report on a discussion meeting: symposium on visibility and air pollution

Atmospheric Environment Pergamon Press 1971. Vol. 5, pp. 155-163. Printed in Great Britain.

REPORT ON A DISCUSSION MEETING

Symposium on: Visibility and Air Pollution

1. Introduction 2. Opening remarks (by D. CLARK, Chief Planning Engineer, C.E.G.B.). 3. Discussion Contributions.

3.1 W. BAILEY and J. GRANT. Measurements of the oxidation of SOz in chimney and plume at Marchwood power station.

3.2 P. M. FOSTER. SO* oxidation in chimney smoke plumes. 3.3 A. E. J. EGGLE-IQN. Comments on various papers. 3.4 E. N. LA~IUXNCE, T. V. IIEALY, M. C. SUBBA RAMU and A. E. J. EGGLETON. Discussion of

Eggleton’s paper. 3.5 M. C. SUBBA RAMU and J. A. GARLAND. Discussion of Garland’s paper.

4. References. 5. English translation of abridged version of paper by 01. JUNGE. 6. H. HORVA~. On the applicability of the Koschmieder visibility formula.

1. INTRODUCTION

A SYMPOSIUM on visibility and air pollution was held on 6 October, 1969, at Sudbury House, the London Headquarters of the Central Electricity Generating Board, who kindly provided the facilities for the meeting.

One of the more spectacular effects of Clean Air Legislation has been the improvement in visibility over the last decade or so in cities like London. The elimination of much of the old soot and dust from the atmosphere has focussed attention on the remaining particulate matter. This enhanced interest in visibility and the nature of the particles responsible for atmospheric opacity has resulted in the proportion of papers submitted to the Journal on these topics rising to about 30 per cent.

The papers under discussion were a selection of those papers which had appeared in this Journal in recent months.

The meeting began with a session devoted to the effects of visible material found either before or soon after emission from the stack on the appearance of the smoke plumes. The second session dealt with the relation between particle distribution and visibility while the final session was concerned primarily with the nature and origin of the obscuring particles.

In addition to the British contingent, authors present to introduce their papers included Dr. JAENICKE who represented Professor JUNGE from Mainz, Dr. HORVATH from Vienna, Dr. CHARLSON from the University of Washington, while Dr. LODGE from the National Center for Atmospheric Research at Boulder took the chair for the tinal session.

The full programme appears on pp. 499-500 of Volume 3 (1969) of the Journal and the papers are listed in the references at the end of the discussion section.

This account of the meeting includes, in addition to the discussion contributions, an abridged translation into English of Professor JUNGE’S paper and a short paper by Dr. HORVATH based on his contribution to the meeting.

2. OPENING REMARKS BY D. CLARK

Mr. CLARK began by welcoming the delegates, especially the visitors from overseas. His credentials for accepting the invitation to open the Symposium were that one way and another

he had been associated with the planning of over half of the electricity generation plant existing in Britain to-day. This plant is now burning over 40 million tons of coal and oil a year. It is at the planning state that the environmental impact of a new power station is largely determined. Therefore he had a long-standing, highly practical interest in problems of the atmospheric environment.

He was glad to see that the organizers had arranged the meteorological conditions for the morning to demonstrate to the delegates that a London fog is now a pale shadow of its former self-thanks to the reduction in particulate air pollution.

Public interest in environmental protection is now higher than ever before. In the U.S. particularly there has been a great upsurge of interest and activity with many people climbing on the bandwagon. In Britain differences of circumstances and temperament led to concern for the environment at a much earlier time and consequently the scope for a new revolution is that much less.

155

156 Report on a Discussion Meeting

The post which is now Chief Alkali Inspector was first created in 1863. He exerts nationwide control of emissions from large installations and is armed with remarkably wide powers. These are used to get on with the job with a minimum of red tape and fuss, in sharp contrast to the procedure for some other consents and licences needed for new power stations. To the developer, trying to strike the right compromise between efficiency and economy and environmental protection, a clear and timely decision is a pearl beyond price.

