The Sierra Club challenged the issuance of a final rule by the United States Environmental Protection Agency (EPA) which stated that nonattainment of National Ambient Air Quality Standards (NAAQS) for particulate matter (PM) in a planning area in California was a result of emissions from Mexico. In its rule, EPA decided that the planning area had attained NAAQS and labeled the area as in "attainment" under the Clean Air Act (CAA).[1] The Ninth Circuit agreed with the Sierra Club that EPA's decision was contrary to the scientific evidence EPA used to make its decision. The court found that further proceedings would not be useful and, as a result, vacated EPA's order and remanded the case to EPA with the instruction to reclassify the planning area as a "serious" nonattainment area.

Under the CAA, states are required to meet standards for levels of particulate matter.[2] One requirement is that the PM concentration in an attainment area cannot exceed 150 g/m3 for more than one day per year (24-hour standard).[3] The other requirement is that the PM concentration cannot exceed a mean of 50 g/m3 for the entire year (annual standard).[4] Areas that exceed the 24-hour standard for PM more than one day a year, or exceed the annual standard for the year, are classified as "nonattainment" areas.[5] Nonattainment areas are designated either "moderate" or "serious."[6] For moderate attainment areas, EPA gives the state a deadline by which it must meet the NAAQS.[7] If the state does not meet this deadline, the area is reclassified as serious[8] and the state must enact more stringent pollution controls.[9] However, if the state demonstrates that emissions from other countries are the source of the nonattainment, the area will not be reclassified as a nonattainment area.[10]

At issue in this challenge was the Imperial Valley Planning Area, an area of southeast California bordered by Mexico. EPA classified Imperial Valley as a moderate nonattainment area, and according to the CAA, California had until December 31, 1994 to comply with the PM NAAQS or be classified as a serious nonattainment area.[11]

When EPA failed to take action during the six years after the attainment date of December 31, 1994, Sierra Club sued EPA in the District Court for the District of Columbia to reclassify the Imperial Valley as a serious nonattainment area. In a consent decree, EPA agreed to make its Imperial Valley determination by October 9, 2001. On August 10, 2001, EPA gave notice that it intended to issue a rule that California had proven that Imperial Valley had exceeded the PM NAAQS only because of emissions from Mexico. This determination was based on wind direction and other sources of evidence. Sierra Club commented on the proposed rule and stated that the evidence cited by EPA did not prove California's contention, that California assumed but did not show causation between emissions from Mexico and CAA violations, and that California did not give reasons for exceeding the PM NAAQS after December 31, 1994. EPA rejected Sierra Club's comments and issued a final rule on October 19, 2001 that emissions from Mexico had caused the PM NAAQS exceedences in the Imperial Valley. Sierra Club petitioned the Ninth Circuit to review EPA's determination under the Administrative Procedure Act (APA).[12]

The court reviewed EPA's rule under the APA's arbitrary and capricious standard[13] to determine whether EPA could "articulate[] a rational connection between the facts found and the choice made."[14] Although the court gave deference to EPA's interpretation of technical information, it agreed with Sierra Club that EPA's evidence failed to show that emissions from Mexico caused the PM NAAQS to be exceeded. The court focused on wind data for two days in 1993 when the PM NAAQS were not met.

The court first addressed whether it had to find that emissions from Mexico caused the exceedances on either or both days to vacate EPA's finding. Because the air quality monitor sampled the air on every sixth day, the number of days the PM exceeded NAAQS was adjusted to account for this sampling rate. Therefore, if the court found that emissions from Mexico did not cause either of the two exceedances, the 24-hour standard was actually exceeded 4.3 days per year. If emissions from Mexico did not cause one of the exceedances, the total was actually 2.14 days per year. As a result, the court had to find that emissions from Mexico caused both exceedances to find that Imperial Valley complied with the CAA 24-hour standards.

For EPA's theory about the emissions to be true, winds on those days must have been blowing from Mexico into California, or from the south. EPA's evidence showed that winds on those days were predominantly from the northwest and only nominally from the south or southwest. The Ninth Circuit found that EPA's definition of southerly winds was "at the least, expansive and, at most, positively incorrect."[15] In addition, the southerly winds identified by EPA were from the west-southwest and did not show that the winds came from the area of Mexico that EPA claimed was the source of the emissions. The evidence did not prove EPA's theory that the emissions came from Mexico and, as a result, did not excuse Imperial Valley from NAAQS violations. The court decided that further proceedings would not be helpful because, based on EPA's evidence, emissions from Mexico could not have caused the PM exceedances. The Ninth Circuit granted Sierra Club's petition for review, vacated EPA's final rule, and remanded the case to EPA with instructions to label the Imperial Valley a serious nonattainment area.