12Draxler, R.R. and Rolph, G.D., 2003. HYSPLIT (HYbrid Single-Particle Lagrangian Integrated
Trajectory) Model access via NOAA ARL READY Website (http://www.arl.noaa.gov/ready/hysplit4.html). NOAA
Air Resources Laboratory, Silver Spring, MD.
that an area petitioning for an OTA classification would complete, and submit to EPA, a full
analysis consisting of evidence from multiple forms of weight of evidence analyses as described
within this guidance. For an area to be classified as an OTA, the large majority of the tests
would have to demonstrate the area meets the criteria in 51.904(a)(2) and (3).
2.1 Ambient Data / Meteorological Analyses
There are numerous types of ambient data analyses that can be completed that show
whether an area is being overwhelmingly influenced by ozone generated outside of the local area
and whether local emissions make a significant contribution to high downwind ozone
concentrations. The following are a subset of possible analyses that States should use in
analyzing whether an overwhelming transport classification is appropriate for an area.
2.1.1 Trajectory analyses and wind roses: One useful analysis to assess the extent of
transport into a given nonattainment region is compiling back trajectories for those days upon
which the ambient NAAQS is exceeded in an area and determining the origin and transport path
of the air masses that result in violations. If a large percentage of days with ozone violations is
associated with air parcel trajectories from areas outside of the area in question, then it may be
concluded that transport is a significant contributor to a large percentage of the violations.
Shorter-path trajectories (i.e., those that originate in or near the area in question) may indicate a
greater role from local/nearby sources, however long-path trajectories (i.e., those that originate
long distances away from the potential OTA) by themselves are not sufficient evidence for an
overwhelming transport classification. One possible tool for use in calculating back trajectories
for days on which exceedance-level ozone was observed locally is the HYSPLIT12 (HYbrid
Single-Particle Lagrangian Integrated Trajectory) model. It is recommended that 24-, 48-, and
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