Acid Rain: Report number 14

The views expressed in this Report are those of the authors of the papers and contributors to the discussion individually and not necessarily those of their institutions or companies or of The Watt Committee on Energy Ltd. Published by: The Watt Committee on Energy Ltd 18 Adam Street London WC2N 6AH Telephone: 01-930 7637 This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” © The Watt Committee on Energy Ltd 1984 ISBN 0-203-21031-X Master e-book ISBN

ISBN 0-203-26813-X (Adobe eReader Format) ISSN 0141-9676

THE WATT COMMITTEE ON ENERGY REPORT NUMBER 14

ACID RAIN

Papers presented at the Fifteenth Consultative Council meeting of the Watt Committee on Energy, London, 1 December 1983

The Watt Committee on Energy Ltd A Company limited by guarantee: Reg. in England No. 1350046 Charity Commissioners Registration No. 279087

AUGUST 1984

Contents

page Members of Acid Rain Working Group

iv

Foreword

vi

Introduction Section 1

The fate of airborne pollution

viii 1

1.1

Introduction

2

1.2

Emissions

5

1.3

Transformations

7

1.4

Contribution of motor vehicles

10

1.6

Loss processes to above the boundary layer

12

1.5

Deposition

10

1.7

Networks and measurements

13

1.8

Trends in acid precipitation

15

1.9

Modelling

16

1.10

Episodes

18

1.11

Some important questions

19

1.12

Recommendations

22

Vegetation and soils

24

2.1

Historical aspects

25

2.2

Effects of ambient air pollution on metabolism and growth of plants

29

2.3

Effects of acid precipitation on plants and soils

37

2.4

The Ulrich Hypothesis on effect of acid depositions on forest ecosystems

42

2.5

Lichens as indicators of atmospheric sulphur pollution

45

2.6

Central European forest die-back and its relevance to Britain

47

Section 2

iv

Section 3

Freshwater

51

3.1

Introduction

52

3.2

Techniques for assessing acidification

53

3.3

Effects of acidification on flora and fauna

54

3.4

Evidence for acidification outside U.K.

55

3.5

Evidence for acidification in U.K.

57

3.6

Extent and distribution of acid waters in Great Britain

59

3.7

Conclusions

63

Section 4

Remedial strategies

65

4.1

Technologies for reducing emissions

66

4.2

Impact of SO2 control on electricity prices

73

4.3

Existing and prospective legislation governing British emissions

76

4.4

Scientific support for an acid rain strategy

79

4.5

Summary

83

Fifteenth Consultative Council meeting of the Watt Committee on Energy

85

Abbreviations

86

Appendices 1. 2

THE WATT COMMITTEE ON ENERGY Member institutions

88

Policy

90

Members of Executive, June 1984 Recent Watt Committee Reports 57

90

Members of Acid Rain Working Group Chairman Professor K.Mellanby CBE Institute of Biology Dr Helen ApSimon Dr R.W.Battarbee Dr J.N.B.Bell Dr W.O.Binns T.R.Carrick Dr M.J.Chadwick A.J.Clarke

Imperial College of Science & Technology, London University College London Imperial College, London Institute of Chartered Foresters Freshwater Biological Association University of York Central Electricity Generating Board

v

Dr A.T.Cocks D.H.Crawshaw C.J.Davies Dr B.E.A.Fisher J.A.Garland D.Hammerton R.Harriman N.H.Highton A.V.Holden A.S.Kallend Dr A.W.C.Keddie Dr J.A.Lee Dr P.S.Maitland Professor T.A.Mansfield Dr A.R.Marsh C.Martin Dr H.G.Miller G.S.Parkinson Dr M.A.Plint Dr P.Roberts Dr R.A.Skeffington Dr F.B.Smith Dr J.H.Stoner Dr M.H.Unsworth (observer) Secretary J.G.Mordue

Royal Society of Chemistry North West Water Authority Operational Research Society Institute of Mathematics and its Applications Royal Meteorological Society Institution of Water Engineers and Scientists Dept of Agriculture & Fisheries for Scotland Beijer Institute, Stockholm Pitlochry Central Electricity Generating Board Warren Spring Laboratory Victoria University of Manchester Institute of Terrestial Ecology University of Lancaster Central Electricity Research Laboratories Institution of Public Health Engineers Institute of Chartered Foresters Institute of Petroleum Royal Geographical Society International Flame Research Foundation Central Electricity Research Laboratories Royal Meteorological Society Institution of Water Engineers and Scientists Institute of Terrestial Ecology

Note The Working Group appointed four sub-groups; the membership of each is listed on the first page of the relevant section of this Report.

