The influence of gas-to-particle conversion on measurements of ammonia exchange over forest

Measurements of vertical gradients of ammonium nitrate aerosol and NH3 are used together with HNO3 concentrations to study the influence of gas-to-particle conversion (gtpc) on surface exchange processes above a forest. A numerical model of surface exchange, in which a description of gtpc was incorporated, is used to interpret the measurements. The model suggests that evaporation of particulate NH4NO3 during the day can lead to situations where upward fluxes of NH3 and HNO3 are observed above the forest, while both gases are deposited at the surface. At the same time, this mechanism provides an explanation for the observed aerosol deposition fluxes, which we found to be larger than theoretically possible. Further support for our hypothesis was found in the analysis of a larger data set which showed that in many cases sub-equilibrium concentrations of NH3 and HNO3 (which may lead to evaporation of NH4NO3 aerosol) coincide with apparent emission fluxes of NH3.