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A mechanistic model was developed to simulate ammonia emissions from cattle feedlot pens. The temperature of the pen surface was determined on an hourly time step by numerical solution of the surface energy balance. The resulting temperature along with a chemical equilibrium submodel for ammoniacal nitrogen was used to approximate the ammonia gas concentration at the surface-air interface. Emissions were calculated using a convective transport equation following standard micrometeorological theory. Weather data from northeast Colorado (Greeley, CO) and the Texas Panhandle (Bushland, TX) were used as environmental inputs. All other input variables (soils, chemistry, roughness, etc) were made identical between the two sites. Results showed summer NH3 fluxes were between 95 and 133 g head-1 d-1, findings that were in good agreement with field measurements reported in the literature. When averaged over three years (2007-2009), fluxes from NE Colorado were 27 % lower than those from the Texas Panhandle. Differences between sites were caused mainly by lower wind speeds and air temperatures in CO. Regional differences in ammonia fluxes were found from cattle feedlots in the High Plains.