Foreword

What is loosely described as ‘acid rain’ is not a new phenomenon. The burning of coal and other fossil fuels must have always resulted in the production of sulphur dioxide, and, where the combustion temperatures are high, of oxides of nitrogen. These may be present in various stages of oxidation and are often referred to as simply SOx and NOx. The Clean Air Act 1956 with its limitations on the burning of raw coal in urban areas has virtually eliminated ‘smog’ in British cities but has not directly reduced the SOx emissions. It is only during the last decade or so that Acid Rain has become a topic of discussion vying with nuclear energy in its emotive power. Initially attention was mainly concerned with the alleged effect of these gases and the acids formed therefrom on lakes and rivers in Scandinavia. This concern was soon followed by reports of serious damage to, for instance, the Black Forest, and, more locally, to lakes in the Galloway area and damage in other parts of Scotland. In the case of these and many other examples, suggestions, still to be verified, have been made about the probable origin of the pollutants. During a general discusion about the future programme of the Watt Committee, Dr A.A.L.Challis, then Chief Scientist at the Department of Energy, told me that acid rain was a field in which we could be of real help. Much of the large amount of data available was self-contradictory, and though Dr Challis did not imagine that we could provide a solution to the problem, we might, he thought, be able to clarify it and give a lead as to the work needed to give a truer picture. A wide range of experts—not just physicists, chemists and mathematicians, but experts in combustion chemistry, meteorology, aerodynamics, biology, water engineering and forestry—was called for, all of whom could be brought together through the existing Watt Committee organisation. A number of our member institutions had, indeed, indicated their support for the idea of a Watt Committee study of acid rain. Knowing of Professor Kenneth Mellanby’s wide knowledge and enthusiasm in this field, I asked him if he would be prepared to lead such a team. After a short hesitation he agreed, and said that he would aim at producing the first draft of a report in about three months, leading to a series of papers for our next Consultative Council meeting on 1st December 1983. This he did, and the general reaction was that the work, having made so much progress in such a short time, must be continued. The first task was to get the present report into print: this would enable us to decide where further work was most needed. In spite of the many hours that he and the thirty or so people involved in the present report had given, Professor Mellanby agreed to continue the study, selecting his team in line with the questions to be answered. As Chairman of the Watt Committee I should like to thank them all for the splendid job that they have done and for their willingness to continue. Finally, I would emphasise again that we are still clarifying the

vii

problem, and can only be any help because of the far deeper studies and more intensive research done by other workers in Europe and North America. April 1984 J.H.Chesters Chairman, The Watt Committee on Energy

Introduction

When I was asked to be Chairman of the Watt Committe’s working group on acid rain, I said that we had three questions to answer. These questions are: (1) Is there an acid rain problem? (2) If so, what is the cause of the problem? (3) How can this problem be solved? The papers here produced show how far we have gone to answer these questions. It will be clear that, though considerable progress has been made, simple and comprehensive answers cannot, as yet, be given. First, is there a problem? There is no doubt that environmental damage—to crops, to trees, to buildings and to freshwater—does occur, and that, at least in some cases, this is caused by acid emissions from the use of energy derived from fossil fuels. However, it is not easy to quantify this damage, or to be sure that ‘acid rain’ is always its main cause. This brings us to the second question. We must be careful not to be misled by spurious correlations between damage and the emission of substances which may, or many not, cause the damage. Here it must be pointed out that it is clearly necessary to consider all the substances emitted when fossil fuels are burned and when the use of energy by industry affects the environment. It is often gaseous emissions that are involved, and the rain, as such, may not always be as important. We must also realise that many different substances—oxides of nitrogen and ozone in particular—may be involved in addition to sulphur dioxide. It is obvious that, if we cannot as yet give clear answers to the first two questions, we cannot answer the third with any precision. Thus we must be very careful not to advocate expensive ‘solutions’ which may, in the end, be found to have little effect. The impression given by the media, that acid rain is a new problem, is quite false. As long ago as 1872 R.A. Smith, the first British Alkali Inspector, published a book on chemical meteorology, in which he described rain from industrial areas as acid as, or even more acidic than, anything found today. For a hundred and fifty years we have recognised the harmful effects of air pollution, a serious local problem in our cities and industrial areas. In fact, of course, many of the harmful effects of air pollution have been greatly reduced in recent years. The pea-soup fogs described by Dickens have disappeared from London and our main cities. The damage to plants and buildings from gaseous pollution, mainly sulphur dioxide, has been greatly reduced. In fact, we now find that roses in city parks are once more attacked by black spot; the fungus that causes this disease was previously killed by sulphur dioxide to which the rose bushes were more resistant. Urban air pollution in Britain has been largely conquered by the policy of ‘dilute and disperse’. By discharging toxic flue gases from high chimneys, ground-level concentrations of sulphur dioxide no longer

