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 (03) concentrations, into tributary valleys to the north of the LFV. Nighttime flows out of the valleys had low 03 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 03 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 N03. 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.