Abstract
Abstract. This study investigates the causes of elevated PM2.5 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 PM2.5 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 PM2.5 > 35 μg m−3 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 (NH4NO3) and water associated with it are found to be main reasons for the PM2.5 exceedances. An effective-variance chemical mass balance (EV-CMB) receptor model using locally-derived geological profiles and inorganic/organic markers confirmed secondary NH4NO3 (27–37%), residential wood combustion (RWC; 11–51%), and diesel engine exhaust (7–22%) as the dominant PM2.5 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 NH4NO3 is also attributed to RWC and diesel engines (including snow removal equipment) through oxides of nitrogen (NOx) emissions from these sources. Findings from this study may apply to similar situations experienced by other urban valleys.
Highlights
The Truckee Meadows is a mountain valley in Northern Nevada of the US with the Sierra Nevadas to its west and the Great Basin to its east
Better effective-variance chemical mass balance (EV-chemical mass balance (CMB)) fits to the data result from a combination of paved road dust (RNOPVRD), road salt (RNOSALT2), mobile source, residential wood combustion (RWC), and secondary (AMNIT and AMSUL) source profiles
The formation of PM2.5 episodes shows meteorological and chemical characteristics that are likely common for many urban valleys in the Western Cordillera
Summary
The Truckee Meadows is a mountain valley in Northern Nevada of the US with the Sierra Nevadas to its west and the Great Basin to its east. The area of the valley floor is ∼ 25 km, containing downtowns of the cities of Reno and Sparks with a population of ∼ 200 000 (2010 census). Similar urban valleys are common in the Western Cordillera region, including Salt Lake City, Utah, BoiseNampa, Idaho, Missoula, Montana, and Kelowna, British Columbia (Canada). Air quality in such valleys is determined by the interaction between synoptic winds and local thermal circulation (Fernando, 2010). A. Chen et al.: Wintertime particulate pollution episodes in the Truckee Meadows are from area (non-point) sources associated with wintertime residential heating. This study investigates interactions between meteorology and emissions that lead to wintertime PM2.5 episodes in the Truckee Meadows valley. Findings from this study provide information for developing strategies to reduce PM2.5 levels in this and similar urban valleys
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