Armosphenc Environment Vol. 23. No. I, pp. 279-286. 1989.

Printed m Great Britain.

MlO&6981;89 $3.oO+O.M) Pcrgamon Press plc

ON COMPOSlTION OF PARTICLES DRY DEPOSITED TO AN INERT SURFACE AT

ITHACA, NEW YORK*

The paper by Butler (1988), concerning the composition of deposited particles, represents a nice, and difficult, piece of work that supports the importance of coarse particles in dry deposition, However, there are a few points in the paper which merit further discussion and clarification. I was inter- ested to note the similarity in the calculated particle number median diameters reported by Butler with those recently reported elsewhere (Coe and Lindberg, 1987): 1.7 pm by Butler, compared to 2.1 pm (range 1.42.9) by Coe. This suggests that coarse particles are indeed a general phenom- enon, not an aberration due to the climate and land use at isolated sites. Similar studies have supported this idea in other areas of North America and Europe as well. However, I continue to be bothered by one problem in our work that may also influence the results of Butler. Did the author attempt to quantify the effect on the calculation of median diameters of the “poor” resolution of the SEM below about 0.3 pm? This was essentially the same effective cutoff reported by Coe. As much as I would like to believe that aerosol dry deposition is controlled by coarse particles, it seems to me that we could be grossly overestimating the median dia- meters due to our “artificial” cutoff size. Coe did attempt to count and identify fine particles at high resolution (5000 x magnification, resolution -0.065 pm), but the area scanned becomes limiting and one has trouble identifying deposited particles from surface irregularities. Does the author have any thoughts on the effect of this problem on the reported number median diameters? On a similar point, did the author compute mass median diameters, since he did report particle masses? Coe’s estimates suggest that deposited mass is dominated by particles of the order of 10 pm. However, our experience with fly ash particles is that not all are hollow and we assumed solid spheres in our calculations.

The ideas and data on CaSO, are also interesting, and I agree on the probable source, which is supported by ion ratios for particles we have collected both in Tennessee, and in Gottingen, F.R.G. However, we and others have also found evidence for the formation of gypsum crystals on plant surfaces following particle deposition (e.g. see Figure Id in Coe and Lindberg, 1987). I believe these form from SO, reactions with parent, previously deposited, Ca-rich particles (CaCO,?). probably after wetting with dew or fog, while Ken Rehfuess at the University of Munich believes these to indicate Ca leaching from foliage in response to acidic deposition (Rehfuess, 1983). In our SEM photographs, the parent particle is easily seen, however. Has the author attempted to calculate the relative roles of pre- vs post- depositional reactions of SO, (or HZS04) with Ca-rich particles based on his surface uptake measurements?

The last sentence in the abstract implies that the author has actually quantified the wet and dry deposition rates of coarse particle SO: _, and that it is minor (“3-5%“) relative to wet depositionjuxes of SO:-. This is clarified somewhat in the text where it is stated that CaSO, represents only 3-5% of SOi- ions in rain. The confusing wording of the abstract is unfortunate in that it gives the impression that

*Butler T. 3. (1988) Atmospheric Environment 22,895-900.

DISCUSSIONS

coarse particle dry deposition of SOi- is itself insignificant (compared to “wet deposition”). Field experiments in Ten- nessee suggest coarse particle SO:- can be important com- pared to total SOi- fluxes (7-8-12% of total annual SO:- deposition in three different forest sites), and certainly signifi- cant relative to wet deposition (15-27-30% of annual wet SOi- deposition) (Lindberg et al., 1986; and unpublished data). Similarly, experiments in a field in Illinois also support the significance of coarse particle SOi- dry deposition (Davidson et al., 1985).

Environmental Science Division, STEVEN E. LINDBERG

Building 1505, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6038, U.S.A.

AUTHOR’S REPLY

I would like to thank Dr Lindberg for his comments, He raises several important issues which I will attempt to clarify.

I did not try to quantify the effect that particles below 0.3 nm would have on the median diameter. Due to the number of samples I was processing and the time involved, a finer resolution was not a practical option. As Lindberg states, the area scanned becomes limiting when an attempt is made to analyze particles in this smaller size range. Also, I did not calculate a mass median diameter because neglecting latger particles over 10.0 pm would significantly under- estimate a mass median diameter calculation. However, since then I have some preliminary results for a wider range of particle sizes (0.05-16.0 pm) than I originally reported (0.3-10.0 pm).

In order to address the question of deposited particle size distribution I have counted and sized 300 particles from 0.05 pm to 5.0 pm (SEM magnification = 3000 x ) and 300 particles from 3.0 to 16.0 pm (SEM magnification = 300 x ). The particle counts for each size range were performed on 5 dry deposition samples collected on “ultra-smooth” electron microscope mounts as described in Butler (1988).

