When NOx and volatile organic compounds
(VOCs) enter the atmosphere, they react in the
presence of sunlight to form ground-level
ozone, which is a major ingredient of smog.
The revised NAAQS for ozone is 0.08 ppm
(eight-hour average). Many urban areas do not
meet this standard and are classified as
nonattainment, and a large number of power
plants are situated within these nonattainment
areas. Nonattainment status is attributable not
only to locally released NOx emissions but also
to significant amounts of ozone and ozone precursors
(NOx and VOCs) transported by wind
over a wide geographical region.