Evaluation of a prediction procedure for stratospheric intrusions in Alberta, Canada

Due to the demonstrated influence of the mountains on lee-side frontogenesis, extreme peaks in downwind ground-level ozone concentrations have often been explained by tropopause folding and the subsequent injection of ozone from the stratosphere. Based on a literature review, a method has been devised to forecast the formation of tropopause folds in Alberta. The forecast procedure criteria were evaluated against the record of weekly ozonesonde flights at one site and compared to ground-level concentrations from several surrounding air monitoring stations in the region (July 1999 through May 2000). This procedure was later used to predict two likely episodes in the spring of 2000. Application of the procedure involved the analyses of additional ozonesonde profiles as well as concurrent ground-level concentrations of beryllium-7 (a cosmogenic tracer), ozone (TECO Model 49C) and relative humidity. The potential contribution of stratospheric intrusions to background ground-level ozone concentrations at two sites during this monitoring period will be also assessed. At a given point, the ozone budget at ground-level consists of three components: anthropogenically derived, stratospheric and natural background. In the relatively pristine Canadian Rocky Mountains, low levels of anthropogenically derived ozone are expected. While several authors have attempted to quantify the magnitude of ozone injection into the troposphere and the possible transport across the atmospheric boundary layer, conditions here could allow a more perceptible isolation of intrusions with the ultimate goal of quantifying the natural background levels.