Author
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Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/15/2013 Publication Date: 4/29/2013 Citation: Koehn, A.C., Leytem, A.B., Bjorneberg, D.L. 2013. Comparison of atmospheric stability methods for calculating ammonia and methane emission rates with WindTrax. Transactions of the ASABE. 56(2):763-768. Interpretive Summary: Inverse dispersion models like the WindTrax model are useful tools for estimating emissions from animal feeding operations. Atmospheric stability is an important input parameter to this model. We conducted a study to determine the differences in estimated methane and ammonia emission rates using five different methods for calculating atmospheric stability. Three methods used wind speed and solar radiation or cloud cover to calculate stability (Pasquill-Gifford). Another method used temperature and wind speed at two different heights (gradient Richardson number). The last method used data from a three-dimensional sonic anemometer, which is considered the best way to calculate atmospheric stability. Differences among methods were only statistically significant for two of the six measurement periods. Predicted emission rates were similar between the gradient Richardson and sonic anemometer methods. Predicted emission rates from the Pasquill-Gifford methods were 30 to 40% greater than emission rates from the sonic anemometer method. Based on this limited data set, using gradient Richardson method to represent atmospheric stability resulted in emission rates that more closely matched emission rates from the sonic method than Pasquill-Gifford methods. Technical Abstract: Inverse dispersion models are useful tools for estimating emissions from animal feeding operations, waste storage ponds, and manure application fields. Atmospheric stability is an important input parameter to such models. The objective of this study was to compare emission rates calculated with a backward Lagrangian stochastic (bLS) inversedispersion model (WindTrax) using three different methods for calculating atmospheric stability: sonic anemometer, gradient Richardson number, and Pasquill-Gifford (P-G) stability class. Ammonia and methane emission data from a compost yard at a 10,000-cow dairy were used for the comparisons. Overall, average emission rates were not significantly different among the stability methods. Emission rates correlated well between the sonic and other methods (r2 > 0.79, p < 0.001). The slopes of the regression lines between the sonic and Richardson methods were 0.95 and 1.0 for CH4 and NH3, respectively. The regression line slopes for the P-G method were about 1.9 for CH4 and 1.6 for NH3, which means emission rates predicted with the P-G method tended to be 50% to 100% greater than rates predicted with sonic anemometer data. Based on this limited data set, using the gradient Richardson method to represent atmospheric stability resulted in emission rates that more closely matched emission rates from the sonic method. Considering the amount of variability inherent in emissions calculations, a three-dimensional sonic anemometer should be used, if possible, to directly provide the necessary data to calculate parameters representing wind properties, rather than inferring values from other stability classification methods. |