It is perhaps appropriate to use the lecture theatre at C.E.G.B. for the meeting since the Electricity Supply Industry has long been concerned with the effects of new projects on the atmospheric environment. It is part of their public duty and is now also written into statutes.

N~ly4Oy~ ago, itwas decided that two newLondon power stations shouldhavefluegas washing plant. Although this process is now seen to have disadvan~~, the fact that such measures were taken so long ago illustrates the seriousness with which the problem was viewed even at that time.

More recently the Supply Industry have taken a lead in (a) Specifying high efficiency dust collectors (b) Doing research into practical achievement of high efficiency (c) Specifying high chimneys (d) Doing research to test their actual effectiveness.

For the past 9 years all new coal and oil fired power stations have been designed to discharge their entire flue gases through one high chimney+zoncentrating all the heat into one plume, yielding maximum buoyancy rise. The largest coal bred station, Drax, with a final capacity of 4000 MW, will have a 850 ft stack.

One reason for the attention paid to emissions is the high proportion of coal in the fuel economy of this country. In 1966, the percentages of electricity generated from coal were

U.K. 75%, U.S.A. less than 55%,

E.E.C. less than 45%.

This situation is changing with the growth of nuclear power and natural gas from the North See. These sources are now meeting the whole growth of national energy consumption. Oil consumption is rising roughly as fast as coal consumption is falling and so environmental air problems are still with us.

The dramatic reductions in smoke, soot and dust emissions have focussed attention on other factors affecting the clarity of the atmosphere, the subject of this Symposium. How wide the interest is, is shown by the inte~ational character of the authorship of papers and the number of overseas partici- pants.

Mr. Clark concluded by wishing those present an interesting day and an enjoyable visit.

3. DISCUSSION CONTRIBUTIONS

3.1 Measurements of the oxidation of SO2 in chimney andplume at Marchwood Power Station

W. BAILEY and J. GRANT (C.E.G.B. S.W. Region Scientific Services Department, Portishead, Bristol).

FURTHEX toF~T~‘s(i969)commentsonoil fired plumes it would perhaps be of interest if we described some experiments in which the plume of Marchwood Power Station was sampIed using apparatus slung sui&iently far beneath a helicopter that there was no visible effect on the plume. The apparatus was located in the phnne by measnring the e.m.f. developed by a fine thermocouple mounted near the sample inlet.

The experiments were carried out at a time when we later learned that condensation of the sutphuric acid had been starting about half way up the stack.

Some idea of the nature of the solids in the plume may be gleaned from a microphotograph of a slide from the first stage of the cascade impactor. A quantity of colourless solid is quite possibly a residue from the injection of dolomite, which was stopped only an hour before sampling. The number and variety of the coloured particles present possibly indicate the presence of a number of transition element ions which may be active catalysts.

Similar slides were exposed close to the stack top and the sulphate on them estimated. It was found that 60-75 % of the sulphate ion was associated with particle sixes greater than l-8 pm (i.e. the sulphate ion on the first three stages plus one haff of that found on the fourth stage of the cascade impactor amounted to 6(t75 % of the total sulphate found). Since condensation occurred within the stack this is

Report on a Discussion Meeting 157

not surprising. The volume (V) of flue gas that the sample originated in was estimated from the area under the graph of thermocoupIe e.m.f. vs. time;

where 0, = Excess temperature of flue gas at stack exit over ambient temperature. T = Absolute temperature of the ambient air, fi = the constant rate of sampling,

E = the instantaneous tbermo-couple e.m.f. k = the constant relating E and amoral.

We found that the total sulphate ion found in the undiluted flue gas was between 3 and 12 times greater than that calculated from the SO, measured at the 20 m level of the stack. The solid burden which was separately measured in the plume was quite insufficient in any way to account for this.

Additionally, and more directly, measurements were made of the ratio of total sulphate ion to sulphur dioxide at various places in the plume. The sample gas was heated to a temperature in excess of its water dew point and then filtered to separate the sulphuric acid droplets, The SOz in the filtered gas was then scrubbed with 50 ml of ~/lo Iodine and any iodine carried over trapped in 50 ml of ~/lo Sodium Thiosulphate. After sampling, the solutions were mixed and the SO1 determined. The ratios found are given in TABLE 1.

