3.2 Identifying surface grid cells near a monitoring site.

There are four reasons why we believe it is appropriate, in the modeled attainment test,

to consider cells “near” a monitor rather than just the cell containing the monitor. First, one

consequence of a control strategy may be “migration” of a predicted peak. If a State were to

confine its attention only to the cell containing a monitor, it might underestimate the RRF (i.e.,

overestimate the effects of a control strategy). Second, monitor siting guidelines recommend

spatial scales (for ozone and PM2.5) which are generally urban or neighborhood scale. In many

cases, the representative spatial scale of a monitor is larger than a single grid cell. Third, we

believe that uncertainty in the formulation of the model and the model inputs is consistent with

17 The average design value should carry one significant figure to the right of the decimal

point for ozone and 24-hour PM2.5 and 2 significant figures for annual PM2.5. There are several

calculations in the modeled attainment test which carry the tenths digit or hundreths digit. We

have found that rounding and/or truncating concentrations and RRFs can lead to an overestimate

or underestimate of the impact of emissions controls. In some cases, a few tenths of a ppb

change (or a few tenths of a percent reduction) can be meaningful. Rounding or truncating can

make the change appear to be equal to a full ppb (or 1%) or equal to zero change. For 8-hour

ozone, it is recommended to round to the tenths digit until the last step in the calculation when

the final future design value is truncated. For annual PM2.5 it is recommended to round to the

hundreths digit until the last step when the final future design value is rounded to the nearest

tenths place. For daily PM2.5 it is recommended to round to the tenths digit until the last step

when the final future design value is rounded to the nearest whole number.

 

 

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