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Vertical mixing of ozone in the very stable nocturnal boundary layer Salmond, Jennifer Ann

Abstract

Significant increases in nocturnal ozone concentration have been observed near the surface in regions of complex terrain. These temporally isolated features are characterised by a sudden increase ('spike') in ozone concentrations superimposed upon spatially uniform background concentrations of near zero. They typically occur during the summer months when synoptic conditions support the development of the very stable nocturnal boundary layer (NBL). Although localised, these 'spikes' in ozone concentration may have environmental and health consequences and play a significant role in determining diurnal pollutant budgets. During the summer of 1998 a field experiment was conducted in the Lower Fraser Valley, British Columbia, to investigate the relationship between near surface ozone concentrations and the structure of the NBL. This thesis examines the hypothesis that nocturnal spikes in surface ozone concentration result from vertical mixing processes which temporarily couple the residual layer to the surface layer, facilitating the transport of ozone stored aloft to the surface. The study uses a comparatively new quantitative analytical tool - wavelet analysis - to objectively isolate periods of intermittent turbulence observed in the near surface layers through the field campaign. A new technique is developed to isolate the flux associated with only the turbulent ('burst') component of each time series, and to provide revised estimates of scalar fluxes. Despite the dominance of ozone sinks in the surface layer, localised spikes in ozone concentration were observed throughout the Lower Fraser Valley during the field campaign. At the field site these were well correlated with periods of increased turbulence associated with the presence or rapid break-down of the nocturnal low-level jet and the development of down-valley winds. However, spikes in surface concentration were also shown to be sensitive to surface nitrogen monoxide concentrations and ozone concentration aloft. Ozone concentrations were highly variable throughout the residual layer, and the largest spikes in surface concentration coincided with periods when ozone concentrations in excess of 80 ppb were observed aloft. Whilst the results demonstrate that vertical mixing processes can play a significant role in determining local surface concentrations, in regions of complex coastal terrain the role of advection in the NBL cannot be ignored.

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