Ammonia emissions occur from manure surfaces on the barn floor, during storage, and following field application. Based upon theoretical principles and associated published information on ammonia emission, relationships were refined for modeling the dissociation constant, Henry's law constant and mass transfer coefficient to better predict ammonia loss from manure surfaces. Expressions were obtained that relate these coefficients to the temperature, pH and ionic strength of the material, and the air velocity over the material. These expressions were tested by comparing predicted ammonia emission rates against values measured in controlled laboratory experiments for buffered ammonium-water solutions and dairy cattle manure. Experimental results compared well to values predicted using these theoretical expressions derived from ammonia volatilization literature. This process-based model provides a basis for developing predictive tools that better quantify management effects on ammonia emissions from farms and thus assist in the development and evaluation of strategies for reducing emissions.