ix

cause acute phytotoxicity. Further from the source of pollution, even greater dilution reduces the sulphur dioxide to apparently harmless levels. Until recently we believed that we had by these means completely conquered this type of pollution. We did not alway recognise that harmful effects may be produced by these very dilute gases at great distances from their sources, probably because we did not fully realise that chemical changes may occur to the substances discharged, producing compounds that have quite different properties. We need to distinguish between primary pollutants, mainly oxides of sulphur and nitrogen, which are emitted as such in industrial areas, and secondary pollutants, the most important of which are sulphates and sulphuric acid derived from SO2 and nitrates and nitric acid derived from oxides of nitrogen. The transformations from primary to secondary pollutants, particularly of sulphur, may be slow, and may take several days during which the contaminated air may be blown a great distance—perhaps from Britain to Norway, or from the Ruhr to Sweden. I have found it useful to describe precipitation containing mainly primary pollutants as ‘primary acid rain’, and that containing secondary pollutants as ‘secondary acid rain’. Not all my colleagues agree with this distinction, which may not always be clear-cut. Rain falling at intermediate distances from pollution sources may contain both primary and secondary pollutants. Nevertheless, I think that the general concept may be useful. Primary acid rain is a local phenomenon, and is often accompanied by high levels of gaseous pollution. Where damage to plants and buildings occurs, it is generally the gases, and not the wet deposition, which are mainly to blame. It is primary acid rain that was described over a hundred years ago by R.A.Smith, and it is its analysis that often hits the headlines today. Though primary acid rain must have some effect on soils and poorly buffered fresh waters, its effects on buildings and vegetation seem generally to be minimal, and so it can sometimes be somewhat of a ‘red herring’. Our working group has therefore devoted its main attention to the important and difficult problem of secondary acid rain, to a study of its production and of its effects. Secondary acid rain is very dilute, so it seldom causes direct phytotoxicity. It usually falls where the air is clean, with low levels of oxides of sulphur and nitrogen. However, in high-rainfall regions soils that contain little calcium and poorly buffered fresh waters may be seriously affected by the great volume of dilute acid falling on them. I may appear to have given greater importance to problems of gaseous pollution, and to have underestimated the importance of ‘acid rain’. This is certainly not my intention. However, I think it is essential that we distinguish between the various processes and reactions involved. We know a good deal about the control of gaseous pollution, very little about how to reduce or eliminate the effects of wet pollution. We still do not fully understand the causes of fishless lakes and dying trees, bu we can be sure that many of the simplistic explanations often circulated do not give us a true picture of the situation. We are still very far from being able to give firm and simple solutions to all the problems concerning acid emissions. We can control, and have controlled, the gross damage caused by gaseous pollution in many areas. We know how, at a cost, emissions may be reduced. We still do not know how the reduction of emissions would reduce the damage from secondary acid rain. We have to unravel the problem, identifying the causes, processes and effects We have to investigate all the methods for reducing emissions, including the improved combustion of fuels, to reduce the amounts of toxic substances in the flue gases. We need to assess the value of remedial methods such as the liming of acid lakes, which may alleviate the situation while a more fundamental solution is being discovered. However, one other important method is clearly available. This is to be more economical in the use of energy and of fossil fuels. The Watt Committee in a previous report* showed that this was possible without reducing our productivity or our standard of living. Also the substitution of renewable sources of energy—

x

solar power, hydroelectric generation, wind power—could make a moderate but significant contribution. The greater use of nuclear power could also be important. So there are many ways of tackling the problem, but first and most important we need to understand just what the problem really is. Hill Farm, Wennington, Huntingdon. March 1984 KENNETH MELLANBY

* The rational use of energy. Watt Committee on Energy Report No. 3, London, 1978.

