3.6 Air Quality

Ambient air quality data are generally used for two purposes:

(1) to specify initial- and boundary-condition concentrations, and

(2) to assess the model performance for each meteorological episode

to be used in the attainment demonstration. These topics are

addressed in the following two subsections.

3.6.1 Initial and boundary conditions

Three general approaches for specifying boundary conditions

for UAM simulations are as follows: (1) use objective/interpolation

techniques with a sufficient amount of measured data (i.e.,

data from an intensive field program), (2) use default background

values and expand the upwind modeling domain and simulation period

to mitigate uncertainties due to paucity of measurements, and (3)

use regional-scale model predictions of ozone and precursor

concentrations. Initial conditions for UAM simulations are handled

in one of two ways: (1) use regional-scale model predictions to

derive initial conditions, and/or (2) begin the UAM simulation

sufficiently far in advance of the primary day to eliminate

sensitivity of results to arbitrary assumptions regarding initial


Clearly, the nature of case-specific applications will

determine what approaches should be taken for establishing initial

and boundary conditions for particular domains. Ideally, the

preferred technique would be based on an intensive field program

with regional-scale modeling used to fill in spatial and temporal

gaps. This approach is seldom feasible, however, particularly for

historical episodes. Presented below are recommendations for

implementing each of the three techniques identified above for

deriving boundary conditions, including discussion of the

advantages and disadvantages of each technique. Default boundarycondition

values for ozone and precursor concentrations are also

provided. Finally, recommendations are provided on the approach

most likely to be feasible for specifying the initial and boundary

conditions for modeling historical episodes in most locations.



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