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Exchange of pollutants between the atmospheric boundary layer and free troposphere is an important (yet often neglected) process that tends to produce distinct layers of pollution in the lower troposphere. These layers represent a potential sink for pollutants from the boundary layer, have the potential to be mixed to ground and likely influence tropospheric chemistry and the global climate system. Factors influencing the vertical distribution of ozone in the troposphere are outlined as a prelude to a more specific discussion of elevated layers and myriad meteorological processes responsible for their development. Evidence from a range of geographical settings suggests that these phenomena are ubiquitous. A rich data set from the Lower Fraser Valley, British Columbia, is used to provide an inventory of layer structures and to highlight their diverse origins and histories. Approaches used to assess the impact of down-mixing of pollutants from elevated layers on ground-level concentrations of ozone are outlined and future research priorities recommended.

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