Annual pattern of ammonia emissions from CAFO hog waste lagoons in Oklahoma

Ammonia (NH3) is often viewed as a major contributor to odor from livestock farms and is a reported gas under the Emergency Planning and Community Right-to-Know Act (EPCRA). NH3 emissions were measured periodically over the course of two years at waste lagoons of a breed-to-wean and a finisher hog farm in Oklahoma. The two farms are within 20 km of each other and therefore have effectively the same climatic influence. Consequently differences in emissions are largely a function of the differences in manure contributions and lagoon operation. At both locations, the path-integrated ammonia concentrations were measured along 12 optical paths along the surface around each lagoon using two scanning tunable diode lasers (each laser measuring two adjacent sides of the lagoon). Emissions were calculated from these concentration measurements and measured turbulence statistics using a backward Lagrangian stochastic model (Windtrax). Emissions were not limited by the gas exchange surface area: the breed-to-wean operation had roughly twice the area but emitted only 1/3 more than the finisher operation (6.8 g NH3 m-2d-1 versus 5.0 g NH3 m-2d-1). However, the mean annual emissions from the two types of hog operation were not significantly different when normalized by animal mass: 113 g NH3 d-1AU-1 for the breed-to-wean farm and 120 g NH3d-1AU-1 for the finisher farm. Annual and diurnal variations in NH3 emissions were in part associated with temperature variation. The annual trend in daily NH3 emissions at the finisher lagoon showed maximum mean monthly emissions in summer and early fall of approximately 1.5 g NH3 s-1 (178 NH3d-1AU-1). The maximum mean monthly emissions at the breed-to-wean lagoon were approximately 4 g NH3 s-1 (214 g NH3d-1AU-1) and also occurred during the summer (no measurements made in the early fall). There were small winter emissions likely resulting from a combination of barn effluent entering the lagoon on top of the frozen surface and the surface not being frozen continuously through the winter. The theoretical temperature influence on solubility of dilute NH3 in water closely matched the observed relationship of emission to air temperature, although the temperature influence on urease activity may also be contributing to the relationship. Approximately 50% of the variation in 1/2 h NH3emissions were associated with this apparent temperature influence. Wind speeds were usually high at these farms (mean annual wind speed of 4.4 and 4.8 ms-1 for the finisher and breed-to-wean farms respectively) and did not correlate with the lagoon NH3 emissions. Operational activities and measured changes in lagoon chemistry also did not correlate with the emissions variability. As expected, emissions from the lagoons were not sensitive to two to three day gaps in active loading.