Pollutant recirculation in the Lower Fraser Valley (British Columbia, Canada)—a numerical study with MC2

Air quality in coastal cities is strongly affected by recirculation processes induced by mesoscale circulations. A typical example is Vancouver (British Columbia, Canada) located at the end of the Lower Fraser Valley near the sea. The atmospheric circulations in the region are affected by the land/sea breeze cycle, as well as by mountain/valley winds (in the main valley and in the tributaries), and slope flows induced by mountain ridges up to 2000 m height a. s. l.. Such complex situation has been studied with a mesoscale atmospheric model (MC2) for few days of the Pacific2001 field experiment (August 2001). Several passive tracer releases have been modelled by emissions in the Vancouver urban area for different periods of the day, in order to analyse the recirculation patterns. The analysis of the results shows the existence of three main recirculation processes: 1) a day-to-night recirculation, where tracers emitted during daytime and pushed inland by sea-breezes, valley winds and upslope flow, are transported back towards the coast by slope flows and mountain winds at night; 2) a night-to-day recirculation, where tracers emitted during night and pushed over the sea by land breezes, are brought back over land during daytime by sea breezes; 3) a day-to-day recirculation where tracers emitted during daytime are transported vertically by up-slope flows, and stored in a reservoir layer. Those tracers are then fumigated back to the ground the following day. The relative importance of these mechanisms will be also analyzed.