|Parnell, jr., C|
Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: 8/10/2007
Publication Date: 8/15/2007
Citation: Lange, J.M., Wanjura, J.D., Skloss, S.J., Parnell Jr, C.B. 2007. Emission factors for cattle feedlots in Texas based on particle size using ISCST3 and AERMOD. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). June 17-20, Minneapolis, Minnesota. Paper No. 074106. Interpretive Summary: Often, an emission factor is developed through research which measures concentrations downwind of the source and back calculates the emission factor using a dispersion model. The problem with this approach is that the resulting emission factor is only valid for use in the dispersion model used in its development. Moreover, the push by EPA for states to use a new dispersion model named AERMOD will likely lead to the over estimation of downwind concentrations from agricultural sources when emission factors developed using the current model, ISCST3, are used. The objective of the research engineers from USDA-ARS in Lubbock, TX and Texas A&M University, College Station, TX, was to develop dust emission factors from cattle feedlots for use in ISCST3 and AERMOD. Concentrations measured downwind of a Texas cattle feedlot over four years were used in the analysis and the results indicate that the emission rate for AERMOD is approximately one half that of the emission rate for ISCST3. This is an important finding from the standpoint of cattle feedlots in that the emissions from their operations could be over-estimated by a factor of two if the wrong emission factor is used. Furthermore, cattle feedlot operations may be inappropriately required to reduce dust emissions via expensive control technologies if regulators justify emission reduction requirements on inaccurate concentration estimates. Air pollution regulators will also be able to use these findings to help protect public health through accurate source impact assessments while providing for the appropriate regulation of agricultural sources.
Technical Abstract: Cattle feedlots in Texas are often assumed to be large sources for particulate matter (PM) emissions. Previously, the Texas Commission on Environmental Quality (TCEQ) used the Industrial Source Complex – Short Term version 3 dispersion model (ISCST3) to estimate downwind concentrations for permitting purposes. Furthermore, researchers have used ISCST3 to back calculate emission factors from cattle feedlots using concentration and meteorological data collected at the source. Recently, the Environmental Protection Agency (EPA) required all state regulatory agencies to switch to the AMS/EPA Regulatory Model (AERMOD). EPA transitioned to AERMOD as the new regulatory model because it incorporates newer science into the Gaussian dispersion model to describe pollutant dispersion in the planetary boundary layer. As such, AERMOD requires more model input data to describe meteorological conditions and terrain features than the older ISCST3. However, previous research has shown that significant differences exist between the concentrations estimated by ISCST3 and AERMOD using identical emission factor and meteorological data. Therefore, the emission factor data used in ISCST3 must be updated for use in AERMOD in order to appropriately estimate the downwind influence of PM emissions from a source. This work shows that using measured data from yearly sampling trips from 2002 to 2005, the ISCST3 emission factor is 6 kg/1000 hd-day (13 lb/1000hd-day). After obtaining meteorological data from the Texas State Climatologist and the Texas Commission on Environmental Quality (TCEQ), the AERMOD emission factor was determined to be 4 kg/1000 hd-day (8 lb/1000hd-day). The difference in the emission factors is due to the method each model uses to calculate the concentrations downwind from the source. ISCST3 is a double Gaussian plume dispersion model which can be approximated using multiple infinite line sources. AERMOD uses a Gaussian distribution for the stable boundary layer and a bi-Gaussian probability distribution function for the convective boundary layer to calculate downwind concentrations.