THE WATT COMMITTEE ON ENERGY REPORT NUMBER 14

Section 1 The fate of airborne pollution F.B.Smith

This paper presents the work of Sub-group 1 (Production, Transformation, Transport and Deposition) of the Watt Committee working group on Acid Rain. Membership of Sub-group 1

Dr F.B.Smith (Chairman) Dr H.ApSimon Dr A.T.Cocks Dr B.Fisher Dr J.Garland Dr A.S.Kallend Dr A.W.C.Keddie Dr A.R.Marsh Professor K.Mellanby G.S.Parkinson Dr M.A.Plint Dr M.H.Unsworth (observer)

The fate of airborne pollution

1.1 Introduction Gaseous and particulate pollution injected into the atmosphere can be subjected to many physical and chemical processes. The main processes, shown schematically in Figure 1.1, are outlined only briefly in this introduction, but are described more fully later. In general, many different pollutant species are found in the atmosphere. Those of principal importance for acid deposition are sulphur dioxide, nitric oxide, ammonia, non-methane hydrocarbons and their derivatives. These are derived from different sources, some natural, others resulting from man’s activities. In Europe, the latter predominate for most of the important species. These ‘anthropogenic’ sources have a spatial distribution that requires careful determination.* In principle, it is important to know the elevation of each source above ground, any inherent plume rise due to buoyancy, the average magnitude of the emission and any trends and cycles that may be evident. Emissions often exhibit diurnal, weekly and annual cycles in response to man’s needs and working patterns, as well as more unpredictable variations in response to weather conditions, for example. There is thus a level of basic uncertainty in emission values from any area at any time, even in those areas of Europe where detailed analyses have been made. Having entered the atmosphere, the pollutants are advected away by the wind. The principal property of the wind in this respect is its direction. Both the wind direction and the wind speed are determined by

Figure 1.1 Processes involved in deposition of atmospheric pollutants.

THE FATE OF AIRBORNE POLLUTION

3

objectively analysing winds and pressures measured at a rather sparse network of meteorological observing stations (typically positioned 50–100 km apart over land) at certain fixed hours of the day to form continuously changing wind fields. Clearly the interpolated wind field is not exact, particularly in the case of complex topography, and this causes errors in pollution trajectories that grow exponentially with distance. There has been only limited testing of our ability to predict the trajectory in the atmosphere of pollution emitted from a single point source, and then usually in relatively simple meteorological situations and out to distances of only a few hundred kilometres. However, most important sources in reality have broad geographical distribution and our interest lies in depositions over some period of time: both these factors tend to reduce the effective errors to within acceptable limits when ecological consequences are assessed. In addition to advection, the motion of the air is usually turbulent. This turbulence causes dispersion, that is, mixing of the pollutant plume with the surrounding atmosphere and with plumes from other sources. In consequence, different species may be brought together, resulting in chemical reactions; in addition, they may be dispersed down to the surface, where they may be partially deposited, and to the top of the mixing layer, where some may be temporarily ‘lost’, often in a sporadic manner, to the less turbulent atmosphere above. Chemical reactions may take place in cloud, often during the uptake into growing raindrops. These reactions are sometimes rapid, implying that what comes out in the rain is physically and chemically very different from that which entered the cloud. Other transformations take place out of cloud in what are called gas-phase reactions. On the whole, these changes take place more slowly, transforming the primary emitted pollutants (e.g. sulphur dioxide (S02) and nitrogen oxide (NO))* to secondary pollutants (e.g. sulphate and nitric acid) at a typical rate of a few per cent per hour. Thus, in very general terms, primary pollutants are important at short range near to the main source regions whereas secondary pollutants become relatively more important at long distances including the more remote parts of Europe. Owing to the transformations involved, the deposition Table 1.1 European and U.K. emissions since 1900, million tonnes per year 1900 SO2 Europe 16 SO2 U.K. 2.8 NOx U.K. 0.68 NOx expressed as equivalent NO2.

