of volatile acids, including isocyanic and alkanoic 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.
Bibliographical noteFunding Information:
We thank William Massman (U.S. Forest Service) and Frank Wania (University of Toronto) for assistance, and NOAA (NA14OAR4310141), the Academy of Finland (310682), and NSF (AGS-1932771, AGS-1428257) for funding.
- biosphere-atmosphere exchange
- isocyanic acid
- organic acids
- surface wetness