A 10-yr record of PM2.5 (particulate matter of aerodynamic diameter ≤ 2.5 μm), collected in Cache Valley near downtown Logan, Utah, reveals a strong peak in the PM2.5 concentration climatology that is tightly localized in mid-January. The cause of this subseasonal variation in the PM2.5 climatology is investigated through dynamical downscaling and large-scale diagnostics. Climatological analysis of the U.S. winter mean ridge reveals a mid-January subseasonal shift in the zonal direction, likely in response to variations in the Rossby wave source over the central North Pacific Ocean. This displacement of the winter mean ridge, in turn, has an impact on regional-scale atmospheric conditions—specifically, subsidence with local leeside enhancements and midlevel warming over Cache Valley. The analyses of this study indicate that the subseasonal peak of long-term mean PM2.5 concentrations in Cache Valley is linked to the large-scale circulations’ subseasonal evolution, which involves remote forcing in the circumpolar circulations as well as possible tropical–midlatitude interactions. This subseasonal evolution of the winter mean circulation also affects precipitation along the West Coast.
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