Wintertime PM2.5 concentrations during persistent, multi-day cold-air pools in a mountain valley

In January and February 2011, PM2.5 concentrations in residential and nonresidential areas of Salt Lake City, Utah, were elevated during days with persistent multi-day stable layers or cold-air pools (CAPs). Under most conditions the PM2.5 concentrations and atmospheric stability increased with time during these events, so that the highest PM2.5 concentrations were observed in long-lived CAPs. PM2.5 concentrations were generally observed to decrease with increasing elevation and were linearly related to the pre-sunrise valley heat deficit, an instantaneous measure of atmospheric stability. Decreases of up to 30 percent were observed as elevation increased from 1300 to 1600 m. During the CAP episode of 23–30 January, concentrations of PM2.5 increased roughly linearly with time at all elevations at the rate of about 6 μg (m3 day)−1. Higher elevation sites also experienced more rapid influxes of clean air during the mix-out of a CAP on 16 January, although short-lived episodes of higher concentrations occurred at times when polluted air was carried upslope from the residual CAP that persisted at lower elevations.