Mandatory Measures for Ozone Nonattainment Areas

Emissions come from a variety of sources, but are generally categorized as mobile (from a vehicle), stationary (from a discrete non-moving source), or biogenic (naturally occurring emissions). Stationary sources include major, minor and non-point sources. Major sources are typically the focus of regulation because they are large, easily identifiable, and can be measured and monitored. Pollution control at these sources is often considered to be cost-effective in comparison to the cost of regulating a multitude of smaller, less identifiable sources that cannot be measured or monitored. Accordingly, under the CAAA, "major" sources of precursors trigger retrofit requirements and specific permitting rules. Mobile sources, however, represent an increasing portion of the inventory of emissions and are becoming a focus of renewed attention from regulators.

The Act defines major sources based on the potential to emit (PTE). The PTE is considered to be the amount of pollution a source could generate if it were run all the time without any pollution controls. This term is described in more detail in the next chapter. The CAAA lowers the threshold for definition as a major source in the more severe nonattainment areas. For example, the threshold in Extreme nonattainment areas is the potential to emit 10 tons per year (tpy) of NOx or VOC. In Marginal and Moderate nonattainment areas, the threshold remains the same at a PTE greater than 100 tpy of VOC and/or NOX The CAAA also defines modifications to a source as major if they result in a net increase in emissions that exceed specific thresholds for each nonattainment classification.

The CAAA requires that new major stationary sources and major modifications at existing major stationary sources of NOX and VOC be offset by reductions either from existing sources or from offsets purchased from another source within the same nonattainment area. The offset ratio varies based on the area's nonattainment classification. Table 15-2 shows major source thresholds and the offset ratios required by nonattainment category.

For example, a facility with a PTE of 100 tpy that is trying to locate or make a major modification in an area classified as Extreme must reduce or purchase 150 tpy of pollution offsets before it can locate the new facility or make the modifications.

Table 15-2

Major Source Thresholds and Offset Ratios for Nonattainment Classifications

Nonattainment Category

Major Source

Cut-off Tons per year

New Source Emissions Offset ratio

Marginal Moderate

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