Phase partitioning of ammonia and ammonium in a multiphase system studied using a new vertical wet denuder technique

The phase partitioning of NH 3/NH + 4 in the atmospheric multiphase system is not completely understood. In order to study these phenomena in more detail, sensitive sample and accumulation methods have been developed which are reliable for separation of the ammonia species in the gaseous-, particle- and droplet phase. The separation of the gaseous- and the particle phase is performed here by a newly developed vertical wet denuder. The separation of the particle- and the droplet phase is done by an impaction technique. Chemical analysis is performed by ion chromatography. This equipment was used during the “CHEMDROP” field experiment, which took place in 1994 in the Po Valley (Italy). In the case of 100% relative humidity, when droplets are formed, most particle material appears in the liquid phase. Further uptake of ammonia is strongly dependent on the pH of the droplets. Especially at lower pH, large deviations from the Henry’s law equilibrium are observed in the samples, with larger interstitial NH 3 concentrations than expected by theory. Although a ground source of NH 3 cannot be ruled out at present, the deviations are consistent with the existence of time constraints that limit the attainment of equilibrium.