for each. Figure 3-3 also displays the locations of these sources in relationship to all 27

original PSD sources (indicated by circles).

Run Time as a Function of Number of Puffs

A series of 24-hour test simulations was performed to ascertain the dependence of

MESOPUFF II execution time on input parameters including puff release rate, number of

sources, number of receptors, ozone data specification and chemistry. The results of these

simulations showed that decreasing the number of puffs, whether by decreasing the puff

release rate or the number of sources, had a major effect on execution time. Changes in the

other parameters had much smaller effects on CPU. Specifically, a 50 percent reduction in the

number of puffs reduced CPU by 56 percent. A 50 percent reduction in the number of

receptors reduced CPU by 6 percent. The use of a single default ozone value rather than

hourly values at monitoring stations had virtually no effect on CPU. Even running the model

in an inert mode (i.e. with all chemistry turned off) resulted in only an 8 percent reduction in

CPU time.

These results show that there is a greater than linear dependence of CPU time on number of

puffs, even within the first 24 hours of a simulation. The departure from linearity increases

for longer simulations. Based on the CPU requirements for the month-long simulations, a

simulation with 17 percent of the original number of puffs required only 6 percent of the CPU

time.

MODELING EPISODE DEFINITION

Following the Phase 1 recommendations, a five-year period was to be modeled. In order to

supply the MESOPUFF II modeling system with the most recent environmental input data, the

IWAQM agreed to the five year period from 1988 through 1992. As planned, the MESOPAC

meteorological preprocessor was run to generate MESOPUFF II input fields for this entire

period. As discussed above, however, a decision was made to reduce the Phase 1 PSD

demonstration to three years (1988-1990), with the remaining 24 months of MESOPUFF II

integrations reserved for the ring source analyses.

The default value and Phase 1 recommended value for output averaging interval is one hour;

however, substantial reductions in disk storage requirements can be achieved by increasing

this interval. Based on the observation that the PSD increments and AQRV parameters are all

based at a minimum on 3-hour averages, we judged that an averaging interval of 3 hours

would be sufficient to produce each desired output.

It should be noted that this deviates somewhat from the Phase 1 recommendations, which state

that visibility effects should be calculated using hourly concentrations and hourly relative

humidities. The intent of this recommendation was to capture the diurnal effects and avoid

calculating visibility on the basis of 24-hour averages. The use of 3-hour averages still allows

for the determination of diurnal trends, and would seem to preserve the intent of the

recommendation.

 

 

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