An IMPROVE monitoring site can have missing data for various reasons, e.g. a power outage at the monitoring site. In some instances, there is enough missing data that the monitoring site fails the completeness criteria to calculate a valid haze metric for the Regional Haze Rule (RHR) baseline period (2000-2004). The RHR guidance documents has procedures for filling in some missing values with “patched” data; however this criteria is strict and few values are actually filled in. A number of States have developed different procedures to fill in additional missing values and submitted these data to the VIEWS database. These data are referred to as “substituted” data to differentiate them from the RHR “patched” data. Currently substituted data are available from 19 monitoring sites. These sites were selected because they either did not have the required 3 years of valid data to calculate the RHR haze metrics for the baseline period or they did not have complete data for the year 2002. The Regional Planning Organizations modeled the year 2002 and monitoring data was needed to evaluate and use the modeling results.
Section 169A of the 1977 Clean Air Act amendments (CAAA) set forth legislative requirements for addressing visibility impairment due to air pollution. It established a national visibility goal to remedy existing impairment and prevent future impairment in 156 National Parks and wilderness areas across the country designated as mandatory Federal Class I areas. It also called for EPA to develop regulations requiring State implementation plans (SIPs) to address visibility. These plans must include a long-term strategy and Best Available Retrofit Technology (BART) on certain existing sources for making “reasonable progress” toward this goal.The EPA issued initial visibility regulations in 1980 that addressed visibility impairment in a mandatory Federal Class I area that is “reasonably attributable” to a single source or small group of sources. The EPA subsequently issued regulations to address regional haze (i.e., visibility impairment caused by emissions from numerous sources located over a broad geographic region), in 1999.
The regional haze rule requires States with mandatory Federal Class I areas to develop state implementation plans (SIPs) that include reasonable progress goals for improving visibility in each mandatory Federal Class I area and emission reduction measures to meet those goals.
After publication of the regional haze rule in 1999, the first step in the implementation process was the upgrade and expansion of the IMPROVE visibility monitoring network to 110 sites nationally. These sites were selected to represent all mandatory Federal Class I areas, except for the Bering Sea Wilderness site. The expanded IMPROVE monitoring network was deployed during the 1999-2001 time frame, and representative monitoring data collected from this network will be used to establish baseline conditions (for the 2000-2004 period) for each Class I area and to track progress toward goals established in future SIPs.
The monitoring data collected by the IMPROVE network is post-processed to include various metrics that help in the analysis and understanding of current and historical visibility conditions at each site in relation to the above-mentioned visibility regulations. These metrics are organized and managed in the Regional Haze Rule Metrics dataset that is disseminated through the FED system.