TABLE 1. RATIO OF SO, : SO, (BY ~EKXIT~ IN THE PLUME POR X%E EA!%T STACIC* AT MARCEIWWD POWER STATION

Date

Time

SO,: SOz (at stack outlet)

1 Q/7/65 19/10/65 27/10/65

15.00 15.30 12.30 13.00 14.45 15.50 12.30 14.30

I:54 1:23 1:26 1:64

SOa: SO2 (1 stack length down wind from stack outlet) 1:31 1:26 1:26

so,: sot (3 stack lengths down wind from stack outlet) 1:13

*The stack is approximately 100 m high

The ratio of SOS: SO2 found at the 20 m level of the stack was about 1: 500. The magnitude of the measured ratio confirms the extensive oxidation within the stack inferred from the previous measurements.

The variation of the ratio SOS : SOz down the length of the plume is somewhat variable as doubtless some relevant parameters e.g. wind-speed, mean ornate etc. varied during the experiments. Nevertheless it appears that there is a systematic increase in the relative quantity of sulphate ion as one progresses down the plume.

To conclude, in the unusual conditions at Marchwood at the time two quite separate and different experiments agree in the order of magnitude of the extensive oxidation occurring within the stack. It seems therefore that catalysts for this reaction must be present.

Oxidation in the plume also occurred, although the extent may not be typical as the condensate particles will have coagulated to an unusually large size in the chimney. We have not considered whether the special circumstances would have slowed or accelerated the reaction.

Acknowledgements-We are grateful for the help of our colleagues at Portishead and at Marchwood Power Station. The help with design and the precise flying of Mr. C. Hosnoooo and Mr. P. GRIQ~~N of theS.W.E.B. Helicopter Aight made the m~su~ments possibie.This note is presented by permission of Mr. R. H. COATES, Director, S.W. Region, C.E.G.B.


3.2 Sulphur Dioxide Oxidation in Chimney Smoke Plumes Reply by P. M. FOSTER (C.E.R.L.).

With reference to Mr. BAILEY and Mr. GRANT'S measurements of sulphur dioxide oxidation observed both within the stack and the plume of an oil fired power station. Oxidation cannot occur within the stack by a solution catalysed process since the effluent temperature is above its water dewpoint; the most probable explanation therefore is a gas/solid phase reaction. Nitrogen oxides are known to catalyse reactions of this type and are present in the effluent in small quantities (i.e. 200-300 ppm, D. J. FOS~R, private communication). They might therefore play some part in inducing sulphur dioxide oxidation in effluent above its water dewpoint. Oxidation within an oil fired power station plume by a solution oxidation mechanism was only discussed briefly in my paper. We may have concluded wrongly that oxidation would be small in these cases since the effluent taken as a whole contains excess acid. It is quite likely, however, that this acid exists mainly in an aerosol form apart from the solid burden of the effluent (JARMAN and DE TURVILLE, 1969) so that the latter is not neces- sarily acidic. This is supported by Mr Bailey’s photographs of dust samples. Under these conditions suitably catalytic particles would then promote solution oxidation of sulphur dioxide in a humid plume.

With reference to Dr NONHEBEL’S comments on the quantities of sulphur dioxide oxidation dealt with in my paper I would like to re-emphasize the point that the T.V.A. plume on which my calculations were based was exceptionally dusty and that for a more typical coal fired station plume we should reduce our estimated rates by a factor of 0.25. Furthermore I would add that there exists some doubt as to the accuracy of the measurements of plume oxidation made by the T.V.A. on account of their sampling technique. This employed a millipore filter at the beginning of the sampler which separated acid mist and particulate matter from the effluent gas. Oxidation was then estimated by analysing the filter contents for sulphate. Since oxidation of sulphur dioxide will occur within the filter during the sampling period this procecure leads to overestimates for plume oxidation.

