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Many populated valleys in the western United States experience increased concentrations of particulate matter with diameter of less than 2.5 μm (PM2.5) during winter stagnation conditions. Further study into the chemical components composing wintertime PM2.5 and how the composition and level of wintertime PM2.5 are related to meteorological conditions can lead to a better understanding of the causes of high PM2.5 and aid in development and application of emission controls. The results can also aid in short-term air-pollution forecasting and implementation of periodic emission controls such as burning bans. This study examines relationships between PM2.5 concentrations and wintertime atmospheric stability (defined by heat deficit) during snow-covered and snow-free conditions from 2000 to 2013 for five western U.S. urbanizations: Salt Lake City, Utah; Reno, Nevada; Boise, Idaho; Missoula, Montana; and Spokane, Washington. Radiosonde data were used where available to calculate daily heat deficit, which was compared with PM2.5 concentration for days with snow cover and days with no snow cover. Chemically speciated PM2.5 data were compared for snow-cover and snow-free days to see whether the chemical abundances varied by day category. Wintertime PM2.5 levels were highly correlated with heat deficit for all cities except Spokane, where the airport sounding does not represent the urban valley. For a given static stability, snow-cover days experienced higher PM2.5 levels than did snow-free days, mainly because of enhanced ammonium nitrate concentrations. Normalizing average PM2.5 to the heat deficit reduced year-to-year PM2.5 variability, resulting in stronger downward trends, mostly because of reduced carbonaceous aerosol concentrations. The study was limited to western U.S. cities, but similar results are expected for other urban areas in mountainous terrain with cold, snowy winters.

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The inclusion of seven wavelengths in thermal/optical carbon analysis of speciated PM (particulate matter with an aerodynamic diameter ≤2.5 μm) samples allows contributions from biomass burning and secondary organic aerosols to be estimated. This separation is useful for evaluating control strategy effectiveness, identifying exceptional events, and determining natural visibility conditions.

Light attenuation (b att ) measured from filter light transmission is compared with elemental carbon (EC) measurements for more than 180,000 collocated PM 2.5 (particulate matter [PM] Յ 2.5 m in aerodynamic diameter) and PM 10 (PM Յ 10 m in aerodynamic diameter) samples from nearly 200 U.S. locations during the past 2 decades. Although there are theoretical reasons for expecting highly variable relationships between b att and EC (such as the effects of "brown carbon" and iron oxides in PM 2.5 ), reasonable correlations are found. These correlations are not a strong function of season or location (e.g., rural vs. urban). Median EC concentrations can be predicted from filter transmittance measurements to within Ϯ15-30%. Although EC predicted from b att shows larger uncertainties (30 -60%), especially at concentrations less than 0.3 g/m 3 , the consistent mass absorption efficiency ( att ) derived from the regression analysis demonstrates the feasibility of using b att as a surrogate for EC. This study demonstrates that a constant factor of 0.1 g/m 2 (equivalent to the 10 m 2 /g att used in the Interagency Monitoring of Protected Visual Environments chemical extinction formula) can be used to estimate EC concentrations from b att through a Teflon-membrane filter sample. Greater accuracy is achieved with site-specific att derived from a period with collocated EC measurements. INTRODUCTIONElemental carbon (EC) or black carbon (BC) in suspended particulate matter (PM) is related to adverse effects on human health, 1-3 visibility degradation, 1,2 the Earth's radiation balance, 3-5 and material damage. 6 Thermal methods for EC analyses quantify PM chemical and physical properties, 7 whereas BC analyses 8 -11 characterize PM light absorption properties.Attenuation of light reflected from or transmitted through a filter onto which PM is collected has long been used to quantify BC as an air pollution indicator. 12 British smoke, 13 also called "black smoke," measures the darkening of a paper filter with reflected light after polluted air is drawn through it. The U.S. coefficient of haze (COH) 14 measures attenuation of light transmitted through a paper tape that advances to a clean portion when the sample becomes too dark. Relationships between the COH "soiling index" and PM mass concentrations were established, 15 but these became less useful as soot from incomplete combustion was reduced in primary emissions. 16,17 Modern filter transmission instruments for BC include the Aethalometer, 18 the particle soot absorption photometer (PSAP), 19 the integrating sphere, 20,21 and several variations of the integrating plate. 20,[22][23][24][25][26][27][28][29][30][31][32] Modern filter reflectance methods include the SX200 black smoke monitor 33 and the M43D smoke stain reflectometer. 32 The multiangle absorption photometer (MAAP) 34,35 is a light transmission and reflection method that also accounts for light scattering effects.Light absorption (b abs in inverse megameters, Mm Ϫ1 ) is approximated by light attenuation (b...