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Air Emissions from Animal Feeding Operations: Current Knowledge, Future Needs
different temperatures, manure solid content, and oxygen content. More research is needed to develop accurate prediction models for quantifying the production rate of ammonia due to mineralization of organic nitrogen in different types of manure management systems.
Once ammonia (NH3) is generated in animal housing or manure storage, its emission from the manure to the atmosphere is controlled by the aqueous chemistry of NH3 in the manure and the convective mass-transfer mechanism at the manure surface. The pH, manure temperature, air temperature, wind velocity, and relative humidity are major factors affecting the emission process. The pH controls the partitioning of ammonia between NH3 and NH4+ (ammonium ion) in the water. The emission rate of ammonia from manure on the kth day can be calculated using the following mass-transfer equation,
where M is g/m2, 86,400 is the number of seconds in a day, KL is the mass transfer coefficient in meters per second, F is the fraction factor for free ammonia in total ammonia and has a value of 0-1, and [TAN]k is the concentration of total ammoniacal nitrogen in milligrams per liter after k days. F can be determined as a function of pH and ionization constant (Ka) of ammonia in water, using the following equation:
Ka is a function of water temperature (Taq, kelvin) as shown in the following equation:
It has been found by researchers that the Ka in wastewater has a different value from that in water (Zhang et al., 1994; Liang et al., 2002). The Ka in animal manure (Ka,m) is 25-50 percent of the Ka in water, depending on the characteristics of manure, such as its solids content.
KL is the convective mass-transfer coefficient and [TAN] is the concentration of total ammoniacal nitrogen at the manure surface. If the stratification of NH3 in the manure is negligible, [TAN] can be assumed to be the concentration in the bulk liquid. For any given day k, [TAN]k can be calculated by the concentra-