Atmospheric Environment, Pergamon Press 1968. Vol. 2, pp. 423-426. Printed in Great Britain.

REPORT OF MEETING

METEOROLOGICAL ASPECTS OF ATMOSPHERIC POLLUTION

AND ATMOSPHERIC CHEMISTRY

(Received 23 January 1968)

IN THE week of 3 June 1967 the Working Group on Atmospheric Pollution and Atmospheric Chemistry, established by the Commission for Atmospheric Sciences (formerly Commission for Aerology) of the World Meteorological Organization met for the tirst time in Geneva.

The following persons act as members of the W.G.:

F. H. Schmidt (Chairman), Netherlands E. Eriksson, Sweden A. G. Forsdyke, England R. E. Munn, Canada Mrs. E. S. Selezneva, U.S.S.R.

They were assisted during the meeting by Mr. G. W. Kronebach and Mr. G. Tarakanov, both of the W.M.O.-Secretariat.

The meeting considered a number of subjects and the most important results of the discussions are contained in the following survey, taken from the report on the session.

One should be aware of the fact, however, that recommendations, contained in this report, have as yet no status in W.M.O. so that the contents of the following pages can only be considered to reflect the views of the members of the W.G.

1. NETWORK OF STATIONS TO MEASURE BACKGROUND POLLUTION

Purpose The group decided that there was a need for a network of background stations which should be set

up to provide information on:

(1) the general level of contamination in the atmosphere, and (2) continuous measurements to determine the long-term changes in the chemical constitutents of

the atmosphere.

Criteria for location of background stations With these objectives in mind, it considered several criteria which might be used in locating back-

ground stations. One proposal was that the density of stations could be proportional to the density of the population. Another considered that sources of pollution could determine the density of stations. Still another proposal was made which stated that the density could be proportional to the gradient of the contamination level of the pollutants. It was noted that 40-50 background stations will be established in Western Europe under the auspices of the Organization for Economic Cooperation and Development (O.E.C.D.), however, the group felt that when considering the problem from a world- wide viewpoint, such a density would be prohibitive. It was finally proposed that countries should endeavour to establish at least one station which would reflect global background contamination only (if possible) and that, depending on the size of the country, other stations could be added, as necessary, to reflect regional conditions.

Inquiry to members concerning establishment of network The group requested that the President of the Commission ask the Secretary-General of W.M.O.

to carry out an inquiry asking members if they would be interested in establishing background stations and, if so, to what extent.

Parameters to be measured at background stations Smoke and suiphur dioxide. It was agreed that concentrations of smoke and sulphur dioxide should

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be measured in accordance with the standard procedures that exist for measuring and analysing these pollutants. The group considered that measurements of carbon dioxide would be very useful for determining the radiation balance and the long-term climatic changes. However, measurements of carbon dioxide at ground level have been found to be unrepresentative in many cases because of local infhtences.

Parficulate matter. Particulate matter in the atmosphere is to a great extent a result of human activities and sampling procedures are available for these constituents. Standardized methods of analyses are, however, available only for determining the average chemical composition whereas mineral identification and size distribution would need standardized procedures.

,Measuremenrs ofprecipitarion. Background stations should also carry out collection of precipitation (rain and snow) on a monthly basis for analyses of the dissolved major constituents. The group recommended that analysis should be made for the following components: S, Cl, NO,-N, NH,-N, Na, K, Mg and Ca. In addition, measurements of pH (negative logarithm of the hydrogen ion con- centration), electrical conductivity, and bicarbonate concentration could be measured. These measure- ments could best be made at laboratories serving many stations.

The group discussed the problem of measurements of dry deposition and whether or not to combine wet and dry deposition. The majority of the group preferred to separate the two because of the un- certainty involved in interpreting the total deposition. One member considered that it would be more practical to combine wet and dry deposition and in so doing a more conservative figure would be obtained.

