Document ID: EPA-HQ-OAR-2007-0225-0083
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2007-08-02T04:00Z

Discussion of 0.079 ppm and 0.075 ppm Benefits Estimation Methodology

EPA employed a monitor rollback approach to estimate the benefits of
attaining an alternative standard of 0.079 ppm nationwide. This approach
likely understates the benefits that would occur due to implementation
of actual controls to reduce ozone precursor emissions because controls
implemented to reduce ozone concentrations at the highest monitor would
likely result in some reductions in ozone concentrations at attaining
monitors down-wind (i.e. the controls would lead to concentrations below
the standard in down-wind locations).  Therefore, air quality
improvements and resulting health benefits from full attainment would be
more widespread than we have estimated in our rollback analysis. 

EPA calculated 0.075 ppm benefits by interpolating the 0.070 ppm
benefits estimates. This interpolation approach may overestimate
benefits relative to a modeled control scenario developed specifically
to attain the 0.075 ppm alternative. The interpolation method scales
down benefits only at the monitors we project to exceed 0.075 ppm—but
it still captures the benefits achieved by the 0.070 ppm regional
control strategy that occur outside of these projected non-attainment
areas. To the extent that a modeled emission control strategy to attain
0.075 ppm did does not include these broader regional emission
reductions, total benefits would be lower than those we have estimated
in this RIA. 

Interpolation and monitor rollback methods of benefits estimation are
inherently different.  As described above, for the purposes of reviewing
this analysis, the reader should understand that the benefits described
for attaining a standard of 0.079 are likely understated, whereas the
estimated benefits of attaining a standard of 0.075 are likely
overstated.  We will develop and present consistent approaches for the
alternative standards for the final RIA.

 This procedure is detailed in Appendix 6A.