Journal of Atmospheric Chemistry 18: 319-357, 1994. 319 (~) 1994 Kluwer Academic Publishers. Printed in the Netherlands. Modelling of the Vertical Fluxes of Nitric Acid, Ammonia, and Ammonium Nitrate GERHARD KRAMM Fraunhofer-lnstitut far Atmosphi~rische Umweltforschung (IFU), Kreuzeckbahnstrafle 19, D-82467 Garmisch-Partenkirchen, Germany and RALPH DLUGI Meteorologisches Institut der Universittit MUnchen, Theresienstrafle 37, D-80333 Munich, Germany (Received: 28 May 1993; in final form: 25 February 1994) Abstract. Results from numerical investigations regarding the exchange of HNO3, NH3, and NH4NO3 between the atmosphere and the biosphere are presented. The investigations were performed with a modified inferential method which is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these nitrogen compounds. This modified inferential method calculates the microm- eteorological quantities (such as the friction velocity and the fluxes of sensible and latent heat), the height-invariant fluxes of the composed chemically conservative trace species with 'group' con- centrations cl = [HNO3] + [NH4NO3] (total nitrate), c2 = [NH3] + [NH4NO3] (total ammonia), and c3 = [HNO3] - [NH3] as well as the fluxes of the 'individual' nitrogen compounds. The parameteriza- tion of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The modified inferential method requires only the data of wind velocity, temperature, humidity and concentrations (HNO3, NH3, and NH4NO3) measured at a reference height by stations of a monitoring network. Key words: Biosphere-atmosphere exchange, dry deposition, flux-gradient relationships, HNO3 fluxes, inferential method, NH3 fluxes, NHnNO3 fluxes, resistance approach, turbulent transfer. 1. Introduction Dry deposition is an important atmospheric removal mechanism for chemical compounds in the gaseous and particulate phase like nitric acid (HNO3), ammo- nia (NH3), and ammonium nitrate (NH4NO3). The deposition of these nitrogen compounds may represent a significant nutrient transport to the biosphere in some areas (see Seinfeld, 1986), where in addition HNO3 is also a strong acid. However, as pointed out by Marshall and Cadle (1989), excess nitrogen from atmospheric deposition of nitrogen compounds is expected to contribute to forest decline (see also McLaughlin, 1985; Schulze, 1989). Hanson and Lindberg (1991) summarized dry deposition data for several reac- tive nitrogen compounds (gases and particles) for both foliar and non-foliar sites of deposition, and differentiated these data with respect to the techniques by which they were obtained. As reported by these authors, dry deposition fluxes of HNO3,