1910

1920

1930

1940

1950

1960

1970

22 3.2 0.69

22 3.2 0.72

25 3.2 0.72

25 3.6 0.82

25 4.6 0.99

32 5.6 1.35

52 6.0 1.64

Table 1.2 U.K. emissions of SO2 and NOx since 1971, million tonnes per year SO2 Power Station s All source s

*

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

2.80

2.87

3.02

2.78

2.82

2.69

2.74

2.81

3.10

2.87

2.71

2.60

5.83

5.64

5.80

5.35

5.13

4.98

4.98

5.02

5.34

4.67

4.23

4.00

Anthropogenesis is the study of the origin of man. The word has sometimes been wrongly used elsewhere, but in this Report the expressions ‘man-made’ and ‘man-induced’ are generally preferred, where appropriate. *See Appendix 2.

4

INTRODUCTION

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

NO2 — 0.73 0.81 0.72 0.76 0.79 0.79 0.81 0.88 0.85 0.82 0.77 Power Station s Vehicl — 0.42 0.45 0.44 0.47 0.45 0.46 0.48 0.49 0.49 0.48 0.49 es* All — 1.73 1.85 1.72 1.70 1.74 1.77 1.80 1.89 1.79 1.71 1.67 source s CO — 7.86 8.30 8.07 7.80 8.06 8.27 8.62 8.78 8.85 8.62 8.83 All source s* ‡ Total — 2.40 2.60 2.73 2.77 2.93 3.03 3.16 3.35 3.37 3.36 3.29 hydroc arbons (metha ne equiva lent weight ) * These are the best estimates currently in a consistent format, but recent W.S.L. work suggests that vehicles emit somewhat more NOx ( 40% of total) and significantly less CO. ‡ Principally vehicles. NOx represented as equivalent NO2.

properties and the ecological effects may be quite different at the two ranges. Different species are absorbed, sedimented and deposited by impaction on to the underlying surface at different rates according to their chemical and physical character and the nature of the surface itself. The whole process is called ‘dry deposition’. Typically, about half the sulphur dioxide emitted into the turbulent mixing layer of the atmosphere is dry-deposited (ignoring its partial conversion to sulphate) in about 30 hours during which time it may have travelled some 1000 km. Other chemically reactive gases tend to be deposited at much the same rate, whereas aerosols, like sulphate, tend to be dry-deposited very much more slowly. As Figure 1.1 shows, the other main deposition process is due to precipitation (rain or snow) and is called ‘wet deposition’. The main route for wet deposition is through the involvement of pollution in developing cloud droplets within the cloud itself. This process, termed ‘rain-out’, tends to be a very efficient removal process, although the exact removal rate is imprecisely known and almost certainly varies with the amount and character of the pollution, the microphysics of the cloud itself and the average size and temperature of the raindrops or snowflakes involved. Less important in general is the take-up of pollution into falling precipitation below cloud. Because precipitation is an occasional event, so is wet deposition. Wet deposition fields over a short period of time, up to a month or two, are therefore intrinsically very patchy and very difficult to assess from conventional meteorological measurements. A very heavy shower occurring between meteorological stations can sometimes affect a heavily polluted airmass and result in an undetected large local deposition (perhaps

THE FATE OF AIRBORNE POLLUTION

5

Figure 1.2 Sulphur dioxide emissions in Great Britain in 1982 in thousands of tonnes; National Grid squares 20 km×20 km. Data by courtesy of Warren Spring Laboratory.

10% or more of the annual total). Situations like this, in which significant deposition occurs within a relatively short period of time, are called ‘episodes’. A third process exists which may be rather important in some mountainous areas. This is the process of fog (or occult) deposition. Fog or low cloud droplets can contain high concentrations of pollution and, when blown by the wind over hills and mountains covered by trees, heather and other scrub, can be deposited on to the vegetation very efficiently. The contribution this makes to the long-term deposition in such areas is not very well known, and it is certainly a contribution that would be very difficult to model except in a statistical way. In general, dry deposition accounts for some two-thirds of the total sulphur deposit in Europe. Wet deposition and fog deposition account for the remainder. These last two are particularly important in areas of high rainfall, especially in mountainous areas to the east or northeast of significant source areas. 1.2 Emissions Table 1.1 gives estimates of sulphur dioxide emission values for the United Kingdom and for Europe since the beginning of the century and corresponding values for nitrogen oxides for the U.K. alone. As would be expected, they all show a very marked increase up to 1970. Table 1.2 gives the U.K. situation in more detail between 1971/72 and 1982. It shows a fairly persistent downward trend in sulphur dioxide emissions from 5830000 tonnes in 1971 to an estimated 4000000 tonnes in 1982, although significant year-to-year variations about this trend clearly exist. Power-station emissions have declined only slightly over this time, and in

