4 4 3

the available NH . Thus, in the presence of both SO and HNO , 3 4 3

=

(NH ) SO will be formed preferentially to NH NO . Essentially, 4 2 4 4 3

NH NO will only be formed when there is an excess of NH 4 3 3

available, relative to SO . The MESOPUFF-II modeling system =

4

accounts for the balance between SO , HNO and NH . Therefore, =

4 3 3

emissions of both SO and NO should be modeled in the same run 2 X

to account for this balance. This balance will not be

accounted for if Level I methods are used; the contribution of

NO to visibility degradation may be overestimated in Level I 

3

analyses, but this is consistent with the rationale for Level I.

As noted above, fine particles are the major contributor

to anthropogenically produced visibility degradation, and

sulfates and organics constitute the highest contributions to

measured fine particle concentrations in most areas of the

country. Organic aerosols are generally considered to be

 

secondary products of chemical reactions in the atmosphere; the

processes which lead to their formation are not well

understood. The sources of organic aerosols can be both

natural and anthropogenic. Current modeling and analysis

techniques are inadequate for providing an estimate of organic

aerosols.

Thus, for the purposes of calculating regional visibility

degradation due to specific sources of air pollution, the

primary focus will be on the contribution to light extinction

of fine particles of sulfate compounds and nitrate compounds,

expressed as (NH ) SO and NH NO . Once these particles are 4 2 4 4 3

formed, however, their size can change, and thus their light

scattering efficiency, due to changes in the relative humidity

of the atmosphere. Therefore, in order to adequately account

for the contribution to light extinction of either (NH ) SO or 4 2 4

NH NO the mass of these constituents and the relative humidity 4 3

of the atmosphere in which these particles reside must be

known. The calculations of the extinction due to primary fine

particulates are assumed to be non-hygroscopic.

Method

1. Apply an appropriate air quality model to obtain hourly

concentrations of SO and/or NO and/or primary fine = 

4 3

particulate.

a. If using MESOPUFF-II concentrations of SO and NO are = 

4 3

obtained as direct model output (refer to

Appendix A).

1) To obtain primary fine particulate

concentrations, The MESOPUFF-II should be run as

an independent run from the SO and NO run, = 

4 3

assuming all of the fine particulate emissions

are SO emissions and that they are the only =

4

emissions in that run, the chemistry options

should be turned off, and the deposition options

should be turned on. The other options should

be set as described in Appendix A.

b. If running a steady-state model, use the methods,

outlined in Inset 1 of the main body of the report,

to convert SO and NO to SO and NO . The primary X X 4 3

= 

fine particulate emissions can be directly modeled.

 

 

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