Ambient Meteorology. The NH3 emissions rate from lagoons is affected by air temperature, wind speed across the lagoon surface and relative humidity (NAS, 2002). The ambient air temperature determines the temperature of the lagoon liquid, particularly near the surface of the lagoon. With increasing ambient air temperature, the resulting increase in liquid temperature is expected to increase NH3 emissions. This positive relationship between liquid temperature and NH3 emissions from swine lagoons was modeled by Xxxxx et al. (2001) and Xxxxxx et al. (1998). Values for ambient temperature were available for all swine and dairy sites. Consequently, the EPA included ambient temperature as a candidate predictor variable. With regard to wind speed, the EPA expected that, in general, higher wind speeds would increase NH3 emissions. As the wind sweeps across the lagoon surface, the concentration gradient between the lagoon liquid and the liquid-air boundary layer at the surface decreases, thereby increasing diffusion of NH3 to the atmosphere. Also, high winds can increase surface agitation, thereby effectively increasing the surface area available for diffusion of NH3 to the atmosphere. The positive relationship of wind speed on NH3 emissions was noted in the model for swine lagoons developed by Xxxxxx et al. (1998). Values for wind speed were available for all swine and dairy sites. Consequently, the EPA included wind speed as a candidate predictor variable. The EPA expected that increases in relative humidity above the lagoon surface would tend to decrease the diffusion of gas molecules from the liquid/gas interface. Values for ambient relative humidity were available for all swine and dairy sites. Therefore, the EPA included relative humidity as a candidate predictor variable. The EPA also expected that increases in atmospheric pressure above the lagoon surface would tend to decrease NH3 emissions. Higher atmospheric pressure will decrease the gradient between the partial pressure of NH3 gas dissolved in the lagoon liquid and the atmosphere above the lagoon surface, thereby reducing diffusion of gas molecules from the liquid. However, the EPA did not use atmospheric pressure as a predictor variable because to do so would have significantly reduced the size of the dataset. The dew point is the temperature at which moisture in the atmosphere condenses to liquid water. This condensed moisture could reabsorb NH3 released from the lagoon and potentially reduce NH3 emissions. The condense...
