Recently, Honorable Judge Ronald M. Gould, writing for a panel of the Ninth Circuit Court of Appeals, found the Environmental Protection Agency’s (“EPA”) 2010 approval of the San Joaquin Valley’s 2004 1-hour ozone National Ambient Air Quality Standard plan (“2004 SIP”) was arbitrary and capricious, citing EPA’s failure to adequately address the potential staleness of mobile source emissions data used to formulate the plan’s emissions inventory.  The court’s decision invalidates EPA’s approval of the plan and requires the agency to conduct its review process anew.  The case potentially signals EPA’s more stringent review of the accuracy and currency of emission inventories during its plan approval process. 

Following EPA’s approval of the 2004 SIP, Sierra Club and several environmental groups petitioned the Ninth Circuit Court Appeals to review EPA’s approval on the basis that mobile source data, current at the time the plan was submitted to EPA in 2004, was outdated and inaccurate by the time the plan, which was amended in 2006 and clarified in 2008, was approved in 2010.  During the six-year period between plan submission and approval, California had replaced the computer modeling tool it used to estimate mobile source emissions with the next generation of that modeling tool, which was better able to capture emissions from heavy-duty trucks.  Also during that time period, California had presented EPA with the Valley’s 2007 SIP for the 8-hour ozone standard (“2007 SIP”), which relied on data compiled through the use of the updated tool.  The court noted that a comparison of the emissions inventories in the 2004 and 2007 plans revealed apparent disparities in emissions estimates for nitrogen oxides (NOx), with the 2004 SIP potentially underpredicting total daily NOx emissions in the Valley.  In the court’s opinion, these disparities, which the court attributed to the state’s change in modeling tools, undermined the accuracy and currency of the 2004 SIP emission inventory data.

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