ARCHIVED REPORTS LEC APPLICATION FOR CERTIFICATION

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In the San Joaquin Valley,PM10 and PM2.5 levels are highest during the late fall and winter. Colder,more stagnant conditions during this time of the year are conducive to the buildup of PM, including the formation of secondary ammonium nitrate. Source Categories Included in the Inventory The goal of the offset ratio analysis is to determine what the impact of directly emitted PM10 from the new source is, and to compare that with the impact of indirect PM10 from the source providing the offsets. Ideally,each source category would be defined as narrowly as possible—for example,individual types of combustion sources, such as power plants and automobiles,would be treated separately—so that the direct and indirect PM10 impacts would be related as closely as possible. However, studies11 that have quantified the contribution of source categories to measured ambient PM have not been able to make distinctions between similar sources. At present,the best that they can do is determine that a fraction comes directly from wood burning, another fraction comes directly from other combustion sources (automobiles,power plants,etc.),fractions form indirectly from nitrates and sulfates, a fraction comes from marine air, and the rest from various other sources. Calculations The interpollutant offset ratio is the number of tons of sulfur dioxide (SO2) emission reductions that would result in the same reduction in ambient PM10 concentration as one ton of direct PM10 emissions. The methodology used to develop an interpollutant offset ratio for SO2 and PM10 uses Chemical Mass Balance (CMB) and rollback modeling from the San Joaquin Valley Air Pollution Control District (SJVAPCD) draft 2007 PM10 plan. This methodology was provided by Jim Sweet of the SJVAPCD's Planning Division for use in previous applications. The data used in this analysis were taken from the District's modeling results for the Modesto 14th Street monitoring station and emission inventories for Stanislaus County. The Modesto station,located 40 miles from Lodi,is the closest station for which all necessary data are available. The analysis calculates the contribution from subregional industrial combustion-related PM10 emissions to PM10 concentrations on a PM10 episode day,and compares that to the contribution from subregional SO2 emissions to ammonium sulfate concentrations. The analysis determines the increase in episode PM10 concentration(in ug/cu m) that results from a ton of direct industrial combustion-related PM10 emissions,and the increase in episode PM10 concentration(in ug/cu m) that results from a ton of SO2 emissions. The ratio of SO2 impact to direct PM10 impact is the interpollutant offset ratio. " Chemical Mass Balance(CMB)modeling is a way of estimating how much various sources contribute to ambient PM10 concentrations. The CMB model uses a computer program whose inputs are source profiles and an ambient PM10 sample or samples which have been analyzed for a variety of chemical components. The CMB model finds the mix of sources whose combined amounts of chemical components best approximates those in the ambient sample. In other words,the output of the CMB model is estimates of the relative contributions from the various emissions sources that would result in the specific profile of chemical components that make up the PM10 in the ambient sample.