T.V.A. suggest that this effect should be independent of distance from the plant and that since oxidation increased with plume travel they believe that filter induced oxidation was not significant. They suggest, however, that further studies might usefully be made on this effect in order to get a definite answer to this comment.

Bailey and Grant prevented solution oxidation occurring within their filter by preheating the effluent above its water dewpoint before filtering. However, this would not prevent oxidation occurring by a gas/solid phase reaction and consequently it is possible that their measurements also over- estimate oxidation.

P. M. FOSTER

3.3 Comments on the papers by NOLL et al., CHARLSON et al. and the contribution by HORVATH* by A. E. J. EGGLETON (partly communicated).

Drs. CHARLSON, HORVATH and other authors have now clearly shown that there is good agreement between the measured light scattering coefficient and the value calculated from the meteorological range using the Koschmieder visibility theory when the meteorological range is correctly estimntedin a homogeneous atmosphere using the right type of target under the light illumination conditions. Differences between the two values are nearly always due to difficulties in estimating the meteorological range because the conditions are not right. Has not the time come to implement the suggestion, originally made by MIDDLETON (1952), that visibility should be reported in terms of extinction coefficient rather than how far the eye can see and that the user should convert this to visual range for the particular conditions of interest to him?

I should also like to suggest that a new term be coined called “specific scattering area” and given by the relation

b a=-

c m2g-’

where a = specific scattering area b = extinction coeff. (due to scattering) (m-l) c = mass concentration of scattering material (g rne3)

It is related to the factor K given in the paper by NOLL, MUELLER and IMADA (1969) by the equation

3.91 a=- K

The specific scattering area measures the scattering effectiveness of unit mass of material and has the advantage of providing a convenient means of comparison between the scattering efficiency of aerosols of different materials at different times and places. Some interesting relationships should become

*This paper appears at the end of this discussion report.


apparent between specific scattering area and the analogous specitic surface area of a particulate material. Furthermore the scattering coefficient of a mixture of materials may be easily obtained by summing the product of the specific scattering area and concentration for each of the component substances.

Values of ‘a’ for the atmospheric aerosol at low relative humidity calculated from the paper by NOLL et al. (1968) lie between 3.9 and 2.1 mzg-’ while CHARLSON et al. (1968) found a modal value of 3.3. In contrast the calculations of GARLAND (1969) for atmospheric ammonium sulphate particles give values between about 10 at 70% r.h. to 70 at 995 % r.h.

A. E. J. EGGLETON

3.4 Discussion ofpaper by EGGLE-~ON

Mr. E. N. LAWRENCE (Met. O&e, Bracknell) (written communication). Dr. EGGLETON, in his concluding written remarks* expressed the intention to remedy many of the imperfections in procedure, and in the subsequent discussion, he mentioned the need for more detailed information on both visibility and aerosol concentrations.

In his calculation of the correlation coefficients between visibility and aerosol concentration, the visibility referred to the ‘daytime” period of 0900 hours to 1900 hours while the concentrations referred to 24-h periods: that is, the period of the mean concentration included the most polluted part of the day while visibility referred, in general, to the clearer part of the day. As the correlation coefficients between 24-h mean concentrations and the 10-h mean concentrations may be significantly different from unity, the quoted correlation coefficients are difhcult to interpret. In any further investigations, the variables should refer to the same part of the day throughout.

The meaning of the correlation coefficient between visibility and sodium concentration is particularly difficult to assess. The author presumed that the sodium was present as sodium chloride and derived from the sea: but whilst this presumption may possibly be supported by the geographical distribution of sodium illustrated in FIG. 10, the latter evidence is hardly conclusive, especially in the absence of data relating sodium to wind speed and direction. Further understanding of the origin and possible effects on visibility of sodium might be obtained by calculating the concentrations for various ranges of wind speed and repeating the calculation of correlation coefficients for each range of wind speed, then reasoning as follows :

If sodium emanated from local sources only, its concentration would correlate negatively with wind speed and hence probably positively with reciprocal visibility, but concentrations of sodium from only marine sources may well correlate positively with wind speed and so negatively with reciprocal visibility. The combination of such opposite effects could result in a low overall correlation coetiicient between visibility and sodium concentration: but coefficients might be quite signCant if calculated separately for low wind speeds and for high wind speeds, though such results in isolation would not indicate any direct causal relationship.

