The total ammonia concentration, on the other hand, is critical to the test case results.

CTNH3 is used to calculate the distribution of total nitrate between nitric acid (HNO ) and 3

aerosol nitrate (NO ). The NO concentration is extremely sensitive to the specification of 3 3

- -

CTNH3. Changing CTNH3 from 1 ppb to 10 ppb results in a tenfold increase in NO in the 3

-

test case.

Correction to the input file can be accomplished simply by moving the CO3B and CTNH3

values one place to the left, and adding decimal points after each value.

We recommended that further thought be given to the default values of CO3B and CTNH3.

The value of 80 ppb for ozone on a regional scale is higher than the values assumed for other

regional modeling applications, particularly in winter. A default value of 40-60 ppb may be

more appropriate for CO3B. The background ozone level is used in the model to calculate the

conversion rates of SO to sulfate and NO to nitrate. The value of 10 ppb for CTNH3 also 2 x

appears quite high for a regional background level. In fact, CTNH3 should not represent the

total background ammonia since the model contains no background sulfate or nitrate for the

sources not being simulated. It represents what could be termed "excess ammonia", the

amount remaining after background sulfate and nitrate have been fully neutralized. Given

this, 10 ppb is almost certainly an upper bound; hence, aerosol nitrate concentrations predicted

by MESOPUFF II may represent upper bounds. A more representative value for the excess

ammonia concentration in the eastern U.S. is between 0.5 and 3 ppb, depending on the season.

For the test case, however, we recommended that the values of 80 ppb (for ozone) and 10 ppb

(for ammonia) be retained, since they are the default values given in the existing

documentation.

Error in MESOPUFF II Subroutine CHEMTF

The model code was not scrutinized in detail for this task. However, one error was found in

the chemistry subroutine CHEMTF. In calculating the concentration of ammonia that is

available to combine with nitric acid to form aerosol nitrate, the amount of ammonia taken up

by sulfate is subtracted from the total ammonia:

CANH3 = CTNH3 - PPB(2)

The equation should read:

CANH3 = CTNH3 - 2.*PPB(2)

because ammonium sulfate contains two ammonium ions per sulfate ion.

Test Case Post-Processing Files

The test case input files for the MESOFILE postprocessor only calculate 24-hour average

concentrations and deposition rates for a single species, SO . If the model is to be used for 2

 

 

n1301 - n1302 - n1303 - n1304 - n1305 - n1306 - n1307 - n1308 - n1309 - n1310 - n1311 - n1312 - n1313 - n1314 - n1315 - n1316 - n1317 - n1318 - n1319 - n1320 - n1321 - n1322 - n1323 - n1324 - n1325 - n1326 - n1327 - n1328 - n1329 - n1330 - n1331 - n1332 - n1333 - n1334 - n1335 - n1336 - n1337 - n1338 - n1339 - n1340 - n1341 - n1342 - n1343 - n1344 - n1345 - n1346 - n13247 - n1348 - n1349 - n1350

 

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