For situations where MSAs cannot be clearly identified, the user may determine the extent of the

area, including the source(s) of interest, where the population density exceeds 750 people per

square kilometer. The combined population within this identified area may then be used for

input to the AERMOD model. Users should avoid using a very fine spatial resolution of

population density for this purpose as this could result in significant gaps within the urban area

due to parks and other unpopulated areas, making it more difficult to define the extent of the

urban area. Population densities by census tract should provide adequate resolution in most

cases, and may still be finer resolution than desired in some cases. Since census tracts vary in

size and shape, another acceptable approach would be to develop gridded estimates of population

data based on census block or block group data. In such cases, a grid resolution on the order of 6

kilometers is suggested. Plotting population density with multiple “contour” levels, such as 0-

500, 500-750, 750-1000, 1000-1500, etc., may also be beneficial in identifying which areas near

the edge of the urban complex to include even though the population density may fall below the

750 threshold. The user should also bear in mind that the urban algorithms in AERMOD are

dependent on population to the one-fourth power, and are therefore not highly sensitive to

variations in population. Population estimates to two significant figures should be sufficiently

accurate for application of AERMOD.



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