Atmospheric Enoironmenr Vol. 13, pp. 1339- 1340 Pergamon Press Ltd 1978. PrInted in Great Britain.

A STUDY OF ATMOSPHERIC MERCURY DISPERSION*

With respect to Methods of Sampling and Analysis (Section 2) we have the following comments.

(1) It iscurious that the authors selected distinctly different analytical techniques for their emission measurements and their ambient air measurements.

In the case of the emission measurements, air samples were drawn through an impinger bottle containing sulphuric acid- potassium permanganate solution the mercury content of which was then determined by flameless atomic absorption spectroscopy. This procedure not only measures the elemen- tal mercury vapour present in the sample, but also measures vapour-phase inorganic mercury compounds and mercury adsorbed onto airborne particulate matter. Hence, this procedure gives, in an operational sense, a value for total inorganic mercury in the air sampled. Organo-mercury com- pounds such as dimethylmercury and diethylmercury (un- likely to be present in significant concentrations in chlor- alkali plant emissions) are not absorbed quantitatively in H,SO,-KMn04 solution (National Academy of Sciences, 1977: Friberg and Vostal. 1972).

On the other hand, in the case of ambient air measurements the method used in this study is said to be similar to that developed by Corte et al. (1973). Specifically, it is based on the adsorption of elemenral mercury rapour on silver followed by thermal desorption and measurement of the liberated mer- cury by flameless AAS.

Thus, Hogstrom et al., as a consequence of the analytical procedure chosen, have obtained total inorganic mercury concentrations during their emission measurements but have obtained elemental mercury vapour concentrations during their ambient air measurements. Because chlorine is often present around chlor-alkali plant production units and in their stack gases at concentrations up to several ppm, and because this Cl, can react with elemental mercury vapour (presumably to produce mercury chlorides, e.g. Hg,CI,, HgCI,) thereby reducing the free elemental mercury vapour concentration in the air sampled, the analytical values of total inorganic mercury concentration may be significantly different from elemental mercury vapour concentration in these environments. Whereas difficulties may be encountered in using the H,SO,-KMnO, absorption train for ambient air measurements of mercury, flameless AAS in conjunction with the silver amalgamation technique has been applied successfully to ambient air measurements (Long et al., 1973; Spittler, 1973) as well as chlor-alkali plant emission measure- ments (Dowd and Hilborn, 1977). Application of the latter method to studies such as the one described here would thus allow a consistent analytical approach to both stack emission and ambient air measurements.

(2) The authors stated that “With the sample air volumes used (approx 0.05 m’) all mercury except the dimethylized form is absorbed by the silver nets”. This statement is not only unsubstantiated by the paper of Corte et al. (1973), whose method was devised for measuring elemental mercury vapour in air, but is also contrary to the findings of other in- vestigators (Braman and Johnson, 1974; Henriques and

* Hogstrom U., Enger L. and Svedung I. (1979) Atmos- pheric Environment 13, 465-476.

DISCUSSION

Isberg, 1975). The scientific evidence available to date indicates that silver effectively removes only elemental mer- cury vapour from air, and is ineffective in removing any organic as well as inorganic mercury compounds.

(3) Little detail is provided on the actual experimental procedures and conditions during sampling and analysis for mercury, neither for the emission measurements nor the ambient air measurements. The authors did point out that known volumes of air were sampled, but fail to mention other relevant experimental parameters such as sampling flow-rate, the mass/weight and temperature of the silver nets (the so- called Hg traps), the concentration of the H,SO,-KMnO, absorbing solution, etc. While it is recognized that stipulation of the total volume of air sample is important in connection with saturation and breakthrough of the mercury from the sorbent medium, the other factors mentioned above are no less important and should be stated explicitly to allow a valid assessment of the analytical procedures by the reader.

(4) Previous experience by several groups (e.g. Galloway and Likens, 1976) have shown that a precipitation sample must not contain dry deposition ifaccurate information on its chemical content is required. Since the authors’, monthly, wet-deposition collector was an open funnel-and-bottle ar- rangement, some comment on contamination between pre- cipitation events is warranted.

In concluding we would like to point out that important information about the physical characteristics of the chlor- alkali plant and its surroundings was not reported. For instance:

(a) The type of surface onto which mercury was deposited was not mentioned. Such details make the observations and results more meaningful and allow the reader to decide whether the conclusions reached are applicable to his area of interest.

(b) The topography and surface roughness of the study area (a circle _ 5 km in radius) was not discussed even though the boundary layer models used in the paper employ wind profile relationships that are valid only for flat, homogeneous terrain. Does the surrounding landscape indeed conform to the limitations of the model?

(c) The height and shape of the mercury source was not mentioned. In any dispersion estimate this should be the first piece of information to be given.

