Assessing the impact of the acid deposition control program

A goal of the acidic deposition control program in the United States has be en to link emissions control policies, such as those mandated under Title I V of the US Clean Air Act Amendments (CAAA) of 1990, to improvements in air and water quality. Recently, several researchers have reported trends in t he time series of pollutant data in an effort to evaluate the effectiveness of the CAAA in reducing the acidic deposition problem. It is well known th at pollutant concentrations are highly influenced by meteorological and cli matic variations. Also, spatial and temporal inhomogeneities in time series of pollutant concentrations, induced by differences in the data collection , reduction, and reporting practices, can significantly affect the trend es timates. We present a method to discern breaks or discontinuities in the ti me series of pollutants stemming from emission reductions in the presence o f meteorological and climatological variability, Using data from a few site s, this paper illustrates that linear trend estimates of concentrations Of SO2, aerosol SO42-, and precipitation-weighted SO42- and NO3- can be biased because of such complex features embedded in pollutant time series. (C) 20 01 Elsevier Science Ltd. All rights reserved.