Fluxes of undissociated acids to terrestrial ecosystems by atmospheric deposition

Undissociated acids in the troposphere are transferred to terrestrial ecosystems as wet deposition in rain and cloud/fog water and as particulate and gaseous dry deposition during interstorm intervals. Wet deposition can be measured directly, and the sum of dry deposition and canopy release of undissociated acids can be estimated from net canopy wash-off into throughfall (TF). Samples of rain, cloud and fog water, and TF were collected from April 1986 through March 1987 at low-elevation (230 m) and high-elevation (1730 m) coniferous forests in Tennessee, USA. The samples were titrated for total acidity by Gran’s method, and undissociated acidity was calculated as total acid (Gran) minus free acidity (pH measurement). During that 12-month period, the undissociated acid fluxes in wet deposition at these sites were both 34 mmol m-², comparable to the free acid fluxes in wet deposition. Fog water fluxes at the lower site were negligible, but at the high-elevation site cloud water was potentially responsible for considerable deposition of undissociated acids to the forest canopy. Dry deposition plus canopy fluxes of undissociated acids were greater at the higher site. Dry deposition plus canopy fluxes averaged 54 mmol m^-2 y^-1 (range 36—77) at the low-elevation site and 82 mmol m^-2 y^-1 (range 77—86) at the high-elevation site. Undissociated acidity in TF consists of biological metabolites and cuticular decomposition products and dry-deposited compounds washed from plant surfaces. Formic and acetic acid concentrations in TF do not exceed those found in rain, therefore dry depositions of those acids cannot account for the undissociated acidity of TF. The relative importance of wet and dry deposition of undissociated acid to terrestrial ecosystems would be further clarified by identifying the weak acids in TF.