Using proportional extrapolations
If new model results are unavailable to establish a target for the mid-course year which is
consistent with attainment, a proportional approach may be needed to assess whether it is
likely that a SIP is on track. This approach uses ozone/emission sensitivity factors from prior
modeling or observed correspondence between normalized ozone trends (which ever is most
appropriate) at mid-course review time with net estimated reduction in emissions between the
SIP’s base period (e.g., 1999) and mid-course review time (e.g., 2004). Either approach may
be used to estimate a unit sensitivity factor of ozone to emission changes.
Unit Sensitivity = (ÄO3 / Ä(emissions)) (1)
The preceding information may be used in concert with (1) information about the net reduction
in emissions anticipated between time of the mid-course review and the attainment date, and
(2) the remaining difference between the design value at mid-course review time and 124 ppb
to estimate if attainment is likely by the required time. If the method suggests attainment is
likely by the required time, the SIP is assumed to be on track toward attainment. This is
shown by the expression below.
(O3)attainment = (O3)@mid-course review + (Unit Sensitivity) (Ä(future emissions)) < 124 ppb
(O3)attainment is the estimated ozone design value for the attainment year.
(O3)@mid-course review is the normalized design value at mid-course review.
Ä(future emissions) is the change in emissions anticipated between mid-course review time
and the required attainment date—note that this is generally a negative number.
Note, if the estimated ozone design value for the attainment year is greater than 124 ppb,
the “Unit Sensitivity” parameter may be used to estimate the level of emission reductions
needed to achieve 124 ppb. However, since this is a linear extrapolation, the farther the
estimated ozone design value is from 124 ppb the less reliable these results are.
For the proportional extrapolation methodology to be applied, it is necessary to make a
series of assumptions about which emissions to consider in the preceding expressions. We
make recommendations about this in the following subsection.
3.2.1 Which Emissions Trends Should I Use In The Proportional Extrapolation
First, use the analysis of transport described in Section 2.2 and results of available
modeling, described in Section 2.3 to assess whether transport is a major reason for
nonattainment of the NAAQS in the designated nonattainment area being reviewed. If it is
not, consider only emission trends within the nonattainment area. If transport and local
emissions both appear to play important roles, then consider emission trends in the
nonattainment area and emissions trends in a larger geographic area, determined using a
method like the one described near the end of Section 2.1. If transport seems to clearly
dominate local emissions, consider only the trends in the larger geographical area.
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