Ammonia concentrations in the air adjacent to and downwind of swine production units are of concern to producers. Most of the existing measurements have been conducted with single point monitors or at locations adjacent to or within the building and are often conducted for a short period of time. We conducted a study on the ammonia concentrations at various positions downwind of a grow-finish unit throughout a production cycle in 2002. Ammonia concentrations were measured with a sensitive ammonia analyzer at four positions. There was a rapid decrease in concentration with distance and with height above the surface for a majority of the observations. The ammonia concentrations decreased during the production cycle because of the more rapid mixing of air between the building and the first sampling location. There were a few periods that covered an entire day in which there was little difference in the ammonia concentrations at the four positions and these were caused by very rapid mixing of the air. On these days the concentrations were not lower than other days, just more uniform. In these studies we found that the variation in mixing of the air surrounding a building will have to be quantified with different techniques than what are currently used. When we measured the rate of mixing with a Lidar system we found the vertical rate of mixing was about 10 times larger than expected which indicates that air flow around buildings is not well understood.
We found ammonia concentrations during and following manure and anhydrous ammonia application to increase rapidly during application and then decrease following application. The patterns were different between the two nutrient sources because of the presence of the manure on the soil surface acting as a continual source of ammonia; however, these concentrations were quite low (less than 100 ppbv) and within 48 hours were near ambient levels of less than 10 ppbv.
These results characterizing a complete growing cycle demonstrate the large changes that occur around a production unit but also the extreme caution that has to be exercised in conducting measurement programs around swine production units. The rapid decrease in ammonia concentrations with distance from a facility shows the ability of the atmosphere to disperse material and act as a transport mechanism.
