Ammonia (NH3/fluxes were estimated from a field being grazed by dairy cattle during spring by applying a backward Lagrangian stochastic model (bLS) model combined with horizontal concentration gradients measured across the field. Continuous concentration measurements at field boundaries were made by open-path miniDOAS (differential optical absorption spectroscopy) instruments while the cattle were present and for 6 subsequent days. The deposition of emitted NH3 to "clean" patches on the field was also simulated, allowing both "net" and "gross" emission estimates, where the dry deposition velocity (vd/was predicted by a canopy resistance (R c/model developed from local NH3 flux and meteorological measurements. Estimated emissions peaked during grazing and decreased after the cattle had left the field, while control on emissions was observed from covariance with temperature, wind speed and humidity and wetness measurements made on the field, revealing a diurnal emission profile. Large concentration differences were observed between downwind receptors, due to spatially heterogeneous emission patterns. This was likely caused by uneven cattle distribution and a low grazing density, where "hotspots" of emissions would arise as the cattle grouped in certain areas, such as around the water trough. The spatial complexity was accounted for by separating the model source area into sub-sections and optimising individual source area coefficients to measured concentrations. The background concentration was the greatest source of uncertainty, and based on a sensitivity/uncertainty analysis the overall uncertainty associated with derived emission factors from this study is at least 30-40 %. Emission factors can be expressed as 6 +/- 2 gNH(3) cow(-1) day(-1), or 9 +/- 3% of excreted urine-N emitted as NH3, when deposition is not simulated and 7 +/- 2 gNH(3) cow 1 day(-1), or 10 +/- 3% of excreted urine-N emitted as NH3, when deposition is included in the gross emission model. The results suggest that around 14 +/- 4% of emitted NH3 was deposited to patches within the field that were not affected by urine or dung.