Elevated ozone layers and vertical down-mixing over the Lower Fraser Valley, BC

Vertical profiling data, including lidar, are used to illustrate elevated layer development during two days of the Pacific'93 field study. Results indicate that multiple processes may produce layers. The ''chimney effect'', where pollutants are vented along the heated sidewalls of the valley, is shown to be important, while evidence is also shown for ''convective debris'' contributing to elevated layer development. These processes are similar to those observed in the Los Angeles Basin. However, the study identified a site-specific process in which a strong elevated layer developed as a result of the propagation of a coastally trapped disturbance (manifested as a low-level marine stratus surge). In addition to distinct elevated layers associated with inversion at the top or above the planetary boundary layer, pollutants (including ozone) are shown to persist overnight in the nocturnal residual layer (RL) that caps the stable boundary layer. In order to investigate the effects of vertical down-mixing on surface ozone concentrations, a transilient turbulence model was applied to vertical profile data for 6 August 1993. After sunrise, this showed significant down-mixing of ozone from the nocturnal RL layer. Furthermore, down-mixing was also shown to contribute to surface concentrations in the early afternoon when the growing mixed layer intercepted the elevated layer created by the coastally trapped disturbance. The mechanisms and structures described show strong similarities to those found in the Los Angeles Basin and suggest that these phenomena likely exist in other polluted coastal environments where they may contribute significantly to the observed ground-level concentrations via vertical mixing processes. (C) 1997 Elsevier Science Ltd.