Increment Tracking

In accordance with Nevada Administrative Code and the Code of Federal Regulation, Nevada is required to track and periodically evaluate increment for planning areas that are triggered for increment consumption.

What is PSD Increment?

PSD stands for “Prevention of Significant Deterioration”. PSD is a federally mandated construction permitting program for large sources (such as large mines, power plants, chemical plants etc.). PSD is implemented in one of three ways:

  1. By the state (or local) air agency through a state permitting program approved into the state’s air quality plan (i.e. SIP);
  2. By the state (or local) air agency through the federally delegated PSD rules contained in Title 40 CFR Part 52.21; OR
  3. Lacking one of the previous two options, directly by U.S. EPA through the federal rules contained in Title 40 CFR Part 52.21.

PSD in Nevada is implemented as a PSD delegated program.

One of the significant components of the PSD program is the requirement to evaluate increment consumption. Increment is the maximum allowed increase in concentration of a pollutant, above a baseline concentration in an area.

Increment standards exist for 3 pollutants: PM10 (24-hour and annual averages), NOx (annual average only), and SO2 (3-hour, 24-hour and annual averages). One the most critical elements of an increment consumption analysis is the establishment of the baseline concentration for an affected pollutant. The Baseline Concentration is simply the ambient concentration that existed in the area before a PSD source submits the first PSD application in the area. The increment consumed, then, is simply the amount of additional air quality impact above the baseline concentration.

How is PSD Increment Triggered?

The requirement to evaluate increment consumption begins when the Baseline Date is triggered. There are two types of baseline dates:

  • The major source baseline date; and
  • The minor source baseline date.

The major source baseline dates were automatically set by the federal PSD rules and reflect the date that the permitting authority is responsible for tracking increment consumption due to changes at majorsources only. The minor source baseline date, however, is arguably the date that carries the greatest impact on industrial growth in an area. The minor source baseline date is triggered when a PSD application (either for a completely new major source or for a major modification at an existing source) is submitted and determined to be complete. When the first PSD application is determined to be complete, the baseline date is set for the pollutants that are being reviewed under the PSD application and for which there is an increment standard established. Once the minor source baseline date is triggered, increment consumption must be evaluated for changes that occur at all sources (including minor, area and mobile sources) after that date. Increment is consumed (or possibly expanded) throughout the entire baseline area.

What is the PSD Baseline Area?

The baseline area is defined as all portions of the planning area in which the new major source or major modification is located, AND all portions of any neighboring planning area in which the new or modified major source results in a significant impact. In Nevada, planning areas are defined by the State Engineer’s hydrographic areas. There are more than 250 of these areas in the state. Click here for a map depicting these areas [PDF]. As a result, it’s possible to trigger the baseline date in multiple planning areas as a result of a single PSD application. Click here for a map of known, triggered, baseline areas in Nevada [PDF].

What are the PSD Increment Standards?

Title 40 CFR Part 52.21(c) defines the “Ambient Air Increments” as the “increases in pollutant concentration over the baseline concentration”. The increment table can be downloaded here [PDF].

Class I areas are those areas designated as pristine or wilderness areas and require more rigorous safeguards to prevent deterioration of the natural pristine air quality. The Jarbidge Wilderness Area [PDF] is the only Class I area located in Nevada.

Class III areas are planning areas set aside for industrial growth. As such, increments are higher in Class III areas. Class II areas are essentially all other areas of the state that are not designated Class I or Class III. All planning areas were initially designated either Class I or Class II. States must request and receive EPA approval for Class III areas. No such Class III designations have been approved (either in Nevada or elsewhere in the country).

How is the Increment Evaluated?

The Typical EPA Evaluation Method:

Because PSD increment is associated with a change in concentration as of a particular date, it can not be effectively monitored with ambient air sampling devices. As a result, increment can only be evaluated through a modeling analysis(es). The standard U.S. EPA approach to assessing increment consumption involves developing two emission inventories. These inventories consist one for the baseline operations, and one for the current operations. The baseline inventory represents the emissions (from point, area and fugitive sources), which existed on the minor source baseline date are generally based on actual emissions. The current year inventory is representative of the emissions, which are due the sources as they exist at some selected year of interest after the minor source baseline date. The typical EPA recommended approach involves subtracting baseline emissions from current emissions (on a source-by-source basis) and modeling the emission change. The model result is then assumed to be the resultant change in concentration (i.e. increment) at each modeled receptor. The results are then directly compared to the increment standards for compliance.

The Nevada Evaluation Approach

As discussed above, increment is defined as the allowable change in concentration above the baseline concentration. This is accomplished by using a “concentration difference” approach. As a result, the baseline concentration is determined at each receptor by modeling the emission inventory for the baseline year (for each applicable pollutant and averaging time). Next, similar modeling analyses are performed for the current year inventory. Finally, the concentration from the baseline modeling at each receptor is subtracted from the current year concentrations at the same receptors. The difference is the amount of increment consumed or expanded. A technical support document [PDF] discussing this approach and the Increment Tracking System (ITS) in detail was developed by BAQ.

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