Lidar investigation of the temporal and spatial distribution of atmospheric aerosols in mountain valleys

Lidar experiments were conducted in the mountainous region of Bulgaria to determine the spatial and temporal distribution of major aerosol sources and the zones of aerosol accumulation. When these lidar data are combined with conventional meteorological observations of temperature and wind profiles they provide a clear picture of the physical processes that lead to the accumulation and subsequent dispersion of aerosols and other pollutants in the valleys. The observations showed that the valley gradually fills with cool air after sunset, producing an inversion that traps aerosols and other pollutants emitted at night. After sunrise a convective boundary layer develops in the valley; its evolution is delayed by the confining valley walls. Insolation causes airflow up the slope, producing divergence near the surface and subsidence of the inversion core. The one winter experiment conducted suggests that weaker winter insolation delays the process until much later than in the summer, sometimes to the extent that the inversion persists throughout the day, or even for several days. The findings described here are in good agreement, qualitatively and quantitatively, with the model described by Whiteman and McKee. The results also demonstrate the power of combining conventional meteorological observations with lidar techniques for determining the nature of boundary layer processes in a valley.