ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-10051-2012 Wintertime particulate pollution episodes in an urban valley of the Western US: a case study Chen L.-W. A. 1 2 Watson J. G. 1 2 Chow J. C. 1 2 Green M. C. 1 Inouye D. 3 Dick K. 3 Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China Washoe County District Health Department Air Quality Management Division, Reno, NV, USA 02 11 2012 12 21 10051 10064 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/10051/2012/acp-12-10051-2012.html The full text article is available as a PDF file from 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.

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