6) Whether the photochemical model utilizes one-way or two-way nested grids.

-The fine grid domain of a model with one-way nested grids may need to be larger

(compared to a model with two-way nested grids). The grid needs to be large enough to

capture re-circulation due to shifting wind directions. A two-way nested grid model

allows for continuous feedback from the fine grid to the coarse grid and vice versa.

7) Computer and time resource issues.

15.2 How are the Initial and Boundary Conditions Specified?

Air quality models require specification of initial conditions for model species in each

grid cell in the model domain (in all layers) and boundary conditions for all grid cells along each

of the boundaries (in all layers). Generation of initial and boundary conditions for individual

model species include gas-phase mechanism species, aerosols, non-reactive species and tracer

species. There is no satisfactory way to specify initial conditions in every grid cell. Thus, we

recommend using a “ramp-up” period by beginning a simulation prior to a period of interest to

diminish the importance of initial conditions103. We recommend a ramp-up period of at least 2-3

days for ozone modeling. Due to longer lifetimes of fine particulate matter, we recommend a

ramp-up period of at least 5-10 days for PM2.5. In this way, the choice of initial conditions are

not likely to influence the modeling results. For nested model applications, initial conditions can

be specified using model predictions from the outer grid if the nested grids are started after the

beginning of the simulation for the outer grid.


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