Boundary conditions can be specified in several ways. One option is to nest the area of

interest within a much larger domain using nested regional models, as described previously. As

noted above in Section 15.1, use of a large regional domain acts to diminish the importance of

boundary conditions. Alternatively, initial and boundary conditions can be derived from another

regional nested modeling application or from a global model104. Another option is to use default

initial and boundary concentration profiles representing relatively clean conditions which have

been formulated from available measurements or results obtained from prior modeling studies,

(e.g. prescribing 35 ppb for annual ozone). Another approach to consider is using the model’s

simulated pollutant values (generated for emissions) averaged over one or more of the upper

layers to specify a value for the lateral and top boundary conditions. This approach allows for

better representation of spatial and temporal variation of boundary conditions (i.e.,

diurnal/seasonal/vertical profiles). This method also avoids arbitrarily guessing at the futureyear

boundary conditions.

If there is no larger regional model application available, then it is recommended that

background boundary conditions be used to specify initial and boundary concentrations for the

attainment demonstration modeling. However, concentration fields derived from a larger

domain regional or global chemistry model (i.e. nesting approach) is considered more credible

than the assumption of static concentrations, since the pollutant concentration fields reflect

simulated atmospheric chemical and physical processes driven by assimilated meteorological

observations. Therefore, we recommend using boundary conditions derived from a regional or

global scale model, whenever possible.


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