One general comment on the symposium is that no attention was given to very low concentrations of air pollution which are strong enough to cause offensive smells and which are associated with poor visibility, presumably because both events occur with stable atmospheric conditions. The public seemingly consider bad smells more obnoxious than either poor visibility or higher levels of less odorous pollutants, judging from press reports of the so-called “Stockton stench”, as for example, that reported in the second week of June 1963 when northeasterly winds (from the Tees-side area) brought both the smell and poor visibility (with a relative humidity which was particularly low for such visibility, in summer).

E. N. LAWRENCE

Following on the results obtained at Tees-side for atmospheric ammonium concentrations and described by ECGLETON (1969), we have measured both ammonia and ammonium concentration on hourly atmospheric samples, obtaining thousands of results at Harwell, which is a rural area. These show that although free ammonia concentrations do occasionally rise above 5 pg m - 3 they are usually of the order of 0.5-2 pg rnm3. Typical results, obtained at hourly intervals over a48-h sampling period. Ammonium peaks, often occur at night, mostly under inversion conditions. In some instances we have recorded peaks of over 20 pg m- j. The visibility correlates well with the ammonium level, except at the latter part of the run where the relative humidity was over 90 %. EGGLETON has already found a correlation with his 24 h samples on Tees-side, and our results show that this holds quite strikingly hour by hour even in a rural area.

Table 2 shows some of our figures (HEALEY and PILBREAM, to be published) for K values (K being the

*EGGLETON (1969)

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specitic scattering area) and 6,,, values (bscac being the light scattering coefficient) and these are compared with those of previous speakers.

The b,, values for smoke are low (CHARLESON et al., 1968; NOLL et al., 1968), and smoke effects are usually less important on visibility: this is certainly true in a rural area such as Harwell. GARLAND’S theoretical calculations (1969) on the effects of ammonium sulphate on visibility are different from our practical results by a factor of 2-3 at lower humidities but about 10 at higher humidities. Perhaps we should consider this fairly good agreement considering the complex factors involved.

T. V. HEALY

THIS is a very interesting paper which throws a considerable amount of light on the dependence of visibility in the Tees-side area. The phenomenon of haze formation is not fully understood and the studies are of current interest. In this connection I would like to make the following comment on this paper.

The study of the variation (FIG. 9) of visibility and relative humidity with time on 12 July, 1969, shows that when relative humidity was 90 per cent or below the visibility improved at 1200 and 1800 hours. But at 90 per cent r.h. from 1300 to 1800 hours, the visibility decreased to a minimum of 400 m. The decrease in the visibility is attributed to an increase of ammonium sulphate concentration and was assumed that the concentration was negligible after the poor visibility period even though the concentrations of ammonium and sulphate ions recorded on this day were the highest. This suggests the possibility of another agency, natural or artificial, also being responsible for the phenomenon of haze formation.

I may mention here the studies carried out by my colleagues and I at the Bhabha Atomic Research Centre, Bombay, India, on the role of ionization in the formation of condensation nuclei in the atmospheric air (VOHRA, SUBBA RAW and VASUDEVAN, 1968). Experiments were conducted to find the effect of ammonia on the formation of condensation nuclei using radon daughters as tracers. These experiments suggest the formation of a substance with an acidic radical, perhaps sulphate, and its subsequent reaction with trace quantities of ammonia to form condensation nuclei. That ionizing radiation results in a higher rate of formation of condensation nuclei has been demonstrated by using an alpha source of 20 prad h-l, which is about twice the natural background radiation. Radiolytic oxidation of SO2 giving rise to H$O., in the presence of water cannot be ruled out.

It is well known that HzS04 has a high al%& for water vapour. A molecule of H2SOb with 150 molecules of Hz0 is known to exist in solution (MELU)R, 1930). The fact that the observed visibility is always less than the calculated visibility (Table 3) supports the above suggestion.

