a) "Baseline concentration" means the ambient concentration level that exists in the baseline area at the time of the applicable minor source baseline date. A baseline concentration is determined for each pollutant for which a minor source baseline date is established and shall include:

1. 1) The actual emissions, as defined in Section 204.210, representative of sources in existence on the applicable minor source baseline date, except as provided in subsection (b); and
2. 2) The allowable emissions of major stationary sources that commenced construction before the major source baseline date, but were not in operation by the applicable minor source baseline date.

b) The following will not be included in the baseline concentration and will affect the applicable maximum allowable increase or increases:

1. 1) Actual emissions, as defined in Section 204.210, from any major stationary source on which construction commenced after the major source baseline date. For a major stationary source in existence on the major source baseline date, "actual emissions" for the purposes of this subsection (b) means increases or decreases in actual emissions resulting from construction commencing after the major source baseline date; and
2. 2) Actual emissions increases and decreases, as defined in Section 204.210, at any stationary source occurring after the minor source baseline date.