Wintertime particulate pollution episodes in an urban valley of the Western US: a case study

• Main Authors: L.-W. A. Chen, J. G. Watson, J. C. Chow, M. C. Green, D. Inouye, K. Dick
• Format: Article
• Language: English
• Published: Copernicus Publications 2012-11-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/12/10051/2012/acp-12-10051-2012.pdf

This study investigates the causes of elevated PM<sub>2.5</sub> 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</sub> 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</sub> > 35 μg m<sup>−3</sup> showed 24-h average temperature inversion of 1.5–4.5 °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</sub>NO<sub>3</sub>) and water associated with it are found to be main reasons for the PM<sub>2.5</sub> 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</sub>NO<sub>3</sub> (27–37%), residential wood combustion (RWC; 11–51%), and diesel engine exhaust (7–22%) as the dominant PM<sub>2.5</sub> 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</sub>NO<sub>3</sub> is also attributed to RWC and diesel engines (including snow removal equipment) through oxides of nitrogen (NO<sub>x</sub>) emissions from these sources. Findings from this study may apply to similar situations experienced by other urban valleys.