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Atmospheric recirculation during ozone episodes in the Lower Fraser Valley, B. C. Seagram, Annie F.

Abstract

The presence of thermo-topographic circulations in areas of complex terrain plays an important role in recirculating pollutants during periods of degraded air quality. In this modelling study, we seek to define and detect atmospheric recirculation in the Lower Fraser Valley (LFV), British Columbia, a region that frequently experiences episodes of degraded air quality despite its modest total emissions and relatively small population size. The Weather Research and Forecasting (WRF) model is used to simulate wind fields during seven severe, three-day summertime ozone episodes occurring over a period of 20 years (1985-2006). These episodes cover the known set of synoptic and mesoscale circulation regimes conducive to ozone episodes in the LFV. A trajectory modelling study is devised where WRF model output is used to compute trajectories using the HYbrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. In order to examine pollutant transport, the starting location of trajectories is selected to coincide with the location of maximum ozone precursor (NOx and VOCs) emissions. Number density maps generated from composite trajectory fields reveal different spatial distributions of trajectories by circulation regime. A generally applicable quantitative definition and objective detection algorithm for recirculation is developed, and then applied to the modelled trajectories to identify recirculating trajectory segments (RTSs). Recirculation is detected during all episodes, though not all circulation regimes result in the same the frequency of detection. Analysis of RTSs shows that recirculation in the LFV is spatially and temporally the same regardless of mesoscale circulation conditions. There is strong evidence that pollutants may be "carried-over" from one day of an episode to the next, and that air parcels frequently return to their origin within less than 12 hours. Results suggest that recirculation is primarily driven by onshore flows and mountain-valley circulations within the main valley floor of the LFV, and secondarily by diurnal flows within tributary valleys. This research adds to our understanding of atmospheric transport during ozone episodes in the LFV, and provides a new framework for studying recirculation elsewhere. Supplementary materials:http://hdl.handle.net/2429/50964

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Attribution-NonCommercial-NoDerivs 2.5 Canada