Evaluation of an 18-year CMAQ simulation: Seasonal variations and long-term temporal changes in sulfate and nitrate Kevin Civerolo a, * , Christian Hogrefe a, b , Eric Zalewsky a , Winston Hao a , Gopal Sistla a , Barry Lynn c , Cynthia Rosenzweig d , Patrick L. Kinney e a New York State Department of Environmental Conservation, Division of Air Resources, 625 Broadway, Albany, NY 12233-3259, USA b Atmospheric Sciences Research Center, University at Albany, Albany, NY, USA c Weather It Is, Ltd., Efrat, Israel d NASA Goddard Institute for Space Studies, New York, NY, USA e Mailman School of Public Health, Columbia University, New York, NY, USA article info Article history: Received 5 April 2010 Received in revised form 25 June 2010 Accepted 28 June 2010 Keywords: CMAQ Model evaluation Atmospheric deposition Emissions abstract This paper compares spatial and seasonal variations and temporal trends in modeled and measured concentrations of sulfur and nitrogen compounds in wet and dry deposition over an 18-year period (1988e2005) over a portion of the northeastern United States. Substantial emissions reduction programs occurred over this time period, including Title IV of the Clean Air Act Amendments of 1990 which primarily resulted in large decreases in sulfur dioxide (SO 2 ) emissions by 1995, and nitrogen oxide (NO x ) trading programs which resulted in large decreases in warm season NO x emissions by 2004. Additionally, NO x emissions from mobile sources declined more gradually over this period. The results presented here illustrate the use of both operational and dynamic model evaluation and suggest that the modeling system largely captures the seasonal and long-term changes in sulfur compounds. The modeling system generally captures the long-term trends in nitrogen compounds, but does not reproduce the average seasonal variation or spatial patterns in nitrate. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In regulatory applications, meteorological and photochemical modeling systems have traditionally been used to simulate air quality and deposition over relatively short time periods, some- times over multiple-day episodes (e.g. Zhang et al., 2004; Hanna et al., 2001) and rarely for longer than a year (e.g. Godowitch et al., 2010; Tesche et al., 2006). Generally, these model simula- tions are performed to test the effects of large-scale changes in emissions, often keeping the meteorological fields unchanged. Such simulations are useful for understanding how the modeling system responds to emissions reductions but do not account for the influence of seasonal and year-to-year meteorological variations that occur in the real world which can have effects that are comparable to, if not larger than, changes in emissions. Here we describe estimates of wet and dry deposition from an 18- year (1988e2005) model simulation over a portion of the north- eastern US. Hogrefe et al. (2009a) previously used these long-term model predictions in combination with available observations to estimate historical fine particulate (PM 2.5 ) mass and species composition. Over this 18-year period several large emission reduc- tions programs were enacted to reduce acidic deposition and improve air quality with respect to ozone (O 3 ), regional haze, and PM 2.5 . Several of these control programs are briefly summarized here. The goal of Title IV of the 1990 Clean Air Act Amendments (CAAA) was to reduce sulfur dioxide (SO 2 ) emissions by 10 million tons below 1980 levels in two phases, targeting large electric utility plants primarily in the eastern US (please see http://www.epa.gov/ airmarkt/progsregs/arp/basic.html). Phase I began in January 1995, and resulted in substantial (nearly 40%) reductions in SO 2 emissions from these facilities. Phase II called for additional reductions in SO 2 emissions and also required a 2 million ton reduction in NO x emis- sions from power plants by 2000. Larger reductions in nitrogen oxide (NO x ) emissions were achieved through the Ozone Transport Commission (OTC) NO x Budget Program implemented in 1999, and the NO x SIP Call Budget Trading program implemented in 2004 which targeted major point sources during the O 3 season (please see http://www.epa.gov/airmarkt/progsregs/nox/index.html). Super- imposed on these changes in utility NO x emissions were more gradual decreases in mobile source emissions due to improved * Corresponding author. Tel.: þ1 518 402 8383; fax: þ1 518 402 9035. E-mail address: kxcivero@gw.dec.state.ny.us (K. Civerolo). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.06.056 Atmospheric Environment 44 (2010) 3745e3752