Trends in ambient precursor concentrations. If a downward trend in ambient ozone

concentrations is accompanied by changes in one precursor (e.g., VOC) but not the other, this

may be considered in judging which precursor is likely affecting observed changes in ozone.

Deciding on the area for which to estimate past and future changes in emissions. In order

to decide whether a nonattainment area is on track toward attainment, a State needs to (1)

review available normalized ozone trend data, (2) review changes in emissions accompanying

these observed trends, (3) note the reduction in ozone still needed to meet the NAAQS, (4)

estimate the corresponding additional reductions in emissions needed to realize the needed

additional reduction in ozone, and (5) compare additional emission reductions provided for in

the SIP revision with the necessary additional reductions estimated as necessary in (4).

Earlier, we discussed how a State may determine whether it needs to focus on VOC

emissions, NOx emissions or both. However, it is also necessary to define the area for which

past and future changes in emissions should be estimated. The nonattainment area is one

such area for which States should estimate changes in emissions. If a State believes that

regional transport is an important factor affecting observed exceedances of 0.12 ppm,

changes in VOC and/or NOx emissions also need to be calculated for an additional, larger

geographic area. To determine how large this area should be, we recommend that States note

every day with one or more observed exceedances within the nonattainment area between the

time of the most current SIP revision (e.g., 1999) and the time of the mid-course review (e.g.,


2004). For each day, states should compute a back trajectory for 36 hours, beginning at the

time and site with the highest observed exceedance. For this purpose, we recommend using

the HY-SPLIT model developed by NOAA (NOAA, 1999). However, other peer reviewed

trajectory models may also be used. This analysis will yield a bundle of 36-hour backtrajectories.

States should use the smallest geographic area for estimating emission changes

which is consistent with including 90% of the total hours represented by the trajectories (i.e.,

90% of 36 hours times number of days with exceedances). This example assumes that most

of the impact at a monitoring site occurs as a result of emissions occurring within 36 hours’

travel time, and provides a means to avoid having to consider a very large area on the basis

of a small number of trajectories. Periods differing from 36 hours may be considered if

justified on a case-by-case basis.



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