0 Milford, C. Theobald, M.R. Nemitz, E. Hargreaves, K J. Horvath, L. Raso, J. Dämmgen, U. Neftel, A. Jones, S. K. Hensen, A. Loubet, B. Cellier, P. Sutton, M. A. 2009 Biogeosciences 6 819-834 <p>Quantification of ammonia (NH3) land-atmosphere exchange is required for atmospheric modelling and assessment of nitrogen deposition, yet flux measurement methods remain highly uncertain. To address this issue, a major inter-comparison of ammonia fluxes over intensively managed grassland was conducted during the GRAMINAE Integrated Experiment held in Braunschweig, Germany. In order to provide a robust dataset of ammonia exchange with the vegetation, four independent continuous flux gradient systems were operated. Three independently operated continuous wet denuders systems (AMANDA) were compared with a Wet Effluent Diffusion Denuder (mini-WEDD) system. Measurements were made at two distances from an adjacent livestock farm, allowing effects of advection to be quantified in a real landscape setting. Data treatment included filtering for instrument failure, disturbed wind sectors and unsuitable micrometeorological conditions, with corrections made for storage and advection errors.</p> <p>The inter-comparison demonstrated good agreement in measured ammonia concentrations and fluxes (relative standard error &lt;20%) for some periods, although the performance of the ammonia analyzers were variable, with much poorer agreement on particular days. However, by using four systems, the inter-comparison was able to provide a robust mean estimate of continuous ammonia fluxes through the experiment. The observed fluxes were: a) small bi-directional fluxes prior to cutting (-64 to 42 ng NH3 m-2 s-1), b) larger diurnally-varying emissions following cutting (-49 to 703 ng NH3 m-2 s-1) and c) much larger emissions following fertilizer application (0 to 3820 ng NH3 m-2 s-1). The results are a salutary reminder of the uncertainty in unreplicated ammonia flux measurements, while the replication of the present study provides a uniquely robust dataset for the evaluation of ammonia exchange processes. It is clear that consistently reliable determination of ammonia concentrations remains the major measurement challenge.</p>