Surface indices of wind, stability, and turbulence at a highly polluted urban site in Santiago, Chile, and their relationship with nocturnal particulate matter concentrations

Abstract

The local meteorological factors of the cold season nocturnal particulate matter (PM) air quality problem in the western urban area of Santiago City are documented and analyzed by means of three years of multi-level temperature and turbulence measurements performed in-situ on a 30 m meteorological tower. For the 20–23 LT period in which the highest PM concentrations commonly occur, high prevalence of low wind and low vertical turbulence conditions are found. With wind speeds less than 1 m s–1, the frequency of calms and the intensity of meandering increase, although the highest PM concentrations are associated more to meandering than to purely calm conditions. In terms of the stability of the near surface layer, while there is a general association between high PM concentrations and high stability, the most extreme PM event in the period occurred with an intermediate value of stability probably affected by the presence of mid-tropospheric clouds. Using a nocturnal vertical height scale based on the stability and the surface turbulent heat flux to compute a nocturnal ventilation factor (VF), a marked inverse relationship between VF and PM concentrations is documented. The prevalence of low winds and high meandering in this sector of Santiago is hypothesized to be related to the local topography that precludes the development of the weak but persistent drainage surface winds observed at other sites of the city. Further investigation of this very stable atmospheric boundary layer is deemed necessary for improving the numerical models used in forecasting and managing the air pollution problem of Santiago.