The Very Basics of Increment

Prevention of Significant Deterioration (PSD) is defined as air quality deterioration relative to a baseline condition

  • Significant deterioration accounted by increment consumption analyses
  • Sources locating near Class I area have to also account for their possible emissionseffects on Air Quality Related Values (AQRVs). AQRVs are defined by the Federal Land Manager, State or Indian Governing Body.

Increment

  • The amount of additional pollution that is allowed over the baseline concentration in a PSD area
  • Called "maximum allowable increases" in the Act
  • There are different increments for 3-hour, 24-hour and annual emission-averaging time periods (measured as increases of ambient concentrations, in g/m3)
  • Most stringent for Class I, less for Class II, least for Class III (if any areas ever designated Class III)
  • Air quality in a region cannot deteriorate to a level in excess of the applicable NAAQS, even if all the applicable increment has not been consumed.

Current Increment Values

Increment per Area Classification (g/m3)
Averaging Period / Pollutant / I / II / III
Annual[1] / PM10[2] / 4 / 17 / 34
SO2 / 2 / 20 / 40
NO2 / 2.5 / 25 / 50
24-hr[3] / PM102 / 8 / 30 / 60
SO2 / 5 / 91 / 182
3-hr3 / SO2 / 25 / 512 / 700

Proposed PM2.5 Increments Values

Increment per Area Classification (g/m3)
Averaging Period / Pollutant / I / II / III
Annual / PM2.5 / 1 / 4 or 5 / 8 or 10
24-hr / PM2.5 / 2 / 9 / 10 or 18

Two ways to think of increment

  • first, conceptually
  • second, practically speaking

Conceptually

  • Baseline concentration is the ambient concentration at the baseline date, the date of the first application for a major permit in a PSD area (....with some exceptions)
  • Increment is additional pollution allowed on top of that

Practically speaking

  • The exceptions to which emissions “consume” increment make calculating increment more complicated than figuring out what the ambient concentration was on the baseline date and when it exceeds the baseline concentration plus the increment. (See CAA §169(4))
  • Thus, increment is calculated by modeling the change (or , delta) in ambient concentration inside a PSD area due to “increment consuming emissions”
  • Defining “increment consuming” depends on three dates:
  • Major source baseline date: set by rule and long past, date after which actual emissions changes at major sources due to construction consume increment
  • Trigger date: set by rule and long past, date after which the minor source baseline date can be triggered
  • Minor source baseline date: the date (after the trigger date) of the first major (i.e. “significant”) NSR permit application in a PSD area, after which ALL actual emissions changes from ALL sources (major, minor, mobile, due to construction or not) consume increment
  • The above dates are analyzed during the date of determination (date name for purposes of this document only), either when a new source is requesting a PSD permit and is looking to see if adequate increment is left for it, or when a State is checking to see how much increment is left for management purposes (rarely done)

Another view of Baseline Dates

Period between MajSBD[4] and Trigger Date / Period between Trigger Date[5] and MinSBD / Period between MinSBD and Date of Determination
Increment Consuming / Emissions associated with construction at a major source / Emissions from major source construction / All actual emission increases at all sources: major, minor and mobile. Includes new construction as well as increases in production.
Not Increment Consuming / Emission from all other sources except major sources / Emission from all other sources except major sources / Not applicable.
Increment Expanding / Not applicable / Not applicable / Emissions reductions at all sources due to physical or operational changes, if Federally or SIP enforceable

What about Baseline Areas?

Basics

  • 107 areas (those State-drawn 107 areas that are classified attainment or unclassified) where the minor source baseline date is triggered
  • Whichever §107 areas contain or are touched by the “impact area.” A source’s “impact area” is defined by rule as the 1g/m3 annual average concentration isopleths
  • Once triggered, increment consumption is also triggered.
  • Sources cannot trigger baseline areas across State lines, but increment is consumed across State lines
  • §107 allows areas to be subdivided into “two or more areas”
  • Minimum limitations for redesignation include (see Section 107 of the CAA):
  • Area cannot be smaller that the 1 g/m3 area of impact of the proposed source
  • The boundaries of the redesignated area cannot intersect the area of impact of the proposed source that establishedthe minor source baseline date for the area proposed for redesignation

Example: Triggering Baseline Dates

  • Source A goes in in1989 and it triggers the minor source baseline date in PSD baseline areas 1 and 2 because source is in area 1 and its 1 g/m3 or more “impact area” straddles to baseline area 2.
  • Source B goes in in 1995, and triggers PSD baseline area 4. Area 2 is already triggered.
  • PSD baseline area 3 remains untriggered.
  • Under present rules, a State could subdivide PSD baseline area 4 (as with the dotted line) and untrigger the part of area 4 not touched by the impact area of Source B.

[1] Annual averages can not be exceeded; the fist exceedance is considered a violation.

[2] EPA proposed PM2.5increments to substitute the current PM10 increments in September 21, 2007. See table.

[3] 24-hr and 3-hr averages can be exceeded once per yea, the second is considered a violation of increment.

[4] MajSBD for PM and SO2 is January 6, 1975. For NO2, February 8, 1988.

[5] Trigger date for PM and SO2 is August 7, 1977. For NO2, February 8, 1988.