ARCHIVED REPORTS LEC APPLICATION FOR CERTIFICATION

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Determining an Offset Ratio Reductions of precursor pollutants as offsets for the pollutant being formed have been approved by the SJVAPCD for other major projects. See Attachment 5.1G-1.1 for two examples of approved offset ratio calculations (SO2 for PM10 and NOx for PM1o). All examples of projects for which PM10 precursors have been accepted as offsets for PM10 emissions have based the offset ratio on the relative effect that the precursor emissions have on ambient PM10 levels versus directly-emitted PM1o. The health benefits due to reductions of ambient concentrations of NOx,SO2,or ozone have not been considered. The determination of an appropriate interpollutant offset ratio begins by determining the air quality impact (i.e., ground level concentration) due to directly emitted PM10 from sources similar to the new source in units of microgram/cubic meter per ton per year (tpy) emitted. The general methodology is to identify the portion of measured average PM10 levels that is attributed to direct PM10 emissions from combustion sources, and divide that concentration by the portion of the emission inventory that contributes to it. The result is the theoretical amount that the regional average PM10 concentrations would go up for every new ton of PM10 directly emitted by the source category. Next,the same calculation is performed for the portion of ambient PM10 that is the result of secondary particulate formation from emissions of the pollutant providing the offsets. If the proposal is to provide SO2 reductions to offset PM10 increases,the sulfate portion of the ambient PM10 levels is divided by the SO2 inventory contributing to formation of those sulfates. S02 for PMlo The SO2 to PM10 interpollutant offset ratio is the amount of SO2 that would result in 1 pg/m3 of ground-level PM10 divided by the amount of directly emitted PM10 that would result in 1 pg/m3 of ground-level PM1o• Ambient PM10 Measurements The SJVAPCD and other jurisdictions recognize that ambient concentrations vary seasonally and require offsets to be provided on a quarterly basis. Recognizing that ambient particulate levels are higher in the winter than other seasons, some districts allow emission reductions from the winter season to be used to offset increases in other seasons,but not vice versa. All of the criteria pollutants subject to offsets for this project are, or are precursors for, PM10—a pollutant for which offsets are required because,on certain days, ambient levels exceed short-term ambient standards. The agency's goal is to reduce the pollutant levels on the worst days to below the ambient standards. In determining whether interpollutant offsets provide a net air quality benefit,therefore, it is reasonable to give greatest consideration to the season of most concern, and set a single ratio based on the days when pollutant levels exceed the standards. This approach has been taken previously by the SJVAPCD in setting PM10 interpollutant offset ratios.