Measurements of concentration gradients of HNO2 and HNO3 over a semi-natural ecosystem

Gradients of HNO2 and HNO3 have been measured over a semi-natural grassland in Central Switzerland. The site was characterised by a high excess of NH3 over HNO3. In such a situation the measured product of NH3 and HNO3 often exceeds the thermodynamic equilibrium product over solid NH4NO3. As a consequence, transient formation and re-evaporation of NH4NO3 can occur leading to a height-dependent flux of HNO3. Consequently, the constant flux approximation, which is a pre-requisite for the determination of HNO3 deposition by the gradient method, is violated. Measured HNO2 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.