California's Central Valley is once again without an approved State Implementation Plan for ozone. On Friday, January 20, the Ninth Circuit held, in Sierra Club v. US EPA, that EPA had acted arbitrarily and capriciously in approving the 2004 plan.
The Valley is an extreme non-attainment area for ozone, and has been a non-attainment area since 1991. California proposed a SIP for the Valley in 2004, but amendments to the plan and EPA's review took so long that by the time EPA approved the plan, which was based on 2004 data, in 2010 there was more current data that EPA chose not to consider in approving the SIP. The Ninth Circuit held that the SIP was required to be based on "current" and "accurate" data, and although that standard doesn't require constant updating, EPA could not ignore data that was collected in 2007 and in EPA's possession when it approved the SIP.