Cache Valley Air Quality Studies

The Cache Valley airshed is in serious threat of exceeding the U.S. National Ambient Air Quality Standard (NAAQS) for Particulate Matter less than 2.5 µm in diameter (PM 2.5). Although the local airshed approaches both the annual (15 µg m-3) and the 24-hr standard (65 µg m-3), it is the 24-hr standard that is in the greatest danger of exceedance. As of June 2006, the regulatory allowable 98 th percentile value for 2006, roughly the eighth highest value) can only be 33.2 µg m-3 without exceeding the NAAQS. To date, this value has been exceeded four times. For comparison, the target value was exceeded 6, 41, and 39 times in the years 2003, 2004, and 2005, respectively. It should also be noted that the U.S. EPA is currently considering lowering the 24-hr standard to 35 µg m-3. The Cache Valley PM 2.5 is somewhat uniquely a wintertime problem, when low lying, persistent radiation and subsidence inversions set up, trapping pollutants in the Valley for extended periods of time, allowing photochemically-derived particulate material to become elevated. Chemical analysis by researchers at Utah's Division of Air Quality and Air Monitoring Center, as well as Utah State University, have shown that 50-95% of the PM 2.5 collected at the Logan site is composed of ammonium nitrate (NH 4 NO 3). In order to understand the local pollutant behavior and identify the most appropriate remediation scenarios, several sequential and parallel research programs have been conducted by Dr. Randy Martin and others at Utah State University. This document summarizes these efforts and attempts to coalesce these studies into a single report. 1) PM Homogeneity-Statistically (95% CI), the Cache Valley is homogeneous with regard to PM 2.5 concentrations during inversion episodes, although some site-to-site variation may exist. The areas which trended lower in concentrations were the extreme southeast (Hyrum) and west (Mendon) portions of the Valley. As a partial result of these studies, FRM regulated monitoring stations were established in Hyrum, 2) Valley Ambient Ammonia-Based on measurements at the Logan location, the Valley's wintertime formation of ammonium nitrate was found to be limited by the availability of nitrate, specifically nitric acid (HNO 3). The ambient Logan wintertime NH 3 averaged (24-hr) 9.2 µg/m 3 (13.9 ppb). Furthermore, following these data, the Cache Valley was found to be NH 3-rich by a factor of approximately two. Comparisons of wintertime ambient NH 3 concentrations between the Valley's urban …