Meteorological conditions and PM2.5 concentrations in winter in a high altitude urbanized valley

Wintertime PM2.5 concentrations in Reno, NV sometimes exceed the 24-hour standards set by the USEPA to protect public health. For the December 2009 - January 2010 period ambient PM2.5 levels were compared by corresponding meteorological conditions. Low wind speeds, strong temperature inversions and presence of snow cover were associated with high PM2.5 levels. A simple stability parameter (difference in temperature between a valley bottom location and valley side wall location) was highly correlated to daily average PM2.5, with a squared correlation coefficient of 0.78 for the two month period. Snow cover appeared to be a necessary condition for PM2.5 concentrations in excess of the standard. On snow covered days the surface based temperature inversion was often maintained throughout the day, while days without snow cover saw at least several hours a days without inversions and presumably enhanced vertical mixing. The results argue for additional restrictions on emissions for days with snow cover and light winds.