Figures 1 and 2 summarize the two separate distributions for the larger and smaller size classes, respectively. Un- fortunately, trying to analyze the whole size distribution (0.05 pm-16.0 pm) at once was not practical. However with a few reasonable assumptions one can merge the two size distribution graphs and calculate median diameters for par- ticle number, surface area, and mass. The two graphs of particle size distribution overlap between 3.0 and 5.0 pm. If we assume that the 3.0pm diameter particle numbers are approximately the same for Figs 1 and 2, then the relative values in Fig. 1, scaled to Fig. 2, are indicated by the hatched bars on Fig. 2. Using this merged data set and making the simplifying assumptions that the inorganic particles are spherical and of similar density, then the number median diameter is 0.4 pm, the surface area median diameter is 8.0 pm and the mass median diameter is 12.5 pm. Thus both Coe and Lindberg (1987), and my work are “grossly over- estimating” the number median diameters because of our “artificial” cutoff point of approximately 0.3 pm. In fact, Fig. 2 shows the highest percentage of particles at the lowest particle diameter measured (0.05 pm). The 0.4 pm number median value reported here appears to be an overestimate also! However, including the smaller particles has little effect

279

280 Discussions

lihtl% IO 15

Average Particle Diameter (pm1

Fig. I. Particle size frequency distribu- tion from five dry deposition samples. Three hundred particles were analyzed in a size range from 3.0 to 16.0 /cm. SEM mag-

nification equals 300 x

IO .30 .50 1.0 3.0 5.0 10.0 16.0 Average Particle Diameter Ipml

Fig. 2. Particle size frequency distribution from the same five dry deposition samples as Fig. 1. Open bars represent three hundred particles analyzed in a size range from 0.05-5.0pm. Hatched bars represent the distribution in Fig. I scaled to match the 0.05~5.0~m distribution. Also shown are the calculated overall number median diameter (NMD), the surface area me- dian diameter (SAMD), and the mass median

diameter (MMD).

on the mass median diameter. The mass median diameter reported here (12.5 pm) is similar to that reported by Coe and Lindberg (l&l I pm). These data further support the argu- ment that indeed dry deposited particle mass is dominated by the larger particles.

The issue of pre- vs post-depositional formation of CaSO, also deserves some comment. My analysis of the conversion of CaCO, to CaSO, is technically a post-depositional phe- nomenon. The CaCO, particles were “deposited” in the laboratory prior to exposure to the atmosphere. Also, they were deposited on an inert surface as opposed to an “active” leaf surface capable of leaching ions. However, I see no reason why the conversion of calcium species to CaSO, could not occur in the atmosphere prior to deposition, as long as the right ambient conditions exist (i.e. sufficient SO, concentration and high relative humidity). This conversion has been shown to occur on suspended particles in labora- tory chamber atmospheres (Liberti et al., 1978).

The presence of dew or fog may accelerate the adsorbtion of SO, and subsequent conversion of CaCO, to CaSO,, but neither are necessary for this conversion to occur. In three

cases when laboratory generated CaCO, particles were exposed to the atmosphere the samples were covered at night specificly to prevent any dew formation on the particles. Also, no fog formation was encountered during these sampling periods. Yet, in two of these three cases significant CaSO, formation occurred. In one of these cases the conversion from CaCO, to CaSO, was 6%-20% per day.

Finally, ii is unfortunate that the wording of the final sentence in my abstract is confusing. I did not mean to imply a quantified dry deposition rate. It might be clearer if the final sentence reads, “While over half of the coarse particle SOi- dry deposition is associated with calcium species, only 3% -5% of the SOi- ions in precipitation are associated with calcium”. In other words, CaSO, dry deposition is a major source ofSO:- dry deposition but CaSO, wet deposition is a minor source of SOi- wet deposition.

TOM BUTLER

Ithuca, NY 14853-2701, U.S.A.

REFERENCES

Butler T. J. (1988) Composition of particles dry deposited to an inert surface at Ithaca, New York. Atmospheric Enriron-

ment 22, 895-900. Coe J. M. and Lindberg S. E. (1987) The morphology and size

distribution of atmospheric particles deposited on foliage and inert surfaces. J. Air Pollut. Control Ass. 37, 237-243.

Davidson C. I., Lindberg S. E., Schmidt J. A., Cartwright L. C. and Landis L. L. (1985) Dry deposition of sulfate onto surrogate surfaces. J. geophys. Em). 90, 2123-2130.

Liberti A., Brocco D. and Possanzini A. (1978) Absorbtion and oxidation of sulfur dioxide on particles. Atmospheric Emkonment 12, 255-261.

Lindberg S. E., Lovett G. M., Richter D. D. and Johnson D. W. (1986) Atmospheric deposition and canopy interactions of major ions in a forest. Sciencr 231, 141-145.

A CLASSIFICATION OF NO OXIDATION RATES IN POWER PLANT PLUMES

BASED ON ATMOSPHERIC CONDITIONS*

Janssen et al. (1988) have published a very useful classili- cation of NO oxidation rates based on a large data base of NO and NO, concentration measurements in the plumes of power plants under widely varying atmospheric conditions. They show that the average NO,/NO, ratio in a plume can be represented by the following expression

NO,/NO,=A(l -em=). (I)

Tabulated values of A and I for various values of 0, background concentration and wind speed at plume height are provided for the winter, spring/autumn and summer seasons. As an example of the application of this classili- cation, they have used Equation (1) in a Gaussian dispersion model in order to calculate NO, concentrations at ground- level.

Some years ago, I co-ordinated a field experiment to determine the effect of plume chemistry on the maximum concentration of SO, at ground-level, however the oxidation rate of SO, is slow enough that it has an insignificant effect

* Janssen L. H. J. M., van Wakeren J. H. A., van Duuren H. and Elshout A. J. (1988) Atmospheric Environmenr 22,43-53.