Correlation of the radon content of the air and the visibility at Tees-side valley, together with the observation of the ammonium and the sulphate contents of the air may perhaps throw further light on the problem. I would be very glad to know the author’s opinion of this suggestion.

M. C. SUBBA RAMU

In reply to Mr. SUBBA RAMU, it should be remembered that our samples were taken on a 24 h basis and only an average value for the 9 h period of the mist of July 12th, 1967 could be calculated. Discussion of differences between observed and calculated visibility during individual hours of that period is therefore fruitless. The overall discrepancy is most likely to be accounted for by the presence of other electrolytes which were present to the extent of some 20 per cent of the ammonium sulphate on a chemical equivalent basis.

Several authors have described the formation of condensation nuclei, probably consisting of sulphuric acid or ammonium sulphate, by radiolysis of air containing trace quantities of impurities. However the mass of material formed is small compared with the amounts discussed in my paper and the particle size, at least when first formed, is too small to have a significant light-scattering effect. I should expect a positive correlation between the radon content of the air, ammonium sulphate concentration, and reciprocal visibility on Tees-side but this would be due to association of these variables with poor atmospheric mixing, not with radiolytic processes.

I accept the criticism of Mr. E. N. LAWRENCE regarding the correlations between visibility and concentration in my paper being based on 10 h and 24 h periods respectively. This is certainly unsatis- factory and in future work we intend to base comparisons on individual hourly data (see below). Unfortunately very few observing stations report visibility throughout the whole of the 24 h period so that comparison with the technically convenient 24 h atmospheric sample is bound to be difficult. In the event I think the correlation with the visibility during the daylight period should be better than that for the whole 24 h since visibility during hours of darkness is affected more by the radiation balanceleading to high values of relative humidity than during daylight when the aerosol concentration is more important.


Our future results will certainly be examined in the manner suggested by Mr. LAWRENCE for correlations between wind speed and the concentration of sodium chloride (and other substances). However the effects of different electrolytes on visibility will be broadly proportional to their concentration in chemical equivalents and examination of FIG. 2 of my paper shows the equivalent concentration of sodium chloride was only about 10-20’~ of the ammonium sulphate present and its contribution to the scattering coefficient is likely to be similar.

In connection with the comments of Mr. T. V. HEALY and Dr. H. A. C. MCKAY I should say that. as suggested in the last paragraph of my paper, we are now carrying out a more intensive investigation at three stations on Tees-side and one at Harwell on the basis of automatic hourly sampling for particulate matter (analysed among other things for ammonium and sulphate ion), sulphur dioxide and ammonia in association with automatic recording of the relevant meteorological parameters including extinction coefficient. When we have gathered sufficient information on Tees-side it is our intention to move this equipment to other places in the U.K. in order to investigate both the formation of ammonium sulphate and the extent to which it is an important contributor, as now appears likely, to the extinction coefficient elsewhere.

We have already analysed a number of hourly samples from our station at Stockton and by separa- ting the results into narrow relative humidity classes (2 %) we find very high correlation coefficients, up to 0.99, between ammonium ion concentration and extinction coefficients for relative humidities between 80% and 97% compared with the values of 0.85 and 0.61 for 24 h samples reported in the paper. For each relative humidity class at least 70 per cent of the extinction coefficient can be accounted for by the ammonium sulphate present on the basis of the calculations given in the paper by GARLAND (1969). These results will be published later.

A. E. J. EGGLETON

3.2 Discussion of paper by GARLAND

The disagreement between the curves (FIG. 4) for the extinction coefficient of ammonium sulphate haze against relative humidity, for size distribution of the Junge type (as illustrated in FIG. 3), and monodisperse aerosol at the mass median diameter, 042 pm, of the Junge distribution suggests that the formation of ammonium sulphate haze in the atmosphere may not be due to the condensation of water vapour directly on ammonium sulphate particles. It may be possible that ammonia reacts with a sulphate compound (say HZS04) that has greater affinity for water, resulting in the formation of ammonium sulphate haze. This is supported by the fact that the extinction coefficients for the two distributions studied are always less than that for the monodisperse ammonium sulphate aerosol at the same mass median diameters corresponding to the distributions studied.

