Process modeling of ammonia volatilization from ammonium solution and manure surfaces: A review with recommended models

Ammonia emissions from animal feeding operations have become a concern due to their potential adverse effects on animal and human health and the environment. Emissions occur from manure surfaces on the barn floor, during storage, and following field application. Process-based modeling with computer simulation is a cost-effective procedure for quantifying and evaluating emissions across diverse production systems. Based upon theoretical principles and associated published information on ammonia emissions, relationships were refined for modeling the dissociation constant, Henry's law constant, and the mass transfer coefficient to improve predictions of ammonia loss from manure surfaces. Inconsistencies found in widely used expressions for the dissociation constant and mass transfer coefficients in ammonia volatilization models from manures are discussed. Refined expressions were developed that relate these parameters to the temperature, pH, and ionic strength of the ammonium-containing material, and the velocity of air flowing over the material. These expressions were tested by comparing predicted ammonia emission rates to 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. These process-based relationships provide a basis for developing predictive tools that quantify management effects on ammonia emissions from farms and thus assist in the development and evaluation of strategies for reducing emissions.