Prevention of Significant Deterioration (PSD)

In the Clean Air Act Amendments of 1977, Congress specified the initial classification of lands for PSD purposes. Certain lands, where existing good air quality is deemed to be of national importance, were designated as Class I and may not be reclassified. These mandatory Class I areas include all international parks, national memorial parks larger than 5,000 acres, and national parks larger than 6,000 acres that were in existence when the Amendments were passed. All other areas to which the PSD provisions apply were classified as Class II.

Wisconsin has a Class I PSD area: the Rainbow Lake Class I Area located in the Checquamagon National Forest. (Click here to view the receptors associated with this area.) Class I areas are the most stringently regulated as these are generally locations that have remained untouched by industry. The Wisconsin Class I area is one of two unique PSD Class I areas in the country where visibility is not recommended as an important value. Consequently, air quality modeling that includes Rainbow Lake as a consideration does not have to conduct an air quality visibility analysis.

New sources that exceed major source thresholds in attainment counties require a PSD permit review. Part of the review includes an increment analysis consumption for PM10, SO2, or NOx if a baseline was set and the new source exceeds significance levels for any of those three pollutants. The PSD baseline counties and the corresponding maps by pollutant (PM10, SO2, and NOx) help determine where PSD analyses are necessary. The increment analysis must consider all sources at the facility that contribute to the PSD increment, which includes all new sources, as well as any sources on site that were constructed or modified after the PSD baseline was set. In addition, sources at other facilities within the area of significant concentration gradient that were constructed or modified after the baseline date, consume increment and should be included in the increment analysis.

The modeled concentration for each applicable baseline pollutant is compared to the increment consumption value for the type of PSD area involved in the analysis. In other words, background concentrations are not added to the concentration when comparing to the increment. The PSD increments are shown in the table below:



For more information regarding PSD increments, click here to go to NR 404.05, Wis. Adm. Code (exit DNR).

Air Quality Modeling