Effects of Snow Cover and Atmospheric Stability on Winter PM2.5 Concentrations in Western U.S. Valleys

Abstract

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.

Detection and variability of combustion-derived vapor in an urban basin

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Synoptic‐flow interaction with valley cold‐air pools and effects on cold‐air pool persistence: Influence of valley size and atmospheric stability

“…; Green et al . ). In addition to increasing daytime albedo, snow cover insulates the surface in contact with the atmosphere from ground heat flux overnight, while constraining the surface temperature to be 0 °C or below, and suppressing sensible heat flux out of the surface during the day as insolation is more likely to go toward melting/evaporating the snow.…”

Fast particulate nitrate formation via N<sub>2</sub>O<sub>5</sub> uptake aloft in winter in Beijing

“…The mass fraction and concentration of pNO − 3 in Beijing was reported higher aloft (260 m) than at the ground level (Chan et al, 2005;Sun et al, 2015b), which was explained by favorable gas-particle partitioning aloft under lower temperature conditions. Overall, the active nighttime chemistry in the upper level plays an important role in surface PM pollution through mixing and dispersing within the planet boundary layer (PBL; Prabhakar et al, 2017), and the pollution was even worse in valley terrain regions coupled with adverse meteorological processes (Baasandorj et al, 2017;Green et al, 2015).…”