If a meteorologically adjusted trend in ozone or PM2.5 can be estimated, then the

information can be used to establish a link between emissions and air quality trends. This is not

always straightforward due to the multitude of emissions precursors that may lead to high ozone

and PM2.5 concentrations. A careful analysis of (meteorologically adjusted) air quality trends

and emissions trends of each of the ozone and PM precursors (as well as primary PM) is needed

to fully establish relationships. Detailed emissions information as well as a solid understanding

of the conceptual model of ozone or PM2.5 formation is needed. If a trend can be established

based on past emissions changes and air quality changes, then future year predicted emissions

levels can be used to extrapolate future air quality.

A simpler (and more uncertain) way to qualitatively assess progress toward attainment is

to examine recently observed air quality and emissions trends. Downward trends in observed air

quality and in emissions (past and projected) are consistent with progress towards attainment.

Strength of the evidence produced by emissions and air quality trends is increased if an extensive

monitoring network exists and if there is a good correlation between past emissions reductions

and current trends in ozone or PM2.5. EPA recently prepared a report that analyzed statistically

significant trends in ozone (U.S. EPA, 2004a and U.S. EPA, 2005c) and ozone precursor


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