Use results of model sensitivity tests

Air quality model applications may be used to indicate whether future reductions in ozone

are most likely to result from reductions in VOC emissions or NOx emissions. If the results

are ambiguous (e.g., differ for different modeled days), then the State should assume that it is

necessary to track projected reductions in emissions of both precursors. For purposes of a

mid-course review, simulations of uniform reductions in anthropogenic VOC emissions and

uniform reductions in anthropogenic NOx emissions may be performed to assess which

precursor to focus on when assessing likelihood of future attainment. The preferred approach

for doing this is to use a photochemical grid model. However, if it appears that an area’s

nonattainment problem is dominated by local emissions and/or there is another means for

considering effects of ozone/precursors transported regionally into the nonattainment area,

the OZIPP/EKMA approach may be used for judging whether local changes in VOC or local

changes in NOx should be tracked to understand observed normalized trends in ozone.

Methods for judging potential significance of transport are discussed in Section 2.2.

It may happen that conclusions reached concerning which of the two precursors to track

depend on the size of the modeling domain used in the sensitivity tests. For example, if a

regional modeling domain is used, a State might conclude that predicted ozone is most


sensitive to changes in NOx emissions. In contrast, if an urban scale domain is used (e.g., < ~

300 km on a side), results might suggest future changes in VOC emissions are more

important. In fact, a finding like this is important if the analysis described in Section 2.2

suggests transport is an important part of an area’s nonattainment problem. It implies that

local changes in VOC emissions along with regional changes in NOx emissions should be

tracked in order to interpret the meaning of observed normalized trends in ambient ozone.

Use measured ambient precursor data

Ratio of indicator species. The indicator species approach is described at length in such

references as Sillman (1995), Sillman (1998), Lu, et al., (1998) and Blanchard, et al., (1999).

The approach relies on past modeling simulations (performed by others) which have noted

that predicted reduction in daily maximum ozone is sensitive to reductions in VOC if the

predicted ratio of certain indicator species (e.g., O3/NOy) is low, but is sensitive to reductions

in NOx if the ratio is high. If measurements of indicators exist, a State can review these data

on days where monitored ozone exceeds 0.12 ppm, to note whether the ratio is in the VOCsensitive

or NOx-sensitive range. There is generally also a range of values for the ratio in

which it is unclear whether maximum observed ozone is limited by availability of VOC or

NOx. If the observations fall in this latter range, it would be necessary to estimate future

changes in both NOx and VOC emissions in order to interpret whether the ozone trend data

suggest timely attainment is likely.


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