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Ammonia (NH3) fluxes were estimated from a field beinggrazed by dairy cattle during spring by applying a backward Lagrangianstochastic model (bLS) model combined with horizontal concentration gradientsmeasured across the field. Continuous concentration measurements at fieldboundaries were made by open-path miniDOAS (differential optical absorptionspectroscopy) instruments while the cattle werepresent and for 6 subsequentdays. The deposition of emitted NH3 to <q>clean</q> patches on the fieldwas also simulated, allowing both <q>net</q> and <q>gross</q> emission estimates,where the dry deposition velocity (vd) was predicted by a canopyresistance (Rc) model developed from local NH3 flux andmeteorological measurements. Estimated emissions peaked during grazing anddecreased after the cattle had left the field, while control on emissions wasobserved from covariance with temperature, wind speed and humidity and wetnessmeasurements made on the field, revealing a diurnal emission profile. Largeconcentration differences were observed between downwind receptors, due tospatially heterogeneous emission patterns. This was likely caused by unevencattle distribution and a low grazing density, where <q>hotspots</q> ofemissions would arise as the cattle grouped in certain areas, such as aroundthe water trough. The spatial complexity was accounted for by separating themodel source area into sub-sections and optimising individual source areacoefficients to measured concentrations. The background concentration was thegreatest source of uncertainty, and based on a sensitivity/uncertaintyanalysis the overall uncertainty associated with derived emission factorsfrom this study is at least 30–40 %. Emission factors can be expressed as 6 ± 2 g NH3 cow−1 day−1, or 9 ± 3 %of excreted urine-N emitted as NH3, when deposition is not simulated and 7 ± 2 g NH3 cow−1 day−1, or 10 ± 3 % of excreted urine-N emitted asNH3,when deposition is included in the gross emission model. The results suggestthat around 14 ± 4 % of emitted NH3 was deposited to patcheswithin the field that were not affected by urine or dung

Ammonia emissions from a grazed field estimated by miniDOAS measurements and inverse dispersion modelling