Atmospheric Environment Service, W. H. SCHROEDER

4905 Dufferin Street, L. A. BARRIE

Downsview, Ontario M3H 5T4, Canada

REFERENCES

Braman R. S. and Johnson D. L. (1974) Selective absorption tubes and emission technique for determination of ambient forms of mercury in air. Enuir. Sci. Tech&. 8, 996-1003.

Corte G., Dubois L. and Monkman J. L. (1973) A reference method for mercury in air. Sci. Tot. Enoir. 2, 89-96.

Dowd G. F. and Hilborn J. C. (1977) A general instrumental method for the measurement of mercury vapor in work- room, stack and adjacent atmospheres. Presented at 174th Natl. Meeting, American Chemical Society, Chicago, Illinois.

A.1. 1319% ” 1339

I340 Discussion

Friberg L. and Vostal J. (Eds.) (1972) Mercury in the Enrironmrnt, Ch. 2. CRC Press, Cleveland, Ohio.

Galloway J. N. and Likens G. E. (1976) Calibration of collection procedures for the determination of precipi- tation chemistry. Wut. 4ir Soil Pollur. 6, 241 258.

Henriques A. and Isberg J. (iY75) A new method for collection and separation of metallic mercury and organo- mercury compounds in air. Chrmiccr Scripru 8, l73-~ 176.

Long S. J., Scott D. R. and Thompson R. J. (1973) Atomic absorption determination of elemental mercury collected from ambient air on silver wool. .4nulrr. C’hen~. 45, 2227 2233.

National Academy of Sciences (1977) An assessment of mercury in the environment. Washington, D.C.

Spittler T. M. (1973) A system for collection and measure- ment of elemental and total mercury in ambient air over a concentration range of 0.004 to 25 pg m-‘. Presented at 165th Natl. Meeting, American Chemical Society. Dallas, Texas.

AUTHOR’S REPLY

Regarding thecomments made by W. H. Schroeder and L. A. Barrie we wish to reply as follows:

The choice of measurement methods for determining the atmospheric mercury concentration was based on the fact that the concentration is about loo0 times higher in the air leaving the chlor-alkali plant than in the ambient air. To carry out emission measurements as well as ambient air measure- ments with one of the two methods is not possible without extensive modifications.

In our report we state that the method developed by Corte er al. using silver as an active sorbent of mercury, at the sampling volume given binds both elementary mercury and the main part of the oxidised mercury present in air. This has been verified by parallel tests carried out by us using gold and silver sorbent materials in the samplers, as well as by personal communication with both G. Corte and R. S. Braman. However, this is valid on the condition that the air sample volume is less than 0.1 m3. In our measurements the air sample volume was approx 0.05 m3.

The mercury fraction which occurs bound to particles in the air was shown by R. S. Braman to be of minor importance compared to other forms of atmospheric mercury.

The two measurement methods used in this context might therefore be regarded as equivalent insofar as both largely indicate the total mercury concentration of the air.

The comments concerning the description of procedure in

the execution of the measurements may be justttied ‘Ihc methods used can be regarded as standard methods, however. and this part of the presentation was therefore deliberately made very brief.

The precipitation sampling is purported to be atiected by possible dry deposition. The investigation comprised both total deposition measurements and measurements in which total deposition was divided into deposition during periods of rain and deposition during dry weather. These measurement\ were carried out with automatic samplers controlled by so called rain sensors. The results of these measurement\ indicated that more than 90”,, of the mercury content of the total sample collected derived from periods of rain.

The particle content of the samples collected was not likely to have notably reduced the mercury content of the samples. as these were acid oxidised throughout the sampling period through the addition of preserving solution.

Drs Schroeder and Barrie are right in pointing out that details should have been given about the topography and surface roughness of the study area. They are given here. The area is a mixture of flat farming land and forested hills (a few tens of meters high) plus scattered houses and gardens. This type of surface is, of course, not homogeneous. a fact which is recognized in section 6. Nevertheless we are fairly contident that the numerical model gives a realistic result in the ,sruti.sriru/ sense it is used in the article. As mentioned in the paper (section 4A) the overall roughness length bar been estimated to ca. 0.6 m.

The height and shape of the mercury source should also have been stated more distinctly. The information I‘. now scattered over the article. In section 2A: “The two plants concerned in this study both have a vent in the ceiling of the production unit, and ventilation is affected through the self- draught resultingfrom the heatingoftheair by thecell$ in this unit. The source contribution. i.e. the amount of mercury emitted via the vents per unit time.. .“. In section 4B: “Our source is a line source with a length of 75 m”. In Section 6 the likely behaviour of the effluent i\ discussed: “We have assumed mixing over the total building height.. .“. The height of the vent above the ground which is 16 m should. ofcourse. have been stated explicitly.

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