Wintertime particulate pollution episodes in an urban valley of the Western US: A case study
This study investigates the causes of elevated PM sub(2.5) episodes and potential exceedences of the US National Ambient Air Quality Standards (NAAQS) in Truckee Meadows, Nevada, an urban valley of the Western US, during winter 2009/2010, an unusually cold and snowy winter. Continuous PM sub(2.5) mass and time-integrated chemical speciation data were acquired from a central valley monitoring site, along with meteorological measurements from nearby sites. All nine days with PM sub(2.5) > 35 mu g m super(-3) showed 24-h average temperature inversion of 1.5-4.5 degree C and snow cover of 8-18 cm. Stagnant atmospheric conditions limited wind ventilation while highly reflective snow cover reduced daytime surface heating creating persistent inversion. Elevated ammonium nitrate (NH sub(4)NO sub(3)) and water associated with it are found to be main reasons for the PM sub(2.5) exceedances. An effective-variance chemical mass balance (EV-CMB) receptor model using locally-derived geological profiles and inorganic/organic markers confirmed secondary NH sub(4)NO sub(3) (27-37%), residential wood combustion (RWC; 11-51%), and diesel engine exhaust (7-22%) as the dominant PM sub(2.5) contributors. Paved road dust and de-icing materials were minor, but detectable contributors. RWC is a more important source than diesel for organic carbon (OC), but vice versa for elemental carbon (EC). A majority of secondary NH sub(4)NO sub(3) is also attributed to RWC and diesel engines (including snow removal equipment) through oxides of nitrogen (NO sub(x)) emissions from these sources. Findings from this study may apply to similar situations experienced by other urban valleys.