Volatile Ammonia Fraction and Flux from Thin Layers of Buffered Ammonium Solution and Dairy Cattle Manure

Process-based models are being used to predict ammonia (NH3) emissions from manure sources, but their accuracy has not been fully evaluated for cattle manure. Laboratory trials were conducted to measure the NH3 volatile fraction (undissociated NH3 fraction divided by Henry's law constant) and volatilization flux from buffered ammonium-water solution and dairy cattle manure. Effects of ionic strength, ammoniacal N concentration, temperature and pH of the media, and air velocity over the media were evaluated. As represented in existing models, media type, temperature, and pH were verified to influence the NH3 volatile fraction and mass transfer. Model prediction underestimated the volatile NH3 fraction by 5% in solution trials and 94% in manure trials. This underestimation was attributed to an increase in surface pH created through the emission of carbon dioxide (CO2). Ammonia emission rate from both the ammonium solution and manure surfaces was affected by temperature and air velocity. A frequently used dairy manure volatilization model predicted NH3 emission rates from manure surfaces more accurately than that from buffered solution with average errors of -16% and +81%, respectively. Temperature significantly affected the difference between predicted and measured rates, with the greatest error at temperatures over 25C. The results imply that further model development is needed to improve the accuracy of predicting NH3 emissions from dairy cattle manure with an emphasis on the measurement and prediction of manure surface pH as affected by CO2 emission.