72-hour backward trajectories be calculated for any days within the past three years in which

observed 8-hour ozone concentrations were greater than 85 ppb. Another way to look for

overwhelming transport would involve the use of pollution wind roses. These combine wind

speed and wind direction with observed ozone concentrations. The ambient data can be

stratified by concentration levels and used with regional emissions analyses, discussed in the

next section, to assess transport effects.

To assess the 51.904(a)(3) criteria, States should identify potential downwind areas that

may be affected by the local area in question and perform similar analyses from the perspective

of the downwind area.

It should be noted that there are some limitations associated with the use of trajectories

and wind roses. In particular, it is not possible to apportion contribution to an area’s ozone



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