Measurements of concentration gradients of HNO 2 and HNO 3 over a semi-natural ecosystem

Abstract

Gradients of HNO 2 and HNO 3 have been measured over a semi-natural grassland in Central Switzerland. The site was characterised by a high excess of NH3 over HNO 3. In such a situation the measured product of NH3 and HNO 3 often exceeds the thermodynamic equilibrium product over solid NH 4NO 3. As a consequence, transient formation and re-evaporation of NH 4NO 3 can occur leading to a height-dependent flux of HNO 3. Consequently, the constant flux approximation, which is a pre-requisite for the determination of HNO 3 deposition by the gradient method, is violated. Measured HNO 2 concentrations are higher as can be explained by production of homogeneous gas-phase reaction alone. But no evidence for a ground-based source from the vegetation-covered soil has been found.