Review of the Air Quality Criteria Document for Particulate Matter

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is directly overhead, the sun and sky look almost white while the sky is blue off to thesides in the direction of the scattered light.” The student might wish to step outside some clearday and check whether the horizon is indeed blue and the sky white.]“The output of the Mie calculations includes efficiency factors for extinction, Q ext , scattering,Q scat , and absorption, Q abs . The Q ext , Q scat , and Q abs give the fraction of the incident radiationfalling on a circle with the same diameter as the particle that is either scattered or absorbed. Thelight scattering or absorption efficiency factor (in units of m 2 /g) is the change in the lightscattering or absorption efficiencies per unit change in mass of the fine particle constituent. ...Multiplying the values of the light scattering efficiency factor by the aerosol volumeconcentration (in units of µm 3 /cm 3 ) gives the value of the light-scattering coefficient, σ sp , (inunits of Mm -1 ) for these particles.” (P 4-89, L 15-26) [Students: find 3 different concepts of‘efficiency factor’ in this paragraph. For extra credit, find 4 or more.]“.. over a 30-year period (1940 to 1990).” (P4-111, L 3)There are misstatements of the Agency’s own key regulatory concepts:“Visibility impairment is defined as any humanly perceptible change in visibility (lightextinction, visual range, contrast, or coloration).” (P 4-85, L 3) [The hypothetical observer in apure Rayleigh atmosphere thus experiences impaired visibility during each sunset and sunrise.Will the Sierra Club have to sue before the Agency addresses the long-standing and pervasiveproblem of twice-daily twilight?]“dv = 10 log 10 (σ ext /10 Mm -1 )” (P 4-95, L 13) [This makes one deciview correspond to a 26%rather than 10% change in extinction, and makes an extinction coefficient of 100 Mm -1correspond to 10 dv rather than the 23 dv indicated in Figure 4-20. To be fair, this error isaccurately reproduced from the 1996 CD, and is faithfully carried into the 2001 Staff Paper.]Currency, competence, and relevance, by subsectionWhat are appropriate standards for review? In terms of currency and competence, a defaultoption for the 2001 CD is to reprint the 6+ page summary of visibility effects from the 1996 CD,section 8.9.1. That text is clear and accurate. If new text is needed, it should be no less clearand accurate. In terms of relevance, I start from the presumption that any secondary standard forPM will be specified in terms of the health-based primary standard, currently PM 2.5 as defined bythe FRM. A key burden of section 4.3, then, is to document a consistent relationship betweenvisibility and measured fine particle mass.4.3.1 Introduction: The second of the two paragraphs is up to date and appropriate (althoughthe citation of the IWAQM document (USEPA 1995a) is puzzling). The first paragraph,in contrast, is confused and unnecessary – why should the 2001 CD open its visibilityupdate with a garbled rehash of the Agency’s 1979 distinction between reasonablyattributable and regional haze?4.3.2 Factors affecting atmospheric visibility: There is nothing in here drawn from workdone since 1996, save for a passing reference to current visibility conditions from theAgency’s latest trend report. Instead, there are odd definitions (e.g. “The visual range isthe closest distance ...”), unused definitions (e.g. multiple scattering), incorrectdefinitions that were treated correctly in the 1996 CD (e.g. Mie scattering, as alreadynoted), and a similarly varied range of ‘facts’. It is dispiriting to find the Agencydiscarding a document that this Committee spent two years reviewing, in order to slaptogether an erratic new assemblage that is no more up-to-date.Is visibility (as crudely indexed by, say, visual range) inversely related to ambientparticle concentration (as crudely indexed by, say, PM 2.5 )? One surely couldn’t establishA - 83
that point from this review! “Visibility impairment may be connected to air pollutantproperties... Human vision is one of the factors ... the appearance of a distant object isdetermined by illumination of the sight path ... Visibility within a sight path longer thanapproximately 100 km .. is affected by changes in the properties of the atmosphere overthe length of the sight path.”4.3.3 Optical properties of particles: Of the 23 different papers cited in this subsection, 17were published by 1994 and 13 were reviewed in the 1996 CD. The technical discussionis very confused, and diverse extinction efficiencies are jumbled together with nocontext.The Staff Paper includes a cross-plot (Figure 5-2) of ASOS airport visibility data versus24-h PM 2.5 concentrations at Fresno, CA. This is exactly the sort of analysis that isneeded to support a PM 2.5 standard for visibility and is missing from the CD. But it isonly the first step: is the rest of the country just like Fresno? The CD instead gives usindigestible factoids: “Richards et al. (1991) reported a scattering efficiency for fineparticles of ammonium sulfate of 1.2 m 2 /g .. Sulfate scattering efficiencies have beenreported to increase by a factor of two when the size distribution went from 0.15 to 0.5µm .. The calculated scattering efficiencies for sulfates were 4.1 m 2 /g for 100% massremoval and 3.4 and 5.6 m 2 /g for 25% mass removal. Calculated scattering efficienciesfor carbon particles ranged from 0.9 to 8.1 m 2 /g ..”4.3.4 Effect of relative humidity: This section cites a higher proportion of recent work and isbetter written.4.3.5 Measures of visibility: Of the 24 different papers cited in this subsection, 17 werepublished by 1994 and 13 were reviewed in the 1996 CD. I don’t see any newinformation.And including “fine particulate matter concentrations” as a “measure of visibility” israther begging the whole question, is it not? The figure (4-22) supporting this subsectionsimply assumes a relationship for which the previous subsections laid no theoretical orempirical basis. (Note that the assumed Koschmieder coefficient in this figure differsfrom that used in the next (4-23).)4.3.6 Visibility monitoring methods and networks: The new ASOS and expandedIMPROVE networks are appropriate topics for inclusion in this CD. The extinctionbudgets in Table 4-7 are problematic, however, because the text has given no theoreticalor empirical basis for constructing and understanding them. It would better support avisibility-based secondary standard to summarize the measured extinction/PM 2.5 ratiosand regression relationships observed at those sites with optical data.4.3.7 Visibility modeling: Modeling can’t be credible until the science is, so I didn’t botherwith this subsection.4.3.8 Trends in visibility impairment: Much of this subsection (P 4-109, L 4-26) concernsextinction budgeting rather than trends in space and time. As noted above at subsection4.3.6, the text has laid no basis for such apportionment. Moreover, some of thecharacterizations are a bit suspect -- for example, the statement “In several areas of thewest, sulfates account for over 50% of the annual average aerosol extinction” is notsupported by Table 4-7.The trend discussion is largely carried over from the 1996 CD; Figure 4-23 is an updateof Figure 6-112 by only three years and Figure 4-24 is a reprint of Figure 6-113.Considering that this is supposed to be an incremental update of the 1996 CD, and thatA - 84
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SCIENCE ADVISORY BOARD STAFFMr. A.
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