D R A F T

OCTOBER 1990

C.10

II.E INCREMENT CONSUMPTION AND EXPANSION

The amount of PSD increment that has been consumed in a PSD area is

determined from the emissions increases and decreases which have occurred from

sources since the applicable baseline date. It is useful to note, however,

that in order to determine the amount of PSD increment consumed (or the amount

of available increment), no determination of the baseline concentration needs

to be made. Instead, increment consumption calculations must reflect only the

ambient pollutant concentration change attributable to increment-affecting

emissions. 

Emissions increases that consume a portion of the applicable increment are,

in general, all those not accounted for in the baseline concentration and

specifically include:

! actual emissions increases occurring after the major source baseline date,

which are associated with physical changes or changes in the method of

operation (i.e., construction) at a major stationary source; and

!actual emissions increases at any stationary source, area source, or

mobile source occurring after the minor source baseline date.

The amount of available increment may be added to, or "expanded," in two

ways. The primary way is through the reduction of actual emissions from any

source after the minor source baseline date. Any such emissions reduction

would increase the amount of available increment to the extent that ambient

concentrations would be reduced.

Increment expansion may also result from the reduction of actual emissions

after the major source baseline date, but before the minor source baseline

date, if the reduction results from a physical change or change in the method

of operation (i.e., construction) at a major stationary source. Moreover, the

reduction will add to the available increment only if the reduction is

included in a federally enforceable permit or SIP provision. Thus, for major

stationary sources, actual emissions reductions made prior to the minor source

baseline date expand the available increment just as increases before the

minor source baseline date consume increment.
D R A F T

OCTOBER 1990

C.11

The creditable increase of an existing stack height or the application of

any other creditable dispersion technique may affect increment consumption or

expansion in the same manner as an actual emissions increase or decrease. 

That is, the effects that a change in the effective stack height would have on

ground level pollutant concentrations generally should be factored into the

increment analysis. For example, this would apply to a raised stack height

occurring in conjunction with a modification at a major stationary source

prior to the minor source baseline date, or to any changed stack height

occurring after the minor source baseline date. It should be noted, however,

that any increase in a stack height, in order to be creditable, must be

consistent with the EPA's stack height regulations; credit cannot be given for

that portion of the new height which exceeds the height demonstrated to be the

good engineering practice (GEP) stack height. 

Increment consumption (and expansion) will generally be based on changes in

actual emissions reflected by the normal source operation for a period of 2

years. However, if little or no operating data are available, as in the case

of permitted emission units not yet in operation at the time of the increment

analysis, the potential to emit must be used instead. Emissions data

requirements for modeling increment consumption are described in

Section IV.D.4. Further guidance for identifying increment-consuming sources

(and emissions) is provided in Section IV.C.2.