PHASE PARTITIONING OF AMMONIA AND AMMONIUM IN A MULTIPHASE SYSTEM STUDIED USING A NEW VERTICAL WET DENUDER TECHNIQUE

The phase partitioning of NH3/NH4+ in the atmospheric multiphase syste m is not completely understood. In order to study these phenomena in m ore detail, sensitive sample and accumulation methods have been develo ped which are reliable for separation of the ammonia species in the ga seous-, particle- and droplet phase. The separation of the gaseous- an d the particle phase is performed here by a newly developed vertical w et denuder. The separation of the particle- and the droplet phase is d one by an impaction technique. Chemical analysis is performed by ion c hromatography. This equipment was used during the ''CHEMDROP'' field e xperiment, which took place in 1994 in the Po Valley (Italy). In the c ase of 100% relative humidity, when droplets are formed, most particle material appears in the liquid phase. Further uptake of ammonia is st rongly dependent on the pH of the droplets. Especially at lower pH, la rge deviations from the Henry's law equilibrium are observed in the sa mples, with larger interstitial NH3 concentrations than expected by th eory. Although a ground source of NH3 cannot be ruled out at present, the deviations are consistent with the existence of time constraints;t hat limit the attainment of equilibrium. (C) 1998 Elsevier Science Ltd . All rights reserved.