Measurernenr ofrurbidify. Measurements of turbidity were discussed by the group, it was noted that the U.S. has a network of stations measuring turbidity. Canada has been making the measurements on an experimental basis but difficulties have been experienced with high thin cirrus clouds which are sometimes almost invisible. The difficulty with turbidity measurements is in interpreting exactly what is being measured. The group decided that further studies and information will be needed before recommending that turbidity measurements be made at background stations.

Sampling carbon dioxide at 10,000 m. One member of the group described a very interesting project designed to obtain measurements of carbon dioxide at 10,000 m, using commercial aircraft. Three major airlines have already agreed to permit technicians to take air samples from the normal air in- takes in the cabin area. See Tellus 17, 1955, p. 398 and 18, 1966, p. 155.

2. CONSIDERATION OF VARIOUS SAMPLING TECHNIQUES

(FROM THE METEOROLOGICAL POINT OF VIEW)

The working group did not wish to evaluate the technical or chemical merits of various sampling methods but wished to restrict its interest to the meteorological aspects of sampling instrumentation and sampling duration and especially siting.

It wished to recommend strongly that meteorologists be consulted by authorities wishing toestablish air sampling sites so that they would obtain a clear idea of the microclimate. The group recognized the complicated problems involved in the siting of instruments for measuring air pollution and the necessity of health authorities to have special data for their special needs. However, it felt very strongly

that meteorological authorities should be consulted when plans are being made to establish all sites- except those which would have only a temporary value.

The group was of the opinion that, generally speaking, a good site for measuring air pollution would be at or near a site chosen for a meteorological observing station. In cities it normally is necessary to establish more than one station to average out the effects from preferential point sources.

When discussing sampling times the group urged that account should be taken of periodicity and non-periodic variability of pollution concentration and that meteorologists could in many cases explain these variations. Chemists, as well as meteorologists, should be aware of space and time variations, both random and periodic, that may occur in the concentration.

3. METEOROLOGICAL FACTORS THAT INFLUENCE THE

DISPERSION OF ATMOSPHERIC POLLUTION

The working group examined the meteorological and other factors that inlIuence the dispersion of atmospheric pollutants, and which factors may be combined in formulae to enable the calculation of concentrations. It felt that distinction should be made between:

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(1) point sources such as isolated chimney stacks and (2) area sources such as large urban communities. In both cases it is wind speed, wind profile, wind structure (turbuience), atmospheric stability and

the roughness of the surface that are the most important factors in determining the dispersion of pollutants. In most cases these factors are only used in a qualitative way. This is especially true for wind structure, atmospheric stability and the roughness of the surface. These three parameters are interrelated to some extent. With respect to point sources it was felt that existing theories or methods of computing concentrations to be experienced during specified meteorological circums~~ and which are based on a qualitative use of a number of the above-mentioned factors can be considered to be useful. The effect of vertical wind shear is not included in these methods and the group felt that improvements might be obtained by the introduction of this effect.

Another shortcoming is the lack of a complete theory concerning the rise of hot plumes in the atmosphere. It was felt that a thorough testing of existing theories would be. very useful.

Another difhculty with respect to the computation of concentrations originating from point sources lies in the relations between maximum concentrations and sampling times. A number of studies- most of which are theoretical-have been devoted to this problem and the group considered that it might be of some use to add a survey of existing theories, as an appendix to the final report.

The problem of estimating the pollution level in cities is a much more important and complicated problem because of the ever-growing urbanization all over the world. The same factors that determine the pollution level of a point source are applicable although the roughness of the terrain is naturally more important. Information on wind protiles and stability in cities is, as a rule, less readily available than in rural regions. The group felt that every possible effort should be made to see that this inform- ation be as complete as possible. It noted that stability could also be inferred in a qualitative way from the diurnal temperature variation, as well as from wind and cloudiness observations.

In principle, concentrations in cities can be determined with order of magnitude accuracy with the help of computers, by applying one of the existing point source models to the various sources in the city and adding the results. Such a procedure is only possible, however, if all source strengths by city blocks are known. It would be desirable to make a better estimate of the effects of terrain roughness and the effects of tratBc. The most important objection to be ma& is, however, that all point source models fail during situations with weak and variable winds and it is especially under these circum- stances that city air shows the largest concentrations of pollutants.

