Impact of vertical atmospheric structure on Beijing aerosol distribution

In August 2004, original informations on Beijing aerosol layers were obtained downtown from experiments achieved during clear and polluted days along the IAP 325-m meteorological tower and a micro-pulse LIDAR operating at BMEMC. Beijing urban boundary layer (UBL) is seen to follow the expected diurnal pattern with low height values at night (80±50 m) and much higher values (up to 3000 m) at daytime. An original process of temperature profiles is shown to closely reconstruct the UBL and sub-layer evolution. UBL vertical extension becomes significant after a short transition period at 10:00 during which at all the tower levels, wind speed increases and wind direction changes dramatically from the synoptic northerly flow to almost the opposite direction (SSW). Additional data from background stations show pollution episodes that may be related to horizontal transport at low altitude during the morning or evening transition periods. Vertical profiles of black carbon (BC) and particle number concentration (N) and LIDAR signal evidence the formation of accumulation layers at 60 m and 90 m probably related to the urban canopy. Using the N/BC ratio, an important chemical reactivity may be observed within the accumulation layers and is still distinguishable during cloudy weather. Reactivity in the vertical column appears to increase with altitude, which might point to the significance of solar radiation shielding by particles. Chemical analyses confirm, in the morning, the presence of a very distinct layer at the top of the tower and show, in the afternoon, a significant mixing (17%) between the UBL and the regional flow above.