Premise PM3. Regional haze is closely related to presence of high concentrations of fine

particulate matter. Light extinction results from scattering and absorption of light. Some

scattering occurs by gas molecules in pristine air (i.e., Rayleigh scattering). Nearly all remaining

light extinction is caused by the presence of aerosols. For any given mass, fine particles (i.e., <

2.5 :m) are more efficient at scattering light than are particles > 2.5 :m aerodynamic diameter.

Further, certain components of PM2.5 are more efficient at scattering or absorbing light than

others. Many of the most efficient are secondary particulate species. For example, sulfates

(secondary), nitrates (secondary) and organic (secondary and primary) components scatter light

more efficiently than do primary particles composed of soil material. Light extinction is also

exacerbated by high relative humidity. Water vapor combines with hygroscopic particulate

matter (e.g., sulfates and nitrates) to greatly increase the light scattering efficiency of these

species. Previously, we noted that secondary particulate matter is likely to comprise an

important fraction of measured PM2.5. Secondary particulate matter will be even more important

as a cause of regional haze. This follows from the greater efficiency with which these already

important components of PM2.5 scatter light. This importance can be enhanced further by high

relative humidity, which is especially relevant in the Eastern U.S.

The discussion in the preceding paragraph suggests that modeling to assess uniform rate

of progress for regional haze will need to address secondary particulate matter. This, in turn,

means that large modeling domains will be necessary. Class I areas are generally likely to be far

removed from most sources of precursors for secondary particulate matter. Additionally, the

measure of visibility (deciviews) with which we are most concerned, addresses maximum range

of visibility. This measure reflects an effect which is integrated over a relatively large distance.

Thus, the need for a large domain, the prevalence of secondary particles, the relative remoteness

of Class I areas from most sources of precursors and the visibility measure of greatest interest

suggest that modeling related to regional haze may be done without a fine degree of spatial




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