Uptake of organic acids by water droplets

Size-resolved measurements of organic acid concentrations (HCOOH, CH3COOH) in cloud and fog droplets are analyzed. Considering their effective Henry's Law Constants it is evident that their equilibrium concentrations are dependent on the pH. At high pH values (pH > pKa) the concentration ratio in large/ small droplets is dependent only on the pH values in the droplets. It is shown that at high pH values (pH = 7) these ratios for acetic acid and formic acid can be predicted fairly well in fog droplets, implying the existence of thermodynamic equilibrium. Deviations from this equilibrium can be caused by mass transfer kinetics and/ or by chemical reactions within the droplets. Based on a resistance model describing the uptake process it is quantified which step delays the achievement of thermodynamic equilibrium: For large droplets r > 5 mm) gas phase diffusion towards the droplet surface is time-limiting, whereas for smaller droplets interfacial mass transport, characterized by the mass accommodation coefficient a, is the time-controlling step. In general, the characteristic times for the transport, i.e. gas phase diffusion and mass accommodation, are much longer than the chemical decay of acids in the aqueous phase. The fast oxidation of the acids by radicals within the aqueous phase causes smaller concentrations than those predicted by thermodynamic equilibrium. At high pH values the formation processes of organic acids within the aqueous phase do not influence the equilibrium due to the low solubility of the precursors (aldehydes) compared to the high effective Henry's Law constants of the acids.