Atmospheric Environment 35 (2001) 4135–4148 Assessing the impact of the acid deposition control program Kevin L. Civerolo a , Elvira Brankov a , S. Trivikrama Rao a,b, *, Igor G. Zurbenko b a New York State Department of Environmental Conservation, Albany, NY 12233, USA b Department of Biometry and Statistics, University at Albany, Rensselaer, NY 12114, USA Received 1 November 2000; accepted 17 March 2001 Abstract A goal of the acidic deposition control program in the United States has been to link emissions control policies, such as those mandated under Title IV of the US Clean Air Act Amendments (CAAA) of 1990, to improvements in air and water quality. Recently, several researchers have reported trends in the 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 that pollutant concentrations are highly influenced by meteorological and climatic 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 estimates. We present a method to discern breaks or discontinuities in the time series of pollutants stemming from emission reductions in the presence of meteorological and climatological variability. Using data from a few sites, this paper illustrates that linear trend estimates of concentrations of SO 2 , aerosol SO 2@ 4 , and precipitation-weighted SO 2@ 4 and NO @ 3 can be biased because of such complex features embedded in pollutant time series. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Acid precipitation/deposition; Trend detection/attribution; Time series analysis; Discontinuities or breaks in time series data; Clean Air Act Amendments of 1990; Air quality management 1. Introduction Since the inception of the Federal Clean Air Act in 1969, numerous emission control programs have been implemented in the United States to meet and maintain ambient air quality standards for the criteria air pollutants. Atmospheric models and air quality mea- surements continue to play an important role in designing and evaluating the efficacy of control strate- gies. This paper illustrates a combined observational and modeling approach to evaluate the effectiveness of emission control strategies. We examine the acidic deposition (‘‘acid rain’’) problem since it is of consider- able debate among the scientific and policymaking communities at the present time. Anthropogenic emis- sions of sulfur dioxide (SO 2 ) in the eastern United States are primarily released from electric power plants, while anthropogenic emissions of nitrogen oxides (NO x ) are attributable to motor vehicles, power plants, and area sources. Once released into the atmosphere, these pollutants are transported downwind and converted to form sulfates ðSO 2@ 4 Þ, nitrates ðNO @ 3 Þ, and other photochemical oxidant species, contributing to the acidic deposition problem. The National Acid Precipitation Assessment Program (NAPAP) was created in the 1980s to coordinate modeling, monitoring, and research efforts relating to the acid rain problem (NAPAP, 1998). The Regional Acid Deposition Model (RADM) (Chang et al., 1987) was developed as part of NAPAP. On the monitoring side, the National Atmospheric Deposition Network *Corresponding author. Present address: 625 Broadway, 2nd Floor, Albany, NY 12233–3259, USA. Tel.: +1-518-402-8402; fax: +1-518-402-9035. E-mail address: kevin@dec.state.ny.us (S.T. Rao). 1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII:S1352-2310(01)00200-X