Long-term Study of the Atmosphere-Biosphere Exchange of Reactive Reduced and Oxidized N-Species at the Melpitz Site (Germany) Motivation and Measurements

The atmosphere-biosphere exchange of main N-containing species over a representative meadow is discussed for the entire year of 2001. Classical aerodynamic profile technique was used to determine the net fluxes of the gaseous compounds NO, NO2 (NOX) and NH3 on a half-hourly basis. Concentrations of NOX, NH3, HNO2 and HNO3 have been measured by chemiluminescence (NOX) and wet annular denuder techniques, respectively (NH3: acidic coating, continuous conductivity detection, Wyers et al. 1993, figure 3; HNO3 and HNO2: alkaline coating, discontinous sampling). Dry deposition of HNO3 was estimated by resistance analogy assuming surface resistances of zero. For HNO2 a deposition velocity of 0.8 cm s-1 was asumed. The N-input by precipitation (NO3–, NH4+) was quantified using a wet-only sampler and subsequent ion chromatographic analysis, Brüggemann and Spindler, 1999. NO3 – and NH4+ concentrations of aerosol particles, sampled with a low flow filter sampler (upstream impactors for PM1, PM2.5 and PM10), were also determined by ion chromatography analysis. The size dependent deposition velocity of particles modeled on the basis of measured friction velocities, Slinn, 1982 (see: Table 1). The seasonal contribution of biogenic NO-emission from soil (dependent on soil moisture and temperature) was estimated by an emission algorithm using the results of laboratory measurements on soil samples taken at the Melpitz site (see: Biogenic NO emissions, Melpitz site, 2001). The influence of chemical reactions of the NO-NO2-O3 triad on the turbulent exchange was discussed in view of the total N-impact (see: Reactions of trace gases during turbulent transport, figure 4). With respect to the annual N-exchange of the individual N-components it appears that NH3 is the most important gaseous N-compound. The N