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Elevated ozone layers and vertical mixing in the lower fraser valley, British Columbia Lundgren, Jeffrey Ross Stanley

Abstract

The vertical distribution of tropospheric ozone in the Lower Fraser Valley (LFV) is examined. The criteria of what constitutes an elevated ozone layer is defined and a total of 105 profiles from aircraft, tethersonde and free ascent balloons obtained during the period of 1993-1996 were examined and a total of 110 elevated features identified according to these criteria. The results show a rich variety of structures present from the boundary layer up to the upper free troposphere. This richness is shown to be a result of a multitude of generation processes, ranging in scope from the local up to synoptic scales, that interact to produce elevated layers. The layers are typed and sorted according to a classification scheme based on the altitude of the layers with respect to significant atmospheric boundaries, such as the height of the mixed layer, and the specific processes of generation as inferred from other studies performed in similar locales, most significantly within the Los Angeles Basin. Layers are classified as Type I,n,ffl or IV ranging in height from within the mixed layer up to the upper troposphere, with Type I through IU being the result of pollution from within the LFV and Type IV layers being present as a result of long range transport from distant sources. Type I and II layers are shown to have the strongest potential for affecting ground level ozone and may be one of the causes of the strong temporal autocorrelation of diurnal near-surface concentrations, over a time scale of 2 to 5 days, during episode conditions. Slope and sea breeze flows are thought to be of great importance in the creation of Type II and HI elevated pollution layers from LFV sources to heights levels nearing 5000m a.g.l. and distances on the order of 100 km from Greater Vancouver. This suggests pollution exists on a vertical and horizontal scale that was not previously believed and is usually only associated with areas that are thought of as seriously polluted, such as the L.A. basin. Type IV layers are present at higher levels and appear to have their sources within the boundary layer, as opposed to being from stratospheric intrusions, possibly as a result of plumes from biomass burning. Trajectory analysis of upper level Type IV features indicates that long range transport of tropospheric ozone is occurring, perhaps on continental scales. This ability to be transported long distances is attributed to a much longer lifetime, up to 90 days versus 4-5 days, for ozone in the free troposphere as compared to ozone in the planetary boundary layer. Transilient turbulence theory is outlined as an appropriate means of mathematically describing mixed layer turbulence. A simplified transilient model, based on the available data, is presented to investigate the extent to which vertical downmixing of elevated ozone features may influence ground level ozone concentrations within the LFV. A case day, August 6t h, 1993, is examined on which both a Type I and a Type II layer are present in the tethersonde profiles over Harris Rd. The results for both of these layers show that vertical downmixing of elevated ozone may account for as much as 50% of the mass increase in ground level concentration during the time of the model run. The influence of both or these layers is apparent in the ground level time series from Harris Rd. for this day.

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