It is, of course, true that the disagreement in the case of the curves for the size distributions corresponding to the observations made by Heard and Wiffen is less. Here it is important to know whether there was any contamination (NH, or SOZ) of the sampling air since the solubility of (NH4)ZS04 in water depends upon its relative concentration with H,SO,.

TABLE 3. (SEIDELL, 1940) Equilibrium in the system (NH&SO.+, H,SO, and HZ0

at 30°C.

G per 100 g of saturated solution

HzSO4 (NH&SO,

0.0 44.03 13.11 44,71 33.88 4544 59.27 17.62 61.50 38.50

TABLE 3 shows that condensation of water vapour takes place efficiently on (NH&SO, when the concentration of ammonium sulphate is more than that of any other sulphate compound (say H2S04) having greater r&in& for water. In this connection it is necessary to know the meteorological para-


meters and the environmental conditions at the sampling location in the case of the measurements made by Heard and Wiffen.

London School of Hygiene and Tropical Medicine, Keppel Street, London. w.c.1

M. C. SUBBA RAMU*

Reply to Mr. Subba Ramu’s comments by J. A. Garland

Mr. Subba Ramu offers an ingenious explanation of the differences between the curves of FIG. 4, which compares the extinction coefficients of two atmospheric size distributions with those of monodisperse aerosols at their mass median diameters. His explanation is based on the misconception that the curves for the hazes are experimental results. I wish to stress that these curves show the results of calculations only, made by taking weighted averages of the curves in Fm. 2 over the range of sizes in each haze. In fact the extinction coefficient of the monodisperse aerosol is greater in each case because of its proximity to the chief maximum of the curves in Fro. 2, while the haze distributions contain particles whose masses fall far away from that maximum. The difference is greater in the case of the Junge distribution simply because this distribution spans a wider range of particle size than that observed by Heard and Wiffen.

Mr. Subba Ramu shows that the presence of sulphuric acid with ammonium sulphate in the same droplet would result in suppressed solubility of ammonium sulphate, and implies that the extinction coefficient would be reduced. In such a situation the growth of the droplet would probably be determined largely by the sulphuric acid present. As this is the more hygroscopic material, the droplet size, and hence the extinction coefficient, would presumably be greater than if all the acid were neutral- ized with ammonia.

J. A. GARLAND

A full exposition of Mr. I-Binel’s contribution to the discussion appeared in the following publica- tion: K. BULLRICH, R. EIDEN, G. ESCHELBACH, K. FISCHER, G. I%NEL, K. HEGER, H. SCHOLLMAYER and G. STEINHORST (1969). Research on atmospheric optical radiation transmission. Scientific Report No. 7, Air Force Cambridge Research Laboratories, Bedford Mass., Contract F 61052 67 C 0046.

REFERENCES

CHARL~~N R. J., AHLQU~ST N. C. and HORVA~TI H. (1968) On the generality of correlation of atmos - pheric aerosol mass concentration and light scatter. Atmos. Environ. 2,455-&I.

EGGLETON A. E. J. (1969) The chemical composition of atmospheric aerosols on Tees-side and its relation to visibility. Atmos. Environ. 3, 355-372.

FOSTER P. (1969) The oxidation of sulphur dioxide in power station plumes. Atmos. Environ. 3, 157-176.

GARLAND, J. A. (1969) Condensation on ammonium sulphate particles and its effect on visibility. Atmos. Environ. 3, 347-354.

HEALY T. V. and PILBEAM A., U.K.A.E.A. Report AERE-R 6232 (to be published). JARMAN, R. T. and DE TURVILLE. C. M. (1969)Visibility and length of chimneyplumes. Atmos. Environ.

3,257-280. JUNGE C. and SCHEICH G. (1969) Studies on the determination of the acid content of aerosol particles.

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* On leave from Bhabha Atomic Research Centre, Trombay, Bombay, India.

report on a discussion meeting: symposium on visibility and air pollution

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