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Mesometeorological modelling and trajectory studies during an air pollution episode in the lower Fraser Valley, British Columbia, Canada Miao, Yuelong


The Lower Fraser Valley, straddling the USA-Canada border in western North America, often experiences episodes of elevated tropospheric ozone in summertime. The meteorology governing those episode days is characterized by a stagnant high pressure system, light wind, strong insolation and the occurrence of sea breezes. To shed some light on the mesometeorology of ozone episodes in the LFV, this study employed the Regional Atmospheric Modelling System from Colorado State University, a non hydrostatic, three-dimensional mesoscale modelling system to simulate the detailed structure of air flows over the valley for one specific episode day. Significant code modifications have been made to enhance the model's ability to represent surface energy fluxes and predict surface temperatures in the surface of complex terrain and land-use patterns. Evaluation of the model performance was made against an extensive set of observation son the episode day. Pollutant transport on the modelling day was explored with a Lagrangian particle dispersion model. Specifically, forward trajectories were calculated for particles released at various locations in the Lower Fraser Valley and at different times. A systematic qualitative and quantitative model evaluation with the statistical method of Willmott showed that the model could adequately simulate the observed sea breeze another interactive terrain-induced flows such as slope wind and channel flow. Trajectory studies indicated that pollutant recirculations occurred largely from sources originating in the northwest part of the valley where most emission sources are located, and ended in a region where the highest ozone concentrations were observed. Recirculations were due to pollutants traveling with the interactive flows of sea breeze and upslope winds, and later being captured and directed back to the valley by the down slope winds. Particles released in other part of the valley all travelled outside the valley, having a minimal contribution to the episode buildup.

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