Assessing vegetation exposure to ozone: is it possible to estimate AOT40 by passive sampling?

A statistical model for estimating AOT40 from time-integrated concentration data is presented and validated. AOT40 is a numerical index that describes the ozone exposure of ecosystems in terms of the hourly accumulated exposure over a threshold of 40 ppb. A rather simple model formulation was achieved by approximating the frequency distribution of hourly concentrations by the Gaussian probability distribution. The model represents a relationship between a time-averaged ozone concentration and the corresponding AOT40. Time-averaged concentration data are obtained when employing the passive sampling technique. For testing the method, passive sampling data with a 14-day sampling time were simulated by averaging continuous monitoring data. Data from eight background stations in Europe were used for calibration and testing. In spite of the simplifying assumption, the model performed well for the accumulated exposure when tested against independent data. The results show that it is possible to obtain a reasonable estimate of AOT40 even in the absence of continuous data, if the time-averaged concentration can be measured reliably.