Are Ozone Exceedance Rates Decreasing Comment On 'Extreme Value Analysis of Environmental Time-Series: An Application to Trend Detection in Ground-Level Ozone'

摘要

R.L. Smith (1989) in his Statistical Science discussion paper, proposed new methods for analyzing extreme values based on the point process view of high-level exceedances, and illustrated them with a detailed analysis of ozone data from Houston, Texas. The methods are powerful and, in particular, the point process of cluster peaks over a high threshold provides a remarkable condensation of the massive data set that he analyzes. It involves little loss of relevant information and permits fairly simple analyses. Smith's conclusion is that there is no trend in the overall levels of the series, but that there is a marked downward trend in the extreme values. It seems hard to find physical explanations for this, and here the evidence is reassessed in terms of a comparison between competing models for the intensity of a Poisson process. This suggests that there is some evidence for a decreasing trend in exceedance rates but that it is rather weak. If there is a trend, it seems more likely to consist of a fairly abrupt change than a gradual decrease. The possibility that such a change is due to an improvement in measurement technology is discussed. The possibility of long-memory dependence is also considered and the clustering method used is discussed. Keywords: Mathematical models; Ozone layer. (KT)