DRAFT 6-29-05

18 ENVIRON, 2002. User's Guide for a Comprehensive Air Quality Model with Extensions (CAMx).

ENVIRON Corp, Novato, CA 94945. Also see, http://www.camx.com.


confirm whether an area is affected by transport, EPA does not believe such modeling would be

necessary to support all such petitions. However, EPA is encouraging the use of modeling as an

element of the weight of evidence to support an OTA petition, where such use is feasible and/or


2.3.1 Source apportionment: One modeling tool that can be used to evaluate the

downwind contributions of emissions in upwind States is source apportionment. The source

apportionment technique was developed to provide a means of estimating the contributions of

many different source areas/categories to ozone formation in a single model run. This is

achieved by using multiple tracer species to track the fate of ozone precursor emissions (VOC

and NOx) and the ozone formation caused by these emissions within a simulation. The

methodology is designed so that all ozone and precursor concentrations are attributed to the

selected source areas/categories at all time steps. Thus, for all receptor locations and times, the

ozone concentrations predicted by the model are attributed to various source areas/categories

selected for analysis. Additional information on the source apportionment technique can be

found in the CAMx User’s Guide.18 Results that indicate large contributions from sources

outside the local area could support a determination of overwhelming transport.

2.3.2 Sensitivity analyses / Zero out modeling: The zero-out modeling technique provides

another technical approach to quantifying the downwind impact of emissions in upwind States.

The zero-out modeling provides an estimate of downwind impacts by calculating the difference

between the model estimates from a base case run and the estimates from a simulation in which

the base case man-made emissions of NOx and VOC are removed from a specific area. This

approach is useful for showing both the effects of transported ozone on a particular area and the

effects of the emissions within a local area on ozone locally and downwind. If the response of

the model is small (i.e., even with zero local emissions, there is still a local ozone problem due to

transport), when emissions from the OTA are removed, it would support a determination that

local ozone is strongly influenced by transport.



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