Detection of statistically significant trends in atmospheric concentrations of semivolatile compounds

This paper examines the sampling and analytical requirements of observing statistically significant temporal trends in atmospheric concentrations of semivolatile organic pollutants. Data precision and study duration are found to be the most important factors that are subject to experimental control. We define a detectable half-life as the longest half-life for which statistical significance can be observed for a given length of study and show how this can be calculated. Finally, we show how to predict the length of a study that will find a statistically significant half-life. These concepts are applied to a recently updated temporal trends study from the Integrated Atmospheric Deposition Network. Updated ha If-lives for gas-phase pesticides at three U.S. sampling sites are given. There are now enough data to observe statistically significant temporal trends for most of the gas-phase pesticides except for alpha-chlordane and trans-nonachlor at Eagle Harbor. Using the concept of detectable half-life and based on observations of half-lives at other lADN sites, at least 2 more years of study are required to observe statistically significant trends for alpha-chlordane and trans-nonachlor. The concepts presented in this paper can also be applied to the design of sampling strategies for measuring long-term temporal trends in media other than air.