For ease in computation, States/Tribes may assume that a monitor is at the center of the
cell in which it is located and that this cell is at the center of an array of “nearby” cells. The
number of cells considered “nearby” (i.e., within about a 15 km radius of) a monitor is a function
of the size of the grid cells used in the modeling. Table 3.2 provides a set of default
recommendations for defining “nearby” cells for grid systems having cells of various sizes.
Thus, if one were using a grid with 4 km grid cells, “nearby” is defined by a 7 x 7 array of cells,
with the monitor located in the center cell.
The use of an array of grid cells near a monitor may have a large impact on the RRFs in
“oxidant limited” areas (areas where NOx decreases may lead to ozone or secondary PM
increases) and in areas with large gradients in primary PM2.5. The array methodology could lead
to unrealistically small or large RRFs, depending on the specific case. Care should be taken in
identifying an appropriate array size for these areas. States/Tribes may consider the presence of
topographic features, demonstrated mesoscale flow patterns (e.g., land/sea, land/lake interfaces),
the density of the monitoring network, the density of emissions (especially primary PM)18, and/or
other factors to deviate from our default definitions for the array of “nearby” grid cells, provided
the justification for doing so is documented.
18It may be inappropriate to use an array as large as 7 X 7 in areas with large gradients of
primary PM2.5 concentrations. Averaging of multiple grid cells may mask changes in local
concentrations between the base year and future years.
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