Deciview Haze Index
The regional haze rule stipulates that reasonable progress is to be measured in terms of
changes in a deciview haze index or simply “deciviews”(dv). Deciviews are defined as the
natural logarithm of the ratio of the extinction coefficient to Rayleigh scattering (Pitchford and
Deciview = 10 ln(bext/10) (6.3)
Where the units of bext and light scattering due to Rayleigh scattering72 (i.e., the “10" in
the denominator of the logarithmic expression) are both expressed in Mm-1.
Achange in deciviews, which is how we track progress is given by Equation (6.4). A 1
deciview change is equivalent to ~10% change in bext.
)dv = dvfuture - dvbaseline (6.4)
A negative number indicates a reduction in deciviews, which is an improvement in visibility.
Estimating Mass Associated With Components Of Particulate Matter.
Regional haze calculations for the modeled uniform rate of progress analysis utilizes
IMPROVE measurements at Class I areas. All Class I areas have on-site speciated PM2.5
measurements or have been assigned a representative IMPROVE monitoring site with
measurements73. Therefore, it is generally not necessary to interpolate measured PM2.5 species
data to a site. The existing IMPROVE database of PM2.5 measurements should be adequate to
provide data for all Class I areas74.
FRM data is not used in the regional haze analysis. Therefore, it is not necessary for the
glidepath (using ambient data) and the modeled visibility changes.
72Even though the “new” IMPROVE equation uses a site specific Rayleigh Scattering
value, the denominator in the deciview equation should always be 10. In this way, the deciview
calculation will be consistent across all Class I areas. Under very clean conditions, the deciview
value can be negative, but this should not be considered a problem.
73See U.S. EPA 2003b, Appendix A, Table A-2.
74There may be a few IMPROVE sites that don’t have enough complete data to provide
baseline condition information for the 2000-2004 period. It may be necessary to substitute data
from other monitoring networks or to interpolate data to the site(s).
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