2.2 Use Air Quality Data To Assess Relative Importance Of Regional Transport And

Local Emissions

States may use this analysis to determine whether to be concerned with past and

projected changes in emissions, (1) only within the nonattainment area , or (2) within a larger

geographical area (including the nonattainment area) as well. The analysis consists of several


(1) determine the resultant wind direction for each day with a monitored exceedance;

(2) identify monitors which are upwind and downwind on each day with an observed


(3) select air quality measures to consider at upwind and downwind sites;

(4) compare and note differences in upwind and downwind ozone as well as trends in

upwind and downwind ozone;

(5) use the information developed in (4) to decide whether it is sufficient to consider

only emission changes occurring within the nonattainment area or whether a larger

geographical area should also be considered.

Determining resultant wind direction. This may be done in one of two ways: (a) using

surface wind velocity measurements, or (b) through use of a meteorological model. If surface

wind velocity measurements are used, the resultant wind direction should be estimated solely

on the basis of daytime measurements (e.g., 7am-8pm) made on each day with a recorded

exceedance. If the resultant wind direction is estimated using a trajectory or other

meteorological model, a 24-hour period (ending at the time of the observed exceedance) may

be considered, so long as an effort is made to account for overnight wind shear.

Identifying upwind and downwind monitors. This step is used to identify which monitored

data are likely to be most impacted by emissions in the nonattainment area. Using previously

estimated resultant wind directions, a group of monitors is identified as downwind and another

group is identified as upwind. To illustrate, for a nonattainment area characterized by large

concentrations of emissions in the center of the designated nonattainment area, downwind



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