Increment Modeling

As part of a successful modeling demonstration, the proposed source not only needs to demonstrate compliance with MAAQS, but with increment standards as well (if applicable). An increment determination will need to be made.

To determine changes in emissions (or other parameters) that affect increment consumption and/or expansion, it is necessary to compare baseline emissions (1977 for SO2 and PM10, 1987 for NOx ) with current emissions. See Chapter 100, definition of "baseline concentration", for more information.

The following basic procedures should be used to determine if the source is increment consuming or expanding:

For sources that did not exist in the baseline year but do currently exist (i.e., new sources or sources that were constructed AFTER the baseline year(s)), assume that the emissions are entirely increment consuming. If the sources are currently in existence and meet the criteria set forth in Air Quality Increment Guideline (MEDEP-BAQ, 1992), use current actual data, otherwise, use maximum licensed allowed. No baseline credits are to be taken.

For sources that existed in the baseline year but do not currently exist (i.e., sources that existed during the baseline year(s), but no longer exist), assume the emissions are retired baseline emissions that expand increment. These emissions reductions may be used, if necessary.
For sources that existed in both the baseline year and are still currently in operation (whose configuration has not changed), compare the stack parameters (i.e., stack height, stack diameter, stack exit velocity, stack temperature, etc.) and geographic coordinates to make sure that they are identical. If so, calculate the difference in actual emissions between the baseline year and current actual year emissions to determine any emission increases (increment consumption) or any emission decreases (increment expansion).
For sources that existed in both the baseline year and the current year (whose configuration has changed), assume that the source that existed in baseline configuration can take credit for these emissions (i.e., model the baseline emission with a negative emission rate and the corresponding baseline stack parameters) and that the current year configuration consumes increment (i.e., model the current year emissions with a positive emission rate using the current year stack parameters). The model will calculate the net increment change (either expanding or consuming).
For sources that were in violation of ambient standards during the baseline year, the baseline year is to be reset to the year in which the sources modeled scenarios can demonstrate compliance with all applicable ambient standards.

These are the basic concepts to use in making increment determinations. MEDEP-BAQ strongly recommends working in close consultation with the appropriate project engineer or meteorologist as one develops such a demonstration.

Additional Increment Information

As stated in Chapters 115 and 140 of the Maine Air Regulations, additional analyses are required to evaluate increment consumption due to non-point (mobile and area) sources.

Population growth in a particular county can typically be used as a surrogate factor for the growth in emissions from residential combustion (area) sources. Populations information/estimates from the Maine State Planning Office can be found at:

http://econ.maine.gov/

Growth in vehicle miles traveled (VMT) can be used to determine growth in NO2 emissions in the general area of the source being modeled. MEDEP-BAQ has performed motor vehicle emission model runs for the period of 1987 to 1998. These figures are available from the Mobile Sources Section of MEDEP-BAQ and may be obtained by calling (207) 287-2437.

Federal guidance and Chapter 140 of the DEP regulations require that any new major source or source undergoing a major modification provide additional analyses of impacts that would occur as a direct result of the general, commercial, residential, industrial and mobile-source growth associated with the construction and operation of that source. Typically, this is done through a series of qualitative analyses that individually addresses each of the growth types listed above.