Comparison of the ISCST Model With Two Alternative U.S. EPA Models in Complex Terrain in Hamilton County, Ohio

Abstract

In order to resolve the inconsistency between SO2 modeled and monitored values in Hamilton County, Ohio, and in response to the U.S. EPA’s requirement to Ohio EPA that the Hamilton County SO2 State Implementation Plan (SIP) be re-evaluated, a coalition of industries who are members of the Greater Cincinnati Chamber of Commerce (GCCC) conducted a technical study. The objective of this study was to compare selected U.S. EPA guideline dispersion models with SO2 monitoring data to determine which model was most accurate in predicting SO2 air quality in Hamilton County. The GCCC developed a protocol to perform this study. The protocol was approved by U.S. EPA and the Ohio EPA. Based upon the geographical proximity and quantity of SO2 emissions, Hamilton County was subdivided into west and east sub-areas. The model comparison was performed for each sub-area. The reference model for this study was the Industrial Source Complex Short-Term (ISCST) model and Rough Terrain Diffusion Model (RTDM) hybrid. Inthis model, ISCST was applied to all receptors upto stack height and RTDM was applied to all receptors above plume height. For the intermediate terrain receptors, i.e., those having elevations between stack height and plume height, the highest concentration from ISCST and RTDM was used. The first alternate model was chosen to be RTDM at all receptors. The second alternate model was chosen to be an ISCST/ RTDM hybrid in which ISCST without downwash was applied to receptors with elevations below stack height and RTDM was applied to receptors with elevations above stack height. Ambient SO2 concentrations were obtained from a network of 13SO2monitors(six in the west sub-area and seven in the east) which were operated for a one-year period beginning November 1, 1990. The monitors were sited based upon dispersion modeling conducted using the ISCST model with and without downwash and the RTDM model. The meteorological data for the one-year study period were obtained from two multi-level meteorological towers,a 100-meter tower in the west sub-area and a 60-meter tower in the eastsub-area. Hourly values of SO2 emission rate, exhaust gas exit temperature, and exhaust gas exit velocity were obtained from the eight major sources which were modeled in the model comparison study. The impact of other SO2, emission sources was accounted for in the hourly background determination. The reference model and the two alternate models were executed using the source and meteorological data to determine predicted SO2 concentrations at each of the 13 SO2 monitor locations. The predicted SO2 data were statistically compared to the observed SO2 data using the methodology referenced in Cox (1987). The test statistic used was the Robust Estimate of the Highest Concentration (RHC). Various RHCs were calculated from which a composite performance measure (CPM) was computed for each model and sub-area. In both sub-areas the winning model was RTDM. Using the Cox “bootstrap” resampling technique, it was shown that, to a 90 percent confidence interval, RTDM was statistically different from the reference model. As discussed in the protocol, the winning model could be used for purposes of either developing a revised Hamilton County SO2 SIP or proposing SO2 SIP revisions for individual facilities in Hamilton County. The protocol included procedures outlining how the winning model would be adjusted if it is shown to underpredicr monitored concentrations. For the west sub-area, there was no underprediction by RTDM at any of the six monitors. For the east sub-area, however, there was under prediction at two low terrain monitors and very small underprediction at one high terrain monitor. Based upon the procedures specified by U.S. EPA, the adjustment factor for RTDM in the east sub-area is 1.68 for 3-hour concentrations and 1.75 for 24-hour concentrations.