Almospheric Em'ironmem Vol 16, No. 10, pp 2367 2368, 1982 0004 69~1 '82 102367 02 SO3.00/0 Primed In Great Britain. Pergamon Press L id

DISCUSSION

VISIBILITY IN ABSORBING AEROSOLS

Roessler and Faxvog (hereafter referred to as R & F) report model calculations of visual range for various objects viewed through light absorbing aerosols. Included are calculations for light and dark objects as well as perfectly black targets viewed against either the horizon sky or an immediately adjacent background. We agree with R & F that the visual range of non-black objects in absorbing and non-absorbing aerosols is much more complex than the simple Koschmeider relation. However, while their model is technically correct, the assumptions they use produce results that can be interpreted as suggesting that visual air quality generally improves as the aerosol becomes more absorbing. We believe that if R & F had used a more realistic set of assumptions they would have found that visual range generally decreases as the aerosol becomes more absorbing. In particular, we disagree with three of their assumptions.

(i) R & F assume a ratio of optical extinction coefficient to fine particle mass concentration, A E, that is independent of the fraction of extinction caused by absorption. In their Fig 2, the same value of AE(5 m 2 g- 1 ) is used to calculate the visual range for both scattering (light scattering to extinction ratio, b~/b, = 0.9) and highly absorbing (bs/b , = 0.1) aerosols. The value of A E and therefore the visual range actually depend strongly on the absorption fraction of extinction in haze size particles. The assumption that A E is constant tends to overestimate the visual range for the highly absorbing case. In R & F's Fig. 2, the value of AE should be about 11 m 2 g - J for small, highly absorbing particles (such as graphite) with b,/b, = 0.l; whereas A E should be about 2 m: g- ~ for weakly absorbing particles (bs/b~ = 0.9) of similar size. Figure 2

Roessler D. M. and Faxvog F. R. (1981) Atmospheric Enrironmem ! 5. 151-155.

shows that the visual range for the light object (object to clear sky brightness ratio, L o / L H = 0.5) is about 2 times greater in the absorbing aerosol than in the weakly absorbing one. In contrast, the visual ranges in both aerosols are about the same for the dark object ( L o l L H = 0.03) and identical for the black object (Lo/L H = 0). However, when our suggested values for A E are used with their target conditions, the visual ranges shown in their Fig. 1 would always be lower for the highly absorbing aerosol. For the values of L o / L H, of 0, 0.03 and 0.5 the respective visual ranges of the highly absorbing aerosol would be about 20, 15 and 30", of that for the weakly absorbing aerosol at each mass concentration.

(ii) R & F model visual range for an aerosol with scattering to extinction ratio between 0.1 and 1.0 and state that "carbonaceous aerosols have been shown to have an absorp- tion to extinction ratio of 0.85 (i.e. a scattering to extinction ratio of0.15)". We feel that their absorbing aerosol used in the model is useful as an extreme example but is so different from ambient aerosols as to produce misleading results. Typical urban aerosols have absorption to extinction ratios of 0.1 to 0.4 and for rural eastern U.S. aerosols the ratio is 0.02 to 0.1 (Weiss et al., 1979; Waggoner et al., 1981). Combustion aerosols from fuel rich flames or diesel engines can have the properties modeled by R & F but after emission into the ambient air the particles initially below 0.1 um in diameter grow by coagulation and are incorporated into larger, tess absorbing particles. Typical urban fine particles (less than 2.5 pm diameter) contain < 20 '.!o graphitic carbon (Groblicki et al., 1981).

(iii) R & F have used a definition for object to horizon brightness ratio that inflates the visual range for non-black objects in absorbing atmospheres. In their Figs 2~4 they re- port visual range for various object to horizon brightness as that for an isotropic pure scattering atmosphere. This has the effect of making the actual object to horizon brightness ratio

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Fig. 1. Calculated visual range for various object to horizon brightness ratios and particle absorption fractions. L o / L H is the ratio of intrinsic object to horizon brightness for a purely scattering aerosol; F ( C ) is the fraction of purely scattering particle volume that is replaced with highly absorbing graphitic carbon: bs/b e is the corresponding fraction of the total optical extinction coefficient that is caused by scattering. The extinction coefficients were calculated with a Mie code for a log normal size distribution with volume mean diameter of 0.2/1m and a geometric standard deviation of 2. The refractive

indices used for the purely scattering material and graphitic carbon were 1.525 and 1.5943.66 i, respectively.

2367

2368 Discuss~or~

inversely proportional to the scattering to extinction ratio. This can he observed in their Fig. 3; visual range becomes infinite as the aerosol scattering to extinction ratio ap- proaches zero i.e., the horizon becomes black. In addition, the curve mislabeled Lo/L H = 0.9 in their Fig. 3 should read Lo/ L N = 0.5.

