Relationship between particulate air pollution and meteorological variables in Utah's Salt Lake Valley

Critical meteorological factors affecting daily particulate concentrations during winter for Utah's urbanized Salt Lake Valley are examined on the basis of forty years of data. In a typical winter, the National Ambient Air Quality Standard for particulate matter with aerodynamic diameter less than 2.5 microns (PM2.5) is exceeded during 6 multi-day events comprising 18 winter days. Multi-day episodes of high stability produce these events, as synoptic-scale high-pressure ridges transit across Utah. The valley heat deficit, a bulk measure of atmospheric stability, exhibits large winter-to-winter variations that are highly related to similar variations in PM2.5. While control strategies have led to downward trends in concentrations for some primary pollutants, no long-term trends in valley heat deficit are evident over the 40 years. PM2.5 concentrations rise gradually over a period of days after a heat deficit threshold is exceeded as the air within the valley becomes decoupled from generally stronger winds aloft. Concentrations climb at a typical rate of about 10 μg m−3 d−1 over a four-day period to about 60 μg m−3 during these episodes. During episodes when PM2.5 concentrations exceed 35 μg m−3, the atmospheric column in the valley is characterized by: temperature below 0 °C; relative humidity in excess of 50%; and light wind speeds. PM2.5 concentrations in excess of 35 μg m−3 are four times more likely when the valley is snow covered than when it is not. A stepwise multiple linear regression based upon selected meteorological variables is used to estimate daily values of PM2.5 during two independent winters. The correlation between observed and estimated PM2.5 for these winters reaches 0.81.