On the drop-size dependence of organic acid and formaldehyde concentrations in fog

Concentration differences between small (r < 8.5 μm) and large droplets (r > 8.5 μm) were observed for formic acid, acetic acid and formaldehyde in fog droplets collected in California's Central Valley. The concentration ratios (large/small droplets) of these compounds were investigated by a stepwise model approach. Assuming thermodynamic equilibrium (K_H^eff) results in an overestimate of the concentration ratios. Considering the time dependence of gas phase diffusion and interfacial mass transport, it appears that the lifetime of fog droplets might be sufficiently long to enable phase equilibrium for formaldehyde and acetic acid, but not for formic acid (at pH ≈7). Oxidation by the OH radical has no effect on formaldehyde concentrations but reduces formic acid concentrations uniformly in all drop size classes. The corresponding reaction for acetic acid is less efficient so that only in large droplets, where replenishment is slowed because the uptake rate of acid from the gas phase is slower, is the acid concentration reduced leading to a smaller concentration ratio. Formaldehyde concentrations in fog can be higher than predicted by Henry's Law due to the formation of hydroxymethanesulfonate. Its formation is dependent on the sulfur(IV) concentration. At high pH values the uptake rate for sulfur(IV) is drop-size dependent. However, the observed concentration ratios for formaldehyde cannot be fully explained by the adduct formation. Finally, it is estimated that mixing effects, i.e., the combination of individual droplets into a bulk sample, have a minor influence (<15%) on the measured heterogeneities.