We have included a figure (Fig. I) to show our estimate of the effects on visual range of target brightness and graphite concentration for a fixed log normal size distribution witl~ particle concentration equal to 100/am 3 cm- 3, volume mean diameter 0.2pro and geometric standard deviation of 2. Figure 1 shows the effect on visual range of varying the graphite (i.e. light absorption} fraction in the particle, f [C° ] , from 0 to 100 ~0 for values of LolL r t, tanging from 0 to 2. We would like to make a few brief comments regarding the principle features of the Fig. 1: (i) For the Koschmeider condition (Lo/L H = 0), the visual range simply decreases inversely with the increase A t as the graphite fraction of the particles increase. (ii) The trough of zero visual range represents the aerosol graphite fraction at which the horizon and object brightness are within 0.02 of each other. The value o f f [ C ° ] at which the visual range is zero is determined by the size distribution. That is, the horizon brightness is propor- tional to bJb, and for a givenf[C°], b,/b, depends on the size of the particles. For smaller particles the trough would move toward lower values of Lo/L H values; larger particles would move the trough towards higher Lo/L n values. (iii) Visual range increases with increas ingf[C ' ] only when the object brightness is near to but greater than or equal to the actual horizon brignmess.

Had R & F included the effects of absorption on A E and on objects of brightness nearly equal to the horizon brightness. they would not have concluded that added absorption often increases visual range. Their results are correct for their choice of model assumptions and it is with these assumptions that we disagree. We believe their results should not be used to predict the visual impacts on urban visual range of future increases in light absorbing particles b2t such sources as wood stoves or a larger fleet of light duty diesel vechicles. This is a timely issue and we encourage more efforts m this area both from a modeling and experimental prespective.

Environmental Engineering & Science Program

Department of Civil Engineering, FC-05

Unit:ersity of Washington Seattle, WA 98195, U.S.A.

R E . WEIss A.P . WAGGONER

REFERENCES

Groblicki P. J., Wolff G. T, and Countess R. J. (1980) Visibility reducing species in the Denver 'Brown Cloud', Part I. R©iationships between extinction and chemical composit ion, General Motors Research Laboratory, Research Publication GMR: 3417 Env. No. 81.

Roessler D. M. and Faxvog F. R. (1981) Visibility in absorbing aerosols. Atmospheric Environment 15, 1510155.

Waggoner A. P., Weiss R. E., Ahlquist N. C., Covert D. S., Will S. and Charlson R. J. (1981) Optical characteristics of

atmospheric aerosols. Atmospheric' Environmen~ ~ ~, 1891-1909.

Weiss R. E., Waggoner A. P., Thorsell D. L., Hall J. S., Rite)' A. and Charlson R. J. (1979) On the nature of light absorbing aerosols, Proc. Conf. on Carbonaceous Particle.s in :he Atmosphere, Lawrence Berkeley Lab., Document LBI.- 9037, edited by T. Novakov.

A U T H O R S ' R E P L Y

The purpose of our paper was to caution against careless application o f the Koschmieder relation without regard to the fraction of aerosol extinction which may be due to absorption rather than scattering. We did this by presenting formulae generalized to include absorption (and also non-black ob- jects) and showing the effects of varying the fractional absorption but keeping the total extinction constant. It was certainly not our intention that this be interpreted as a claim that all aerosols have the same specific extinction. It was our intention that those interested in computing visual range use the correct formulae and the values of extinction, absorpI~on and brightness appropriate to aerosols of their concern.

With specific reference to Weiss and Waggoner's points: (11 To have varied both A E and b,/b, in our model would

have defeated the purpose of extracting just the effects of bs/b e. Our choice of of A E = 5 m 2 g - ~ was simply a value intermediate between that o f carbon and that of many dominantly scattering aerosols: It was not meant to be interpreted as some universal value for al aerosols.

(2) We did indeed choose an extreme example of an absorbing aerosol (carbon). It indicates how significant the effect can he and draws attention to the role that carbonaceous aerosols may play in the future.

(3) The assumption of isotropic scattering was made for simplicity.

We are glad that Weiss and Waggoner agree with the technical correctness of our model and thank them for noting the mislabelling of our Fig. 3. Here, and in the supporting text. the value o f L o / L H should be 0.1 for the dark object and 0.5 for the light object (not 0.9).

It should be noted that a carbon aerosol certainly does have a strong effect on visual range, largely because of its very high specific extinction. It is precisely because only a small fraction of the extinction is due to scattering thar the effect is not even greater.

On a final note we add that carbon aerosols are becoming important in environmental considerations, and there is still need for better documentation and understanding of their optical properties. Both the specific extinction and the refractive index deserve further study.

Physics Department DAVID M. ROESSt-ER General Motors Research Laboratories Warren, MI 48090, U.S.A.

(currently) Honeywell Systems and Research Center

Minneapolis, MN 55413, U.S.A.

FREDERICK R. FAXVOG