Pocatello Region Air Quality Plans & Reports
The Portneuf Valley PM10 Non-Attainment Area (PVNAA) contains 96.6 square miles of Pocatello, Chubbuck, and surrounding areas. It includes federal land managed by the Bureau of Land Management (BLM) and the Caribou National Forest, as well as privately owned land in the cities of Pocatello and Chubbuck. The combined population of the two cities is approximately 76,000.
The topography of the PVNAA is complex and aids in the formation of strong inversion layers during winter months. This is the period when most of the air pollution events occur. During these inversions, the major pollutant has been ammonium sulfate, which is formed under high humidity (winter inversions) from gasses, ammonia, and sulfur dioxide.
The PVNAA PM10 State Implementation Plan (SIP) was submitted to the United States Environmental Protection Agency (EPA) in 2004. The plan relies on control measures on point and area sources to attain and maintain the National Ambient Air Quality Standards for PM10, primarily measures for control of emissions from residential wood combustion, residential road dust, and the industrial source, J.R. Simplot. In July 2006, EPA approved the plan and granted the redesignation request. As a result, the area has been redesignated a PM10 Maintenance Area. The plan was amended in 2014 incorporate emission estimates made with the Motor Vehicle Emission Simulator (MOVES), the latest EPA-required motor vehicle emissions factor model, and with an updated road dust estimation method.
The Portneuf Valley is in attainment for PM2.5 and SO2.
The Cache Valley PM2.5 Non-Attainment Area (NAA) lies within Cache County, Utah (northern Utah) and Franklin County, Idaho (southeastern Idaho), which is part of the Logan UT-ID NAA. The Logan UT-ID NAA encompasses a bowl-shaped, topographically isolated valley measuring approximately 37.3 miles north to south and 12.4 miles east to west.
The Cache Valley experiences air stagnation events in the wintertime. During these periods, the stable layer above the ground is much deeper than a typical nocturnal inversion. Cold air is trapped in the basins, and the air mass stabilizes as high pressure aloft overtakes the region. Under these circumstances, a prolonged strong inversion layer (or layers) limits vertical mixing, trapping local pollutants in a thin layer against the valley floor. The low sun angle, short length of the days during winter months, and strong likelihood of snow cover to reflect the solar radiation are all factors that limit daytime surface heating and aggravate the situation. As a result, some inversions may not break for many days. The scenario described above leads to exceedances and violations of the 24‐hour health standard for PM2.5.
In December 2012, DEQ completed a PM2.5 SIP for the Idaho side of Cache Valley and submitted it to the US Environmental Protection Agency (EPA) aimed at demonstrating that the area would attain the national PM2.5 standard by January 1, 2015. The plan satisfied requirements from EPA’s 2007 Fine Particle Implementation Rule, which was grounded in Title I, Part D (Plan Requirements for Nonattainment Areas), Subpart 1 (Nonattainment Areas in General), of the Clean Air Act (CAA).
On January 4, 2013, the District of Columbia Circuit Court found that EPA incorrectly interpreted the CAA when determining how to implement the PM2.5 standard. The court ruled that EPA should have implemented the PM2.5 standard based on both Subpart 1 and Subpart 4 (Additional Provisions for Particulate Matter Nonattainment). Therefore, as of January 4, 2013, Idaho's PM2.5 State SIP was required to meet the requirements of Subpart 4 as well.
In December 2014, DEQ completed and submitted to EPA an amendment to Idaho's PM2.5 SIP addressing the requirements of Subpart 4 and containing the necessary evidence, analysis, and Idaho control strategies (in conjunction with the Utah SIP) to demonstrate attainment with the PM2.5 standard by December 31, 2015.