On Wednesday, EPA Region IX disapproved ADEQ's request to treat four exceedances of the National Ambient Air Quality Standard ("NAAQS") for PM-10 during 2008 as "exceptional events". A large portion of Maricopa County has been classified as a non-attainment area for PM-10 (particles of dust 10 microns or smaller) since 1996. Under the Maricopa Association of Governments ("MAG") 2007 Five Percent Plan for PM-10, the goal for the non-attainment area was to have no more than three violations of the PM-10 NAAQS in the three-year period 2008-2010. After registering these violations at the West 43rd Avenue monitoring station in southwestern Phoenix, ADEQ concluded that the high levels of dust were due to "exceptional events" (i.e., dust storms) rather than human activity and requested EPA to concur in that determination. EPA's finding that the exceedances were not caused by exceptional events, means that the Maricopa County non-attainment area will not be able to demonstrate attainment of the PM-10 NAAQS by 2010.

The failure of MAG's Five Percent Plan to bring the area into attainment means that EPA will also most likely reject the plan itself. In developing and submitting a new plan, MAG will undoubtedly urge local governments (most notably the Air Quality Department of Maricopa County) to impose tighter controls over dust generating activities. If the problem is not corrected to EPA's satisfaction within two years, the State could face sanctions under the Clear Air Act, including the loss of over $1 billion of federal highway funds.

Here is an article on this issue from the Arizona Republic and here is a great presentation by MAG. You can read EPA's technical support document for this decision here.