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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