The quantification of ammonia (NH3) emissions at ambient air conditions is still a challenge and emission factors for ammonia have therefore a large uncertainty. We present first results of a pasture experiment carried out in western Switzerland in 2016. During the measurement campaign, the pasture was grazed by 24 dairy cows in an intensive rotational management. NH3 concentrations were measured with lineintegrating open-path instruments. The NH3 emission fluxes were calculated by applying a backward Lagrangian Stochastic dispersion model (bLS) to the difference of paired concentration measurements upwind and downwind of a grazed sub-plot. The instruments were able to retrieve small horizontal concentration differences (as small as 0.5 μg NH3 m-3) and the resulting fluxes were within a range of 0 to 3 μg N-NH3 m-2 s-1. We found, that the fluxes decreased to values below 0.5 μg N-NH3 m-2 s-1 typically within 48 hours. First flux evaluation showed, that rain events during the grazing period had a major effect on the cumulative emissions.