47In the CAIR analysis, the DON was assumed to stay constant through time due to the

uncertainty in the ammonium measurements. The water calculation is sensitive to the ammonium (and

therefore the DON value) concentrations. Keeping the DON constant allows for the future year

ammonium and water values to be solely a function of the change in sulfate and nitrate concentrations.

Otherwise, the water concentration can go up when the sulfate and nitrate concentrations go down. This

may occur if sulfate becomes more neutralized in the future. It is a somewhat illogical outcome (although

scientifically possible) and is highly dependent on an uncertain measurement (ammonium). Therefore,

use of a constant DON creates a more stable set of calculations. If the measured and modeled ammonium

concentrations are believed to be accurate and respond in a reasonable way to emissions controls, then it

would be more scientifically credible to use the model predicted change in ammonium. Otherwise, it is a

reasonable assumption to keep the DON constant over time.

AmmoniumFuture= 0.336 * 6.69 + 0.29 * 0.93 = 2.52 ug/m3

The NH4future, SO4future, and NO3future concentrations can then be input into an equilibrium

model (AIM or another alternative model) or through a polynomial equation to predict future

year particle bound water concentration.

(9) The future species concentrations at each FRM site are then summed over the seven species

plus blank mass to estimate the future quarterly average PM2.5 concentration.

In the example above, the total PM2.5Future=

6.69 + 0.93 + 4.53 + 0.75 + 0.66 + 2.52 + 1.73 + 0.5 = 18.31 ug/m3

(assuming that the future year water concentration is equal to 1.73 ug/m3)

(10) The same calculations are completed for the other 3 quarters to get a future year PM2.5

concentration for each quarter. The 4 quarters are then averaged to get a final future year annual

average PM2.5 concentration for each FRM site.

(11) The future year annual average concentration is compared to 15.0 ug/m348.

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