the two retrofits are combined, the emissions reductions obtained from each can be considered additive.

[38 FR 15197, June 8, 1973]

APPENDIX O

The following guidelines are intended to assist in the development of regulations and procedures to comply with the requirements of § 51.18.

1. With respect to facilities which would significantly affect air quality because of emisisons arising from associated mobile source activity, review procedures should cover any facility which can reasonably be expected to cause or induce sufficient mobile source activity so that the resulting emissions might be expected to interfere with the attainment or maintenance of a national standard. The likelihood that there will be such interference will vary with local conditions, such as current air quality, meteorology, topography, and growth rates. For this reason, it is not practicable to establish definitive nationally applicable criteria as to the types or sizes of such facilities which should be reviewed. There are, in however, certain types of facilities which generally should be considered for review. Experience and estimating techniques have indicated that the air quality impact of certain types and sizes of facilities is potentially significant regardless of their location. They include major highways and airports, large regional shopping centers, major municipal sports complexes or stadiums, major parking facilities, and large amusement and recreational facilities. The above examples are not meant to be exhaustive. Local conditions must be considered in determining which types of facilities will be subject to new source review.

New source review procedures must also consider the impact of a new or modified source in political jurisdictions other than the one in which it is located. Construction or modification of that source must be prevented if the impact in another political jurisdiction is great enough to interfere with attainment or maintenance of a national standard, whether or not there is significant impact in the political jurisdiction of the facility.

2. Frequently, a substantial amount of information will be needed to make the determinations required by § 51.18. In addition to general information on the nature, design, and size of a facility, data on its expected mode of operation also will be needed in order to estimate the types and amounts of air pollutant emissions likely to be associated with it. The operational data needed to make such estimates may include time periods of operation, antipicated numbers of employees and/or patrons, expected transportation routes, modes, and habits of employees and/ or patrons, and so on.

Data on present air quality, topography, and meteorology and on emissions from other sources in the affected area may also be necessary.

In those cases where an environmental impact statement (EIS) has been or will be prepared under the National Environmental Policy Act or similar State or local laws, the EIS may well be an excellent source of information to aid in making the determinations required by § 51.18. Accordingly, agencies responsible for new source reviews are encouraged to make such use of EIS wherever possible in order to avoid needless duplication of information gathering and analysis.

3. Wherever possible, modeling techniques for approximating the effects of facilities with associated mobile source activity on air quality should be used. A simplified relationship between emission density (pollutant mass/ time/area), size of an area (such as a parking lot) and maximum downwind concentration of carbon monoxide is given in figure 1. This relationship was derived using a technique similar to one used by Hanna.1 The relationships depicted in figure 1 are based on assumptions of flat terrain, average atmospheric stability (class D) with a steady wind speed of 1 meter/second, constant wind direction, even distribution of emissions at ground level over the area, and insignificant edge effects. Various assumptions are needed to calculate precisely the emission density from a facility, including vehicle speeds within the area, the distribution of automobile ages (which will determine which vehicle emission factor to use), the average area occupied by a vehicle, the fraction of the total area which may be occupied by vehicles, and the maximum number of vehicles running simultaneously for 1-hour and 8hour periods (to determine if either carbon monoxide ambient air quality standard will be exceeded).

Prior to employing the emission density-air quality relationship in figure 1, other factors may first have to be considered in determining whether ambient air quality standards will be exceeded. These factors include measured or estimated existing air quality, the impact of any point sources planned on or near the facility and the impact of any traffic routes on or near the facility passing within the close proximity of critical receptors. Also, consideration should be given to any factors which differ substantially from the assumptions made in the figure 1 relationship, such as topography, meteorology, aerodynamic effects, and spatial distribution of motor vehicles, height of emission, and any facility configuration which would constrain the dispersion of pollutants (such as a parking deck).

1 Hanna, S. R., A Simple Method of Calculating Dispersion from Urban Area Sources, Journal of the Air Pollution Control Association, vol. 21, pp. 714-777 (1971).