Nocturnal cleansing flows in a tributary valley

During photochemical air pollution episodes in the Lower Fraser Valley (LFV) near Vancouver, BC, daytime upvalley flows carried polluted air, with high ozone (O₃) concentrations, into tributary valleys to the north of the LFV. Nighttime flows out of the valleys had low O₃ concentrations, according to surface measurements, and also had low aerosol concentrations, as measured by a scanning Doppler lidar. Analysis of lidar scan data showed that the flows were highly complex, that the relatively clean flow was confined to the lower levels (lowest ~ 500 m) of the valley, and that regions of strongest outflow were also the regions of 'cleanest' air. Measurements of NO₂ concentrations well above background levels in the outflow indicate that it was formerly polluted air from which O₃ and aerosols had been removed. Possible removal mechanisms were found to be dry deposition in the katabatic (downslope) flows down the valley sidewalls, in agreement with a previous study in a Swiss valley, or fast chemical reactions with NO and NO₃. Nearly horizontal lidar scans showed that the valley exit flows penetrated into the LFV, where they merged with the downvalley/land-breeze system along the Fraser River.