Another possibility would be to investigate under what general meteorolo8&lconditions (character- ized for example by circulation types such as those that have been delined for Central Europe by Hess and Brezowsky) high conceutrations occur in urban regions. This semismpirical method has been investigated in a number of countries and it is thought that it mi& be helpful in detecting initial indications of high pollution levels.

Here again the problem is that concentrations vary with sampling time. According to some com- putations made, concentrations in large cities tend to iucrease ~roportion~ly with the square root of time when stability increases suddenly to a constant value. Ahho@ this result was confirmed by the course of the air pollution level during the great smog in London (December 1952) it would be worth while to 8et information on the correlations found in general between concentration and sampling time in cities.

The group noted that the problem of air pollution in cities is becoming further complicated as cities spread in size towards each other creating a multisource urban problem. Another compfication arises when poflution spreads across national boundaries.

4. BOUNDARY LAYER QBSERVATIONS

The group noted that instrumentation of towers is one way of determining the vertical profiles of pollution and of meteorological elements which a%& atmospheric pollution. It recommended that standardization of the height at which measurements are to be taken is a prerequisite to a comparison programme and suggested that one scale which could be used is 25 m. 5 m, 10 m, 20 m, 40 m, 80 m, 160 m, etc. Further levels might be used, such as 100 m, 120 m, etc. In addition to towers, tethered balloons and helicopters can be used to obtain the vertical pollution and meteorological profiles.

The group expressed the desire that vertical measurements be made continuously and averaged over short time-intervals as appropriate to the element.

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5. SPECIAL POLLUTANTS

It was noted that some scientists have published information which indicates that there has been an alarming increase in certain special pollutants in the atmosphere. One such study reveals that the lead concentration in Greenland had increased greatly, other studies reveal that DDT has been found in measurable quantities in the Antarctic and also in the centre of Pitrsburgh during the winter season. It was further noted that oxidant concentrations as high as 15 parts per 100 million have been detected in Hamilton, Canada. Pollution by oxidants appears to be especially a problem in North America, although the reason for this is not precisely known. There are indications, however, that this form of pollution also occurs elsewhere.

The group realized that non-meteorological agencies have established stations to make measure- ments of special pollutants. However, it is felt that to ensure that attention be given to long-term changes in the concentrations of special pollutants the responsibility for arranging for proper observa- tions should be assigned to the meteorological services.

6. FORECASTING PROBLEMS

The group considered the value or purpose of a forecast of pollution concentration. In this con- nexion, two kinds of forecasts were recognized,

(1) climatological forecasts for engineering design and town planning, and (2) synoptic forecasts for short-term alerts.

The probtem in synoptic forecasting at the moment in most countries is that there is no legal machinery that would require emission reductions when unfavourable meteorological conditions are forecast. However, forecasts are valuable in some countries for alerting various agencies and encourag- ing voluntary control. National health authorities should realize that many national meteorological services are in a position to assist them in making forecasts of high pollution levels.

In many countries (incIuding Canada, the Netherlands, Sweden, the U.K., the U.S.A. and the U.S.S.R.) meteorological services are asked for advice concerning location of new polIutant sources. Climatological forecasts are made by relating the emission rate and the height of the chimney to the atmospheric dispersion factors and the climatological wind distribution. In this regard it would be very useful if more climatological studies of dispersion factors are made.

The question of who shouId issue forecasts of meteorological conditions favourable to high polIution was discussed. The group conciuded that this woufd depend upon the meteorological service in each country or region. Both large+cale meteorological factors and small-scale factors are involved.

Finally the group wished to state that forecasting for air pollution did not necessarily involve highly sophisti~ted meteorological techniques and that the various systems in current use could meet the needs of the authorities concerned with atmospheric pollution.

Koninklijk Nederlands Meteorologisch Institurr! F. H. %XMDT