The influence of thermally driven circulation on PM10 concentration in the Salt Lake Valley

Ground level and vertical measurements of aerosols (PM10) and meteorological parameters in the Salt Lake Valley were conducted as a part of the Vertical Transport and MiXing in the atmospheric boundary layer (VTMX) experiment, conducted during the period 1–18 October 2000. This component of the VTMX experiment was directed toward developing a deeper understanding of the factors affecting the distribution and transport of pollutants in urban areas within complex terrain. The present study focused on the temporal (diurnal) and vertical variation of PM10 concentration during the stable nighttime period as well as the morning and evening transition periods. The temporal variations in the aerosol concentration during these periods was dominated by thermally driven diurnal winds, with only minor influences from background synoptic winds. The increased aerosol concentration between sunrise and the afternoon was associated with the westerly up-slope winds that advected PM10 from the nearest source, the downtown area of Salt Lake City. In mid-afternoon the PM10 concentration decreased significantly due to the arrival of a northwesterly lake breeze near the ground. The lowest aerosol concentrations during the period from late afternoon to early morning are the result of the draining of cleaner air from the mountain slope after the wind shifted from westerly to easterly. The temporal variations in the wind direction and aerosol levels during the nighttime are discussed. A hypothesis is advanced, with some supporting evidence, that periodic variations of the nighttime PM10 concentrations owe to the shedding of vortices from the Wasatch mountain range bordering the lake. The vertical distribution of aerosols is sensitive to the thermal forcing, which exerts the driving influence on thermal circulation within the Salt Lake Valley.