Ozone and the clean troposphere: ecological implications

In recent years numerous investigations have examined the relationships between chronic ozone (O(3)) exposure and plant responses using statistical or empirical cause-effect models. One of the critically important decisions underlying these studies has been the choice of the control or reference O(3) exposure to which all other treatments are compared. The issue of the control or reference O(3) exposure is a complex one. Does a single ambient control or reference O(3) exposure exist? The approach taken in this paper is to examine the O(3) concentrations in the 'clean troposphere.' One must ask what a clean troposphere is and whether there are any geographic locations in the world today that are totally free of human influences. Data on photochemical oxidants from Fortress Mountain, Alberta, Canada, are examined. A comparison of these data with air quality data from other locations in North America is used to examine the design of vegetation response studies. The need for integrated research design, execution, and evaluation is stressed.