Diagnostic testing which indicates a large impact on the model results from initial or

boundary conditions may indicate that the domain is not large enough or the ramp-up period is

too short. In either case, it should generally be assumed that initial and boundary conditions do

103Sensitivity simulations can be completed to determine the necessary length of the

ramp-up period. A longer ramp-up period may be needed for very large domains where the

characterization of long range transport is important.

104One atmosphere model applications for ozone and PM may commonly use global

models to specify boundary conditions. This is especially important for PM species due to their

long lifetimes. A number of recent studies show that long-range, intercontinental transport of

pollutants is important for simulating seasonal/annual ozone and particulate matter (Jacob, 1999;

Jaffe, 2003; Fiore, 2003).

not change in the future. The use of altered initial or boundary conditions in the future year

should be documented and justified.

15.3 What Horizontal Grid Cell Size Is Necessary?

As we discuss in Section 16, most applications will use a prognostic meteorological

model to provide meteorological inputs needed to make air quality estimates. Typically, these

models are set up to produce meteorological fields for nested grids with a 3:1 ratio. In past

modeling applications, the most commonly used grid cell sizes have been 108, 36, 12 and 4 km

cells. In this section we provide recommendations for choosing the grid size to use as an upper

limit for regional and urban scale models or for fine portions of nested regional grids.

This section only applies to the use of numerical grid models. If a Lagrangian dispersion

model is used, estimates at (or near) the monitor locations should be used to calculate RRF

values for primary PM.


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