AN ENGINEERING ANALYSIS OF A METHOD USED TO DETERMINE PROPERTY LINE SETBACK DISTANCES FOR COTTON GINS

The US Environmental Protecti on Agency (EPA) under authority granted by the federal Clean Air Act delegates the responsibility of regulating and enforcing ambient air quality standards to the states. States are required by EPA to submit state implementation plans (SIP) detailing how a state will implement and enforce regulatory actions to ensure that the National Ambient Air Quality Standards (NAAQS) are achieved. The NAAQS list federal ambient concentration limits for six criteria pollutants including particulate matter less than or equal to 10 micrometers ( μ m) aerodynamic equivalent diameter (PM 10 ) . The primary criteria pollutant emitted by a cotton gin is PM 10 . State air pollution regulatory agencies (SAPRA) regulate cotton gins by requiring minor sour ce permits under the new sou rce review (NSR) process. When granted the NSR minor source permit, the owner/operator has permission from the state to begin construction on either a new facility or modifications to an existing facility. States have granted cotton gin pre - construction permits based on permit by rule, standard permit, or individual facility permit. Recently, the state of New Mexico developed a new tool to use in permit ting cotton gins. This new tool involves a property line setback distance. The property line setback distance is the distance from the emission point to the nearest property line. The results of field sampling and dispersion modeling taken from four days identified as worst case scenarios were used to determine the required setback distance for a New Mex ico gin. The ratio of the modeled concentrations to the measured concentration (using FRM PM10 samplers) was determined for the four days and used to correct the concentration predictions from the model. The required setback distance for the gin was dete rmined to be 41 meters corresponding to a 12 lb/hr PM10 emission rate. The method used to determine this setback distance did not account for the sampler errors that occur when using FRM PM10 samplers to measure concentrations of PM emitted from agricultu ral sources. The purpose of this manuscript is to evaluate the methods used by the NMED to determine the required property line setback distance and to provide guidance based on sound science and engineering for all parties involved in the permit applicat ion process for a “permit by rule” via a minimum property line setback distance regulation. The method described in this manuscript shows that a gin emitting 24 lbs/hr PM10 would have a required setback distance of 58 meters if only modeling errors were a ccounted for. The same gin would have a required setback distance of 13 meters if both modeling errors and sampler errors were accounted for. Accounting for modeling and sampling errors results in a more appropriate minimum setback distance for cotton gi ns.