sites on a day might be characterized as those included within a relatively large arc (e.g., 120o)

whose vertex is at the center of the emissions and whose axis is the resultant wind direction.

All other sites in this example would be upwind sites.

Select air quality measures to consider at upwind and downwind sites. Since the identity

of downwind and upwind sites is a function of the resultant wind direction, individual

monitoring locations may be upwind on some days and downwind on others. Thus, to assess

relative importance of transport, it is more appropriate to look at concentrations and trends

for the group of upwind vs. the group of downwind sites rather than focus on all observations

at individual sites. We recommend looking at measures such as the highest, second highest

and spatially averaged daily maximum observed ozone concentrations at downwind vs. upwind

sites on individual days, as well as values for these variables which have been averaged over

an ozone season. In addition, we recommend looking at trends in these parameters observed

over the period between the SIP revision and the mid-course review and, perhaps, over a

longer period stretching back 10 years from the mid-course review.

Compare absolute ozone concentrations and trends at upwind and downwind sites. The

purpose of this step is to contrast ozone concentrations and trends in ozone concentrations for

each of the air quality measures selected in the previous step. This comparison is used to

judge whether transport is an important factor affecting a nonattainment area’s attainment

status. Large contrasts between upwind and downwind sites suggest that local emissions are

an important factor affecting the area’s attainment status. A mixed set of results is probably

consistent with both local emissions and transport playing a significant role. If results for

upwind and downwind monitors are nearly identical, this suggests that an area’s

nonattainment status is dominated by regional transport.



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