Daytime Photochemical Pollutant Transport over a Tributary Valley Lake in Southwestern British Columbia

Abstract

Tethersonde, lidar, aircraft, and surface chemistry measurements from an intensive field campaign (Pacific’93) in the Lower Fraser Valley (LFV) demonstrate the daytime advection of pollutants into a lake-filled valley adjoining a broad urbanized coastal valley. On three separate days (immediately before, during, and after a pollutant episode), elevated concentrations of ozone (O3) in the narrow tributary valley could be attributed to the advection of pollutants northward from sources in the LFV (primarily metropolitan Vancouver). On 5 August, the highest concentrations of O3 observed in the region during the entire episode were observed over the tributary lake. Simple Lagrangian mass budget calculations suggest that the unusually high concentrations observed on 5 August over the lake were physically reasonable and consistent with the known chemistry of the air advected into the valley. They also indicate that reductions in O3 flux divergence during the overlake trajectory in the Pitt Valley, primarily as a result of reduced surface deposition, may contribute to the relatively high concentrations observed in the tributary valley. Observations immediately after the episode show that chemically aged polluted air masses can persist within the tributary valleys from the previous day. These results have implications for the understanding of air pollution in other regions of complex terrain and show that the predominance of daytime upvalley pollutant transport in such tributary valleys is likely to have significant impacts on the local ecology and visibility.