F
Ammonia Emissions from Manure Storage
The production of ammonia from animal manure via mineralization of organic nitrogen on the kth day of storage can be predicted using the following equation:
TM_{TAN,}_{k} = ΓN_{i} · N_{0}_{i} · NCF_{jk} · WS_{ij} · CAF,
where
TM_{TAN,}_{k} = 
production of total ammoniacal nitrogen (TAN) from manure storage on the farm in grams per day on the kth day of storage; 
N_{i} = 
total nitrogen excreted by animal i in grams per day; 
N_{0}_{i} = 
maximum TAN production potential of the manure from animal i in grams per grams of total nitrogen; 
NCF_{jk} = 
nitrogen conversion factor for manure storage j, for k days of storage time, which represents the extent to which the N_{0} is realized (note: 0 ≤ NCF ≤ 1); 
WS_{ij} = 
fraction of waste of animal i handled in manure storage j; 
CAF = 
climate adjustment factor for the farm, which represents the extent to which N_{0} is realized under climatic conditions (e.g., temperature and rainfall) on the farm (note: 0 ≤ CAF ≤ 1). 
Quantitative information on the production of ammonia in animal manure is scarce in the literature. Zhang et al. (1994) developed equations for predicting the production rate of ammonia in swine manure storage as a function of time and depth in the manure. However, their study did not account for the influence of
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 (NH_{3}) is generated in animal housing or manure storage, its emission from the manure to the atmosphere is controlled by the aqueous chemistry of NH_{3} in the manure and the convective masstransfer 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 NH_{3} and NH_{4}^{+} (ammonium ion) in the water. The emission rate of ammonia from manure on the kth day can be calculated using the following masstransfer equation,
where M is g/m^{2}, 86,400 is the number of seconds in a day, K_{L} 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 01, 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 (K_{a}) of ammonia in water, using the following equation:
K_{a} is a function of water temperature (T_{aq}, kelvin) as shown in the following equation:
It has been found by researchers that the K_{a} in wastewater has a different value from that in water (Zhang et al., 1994; Liang et al., 2002). The K_{a} in animal manure (K_{a,m}) is 2550 percent of the K_{a} in water, depending on the characteristics of manure, such as its solids content.
K_{L} is the convective masstransfer coefficient and [TAN] is the concentration of total ammoniacal nitrogen at the manure surface. If the stratification of NH_{3} 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
tion at the end of previous day, [TAN]_{k}_{−1}, and the ammonia generated on the kth day (TM_{TAN,}_{k}, grams) divided by the volume (V, cubic meters) of the liquid manure in storage, as shown below:
[TAN] _{k} = [TAN]_{k}_{−1} + TM_{TAN,}_{k} /V.
The masstransfer coefficient K_{L} is a function of manure temperature, air temperature, wind velocity, and relative humidity. Various equations for K_{L} are available in the literature. However, most of them were developed using controlled experiments by means of convective masstransfer chambers, and have not been well validated using fieldscale experiments. More research is needed to calibrate and validate them. An example is given below is based on the twofilm theory.
AMMONIA MASSTRANSFER COEFFICIENT
The masstransfer coefficient for ammonia as derived from the twofilm theory (Whitman, 1923) is given as follows:
where K_{L} is the overall masstransfer coefficient in meters per second, K_{H} is Henry’s law constant (dimensionless) calculated as a function of water temperature (T_{aq}, kelvin),
and and are masstransfer coefficients (meters per second) through gaseous and liquid films, respectively, at the interface of water and air, and are related to the diffusivities (square meters per second) of ammonia and water in air ( and ), and of ammonia and oxygen in water ( and ):
where u_{8} is the wind velocity (meters per second) at 8 m above the water surface. The diffusivities are calculated using the following equations:
where M is the molecular weight (grams per mole), T_{a} and T_{aq} are the air and water temperatures (kelvin), and v is the molecular diffusion volume (cubic centimeters per mole).
SYMBOL DEFINITIONS, UNITS
K_{L} 
Overall masstransfer coefficient of ammonia, cm/h 
K_{H} 
Henry’s coefficient, dimensionless 
Masstransfer coefficient of ammonia in liquid phase, cm/h 

Masstransfer coefficient of ammonia in gas phase, cm/h 

Masstransfer coefficient of oxygen in liquid phase, cm/h 

Masstransfer coefficient of water in gas phase, cm/h 

u_{8} 
Wind speed at 8m height, m/s 
u_{z} 
Wind speed at an anemometer height z, m/s 
z_{0} 
Roughness height, m 
P 
Atmospheric pressure, atm 
NH_{3} diffusion coefficient in air, m^{2}/s 

H_{2}O diffusion coefficient in air, m^{2}/s 

O_{2} diffusion coefficient in water, m^{2}/s 

NH_{3} diffusion coefficient in water, m^{2}/s 

M_{air} 
Molecular weight of air (average), g/mol (29) 
Molecular weight of NH_{3}, g/mol (17) 

Molecular weight of H_{2}O, g/mol (18) 

(Σv)_{air} 
Air molecular diffusion volume, 20.1 cm^{3}/mol 
NH_{3} molecular diffusion volume, 14.9 cm^{3}/mol 

T_{a} 
Air temperature, K, mg/l 
[TAN] _{k} 
Total ammoniacal concentration at the manure surface on the kth day of storage 
T_{aq} 
Water (manure) temperature, K 