6

INTRODUCTION

consequence they constitute now roughly two-thirds of all emissions compared with less than a half in 1971. Figure 1.2 shows how the total SO2 emissions were distributed spatially across the U.K. in 1982. The nitrogen oxide emissions (given as equivalent NO2) are also given in Table 1.2. In reality the emissions include Table 1.3 Annual emissions of sulphur dioxide, oxides of nitrogen and non-methane hydrocarbons for European countries about 1 978 SO2

NOx

NMHC

Albania 100 10 10 Austria 430 275 280 Belgium 760 410 390 Bulgaria 1000 240 240 Czechoslovkia 3000 600 600 Denmark 456 240 220 Finland 540 200 200 France 3600 1650 2000 G.D.R. 4000 680 680 F.R.G. 3630 3350 2450 Greece 704 500 260 Hungary 1500 220 220 Iceland 12 10 15 Ireland 174 90 105 Italy 4400 1550 1750 Luxembourg 48 50 30 Holland 480 700 600 Norway 150 110 170 Poland 3000 1000 1000 Portugal 168 110 200 Romania 2000 460 460 Spain 2000 850 1050 Sweden 550 260 380 Switzerland 116 160 260 Turkey 483 600 600 Western U.S.S.R. 16000 5000 5000 U.K. 5020 1800 1158 Yugoslavia 2950 210 210 Estimated values in thousands of tonnes. NOx expressed as equivalent NO2. NMHC values represented by total mass.

both NO and NO2, the former tending to predominate. The totals show little overall trend, although from 1975 the levels first increased systematically until a maximum was reached in 1979, since which time they have decreased, mainly as a result of reductions from industry and from power stations. The contribution from vehicles has shown a small increase over the period—of the order of 15% — and now constitutes

THE FATE OF AIRBORNE POLLUTION

7

roughly 30% of the total emissions, compared with about 45% from power stations (but see the footnote to Table 1.2). Emissions of carbon monoxide have increased by roughly 10% and those of total hydrocarbons by about 30% over the same period. Of the latter, it seems that the relevant non-methane hydrocarbons (NMHC) constitute roughly one-third of the total when expressed as an equivalent weight of methane. The remaining methane is chemically very stable and is therefore fairly unimportant as far as the formation of acidic species in the atmosphere is concerned. At present it is not known for certain how much detail is required concerning the composition of NMHC, although some, but not all, modellers consider it likely that they can be ‘lumped’ together as one species beyond a few tens of kilometres from significant sources. It is possible that control of NMHC emissions could be a way of controlling the formation of acidic species. European emissions are given by country for a period around 1978 in Table 1.3. The uncertainty in the SO2 emissions is estimated to be 10–15% at best, and considerably larger for many countries, especially those in eastern Europe. The NOx and NMHC emissions are even more uncertain. Considered as average emissions per head of population, in Western Europe, values for SO2 all range below 0.1 t per year, whereas in Eastern Europe values range up to 0.25 t per year, largely reflecting the nature of the fuel used. Emissions of NOx are much more uniform, however, at around 0.03–0.04 t per year, with only the F.R.G.* standing out as exceptional with 0.055 tonnes. Similarly, for NMHC the U.K. emissions at 0.02 t per year per head contrast with other countries’ values of around 0.04 t per year, for reasons that are not entirely clear, although different assumptions about sources and emission factors are a probable reason. The tables presented here do not contain the contribution of natural emissions. These are estimated to be some 10% of the total sulphur emissions for Europe. Background air coming into Europe from the Atlantic carries some further sulphate (typically at a concentration of 0.5–2 µg m−3). The origins of this sulphate are still being debated; it is possible some of it comes from North America and other very distant man-induced sources. This sulphur comprises a small fraction of the total airborne amount available for deposition to the ground surface of Europe (typically 5%), although in more remote areas, like northern Scandinavia, the background contribution can be dominant. Natural emissions of NOx are not thoroughly quantified. Sometimes air concentrations at remote sites are surprisingly large and are not readily explained. Lightning is thought to contribute some 20% of the natural generation over the world as a whole, but its incidence obviously varies considerably in time and space. Figure 1.3 shows the sulphur dioxide emissions for Europe in about 1978 in 150-km grid squares prepared in connection with the EMEP Programme (see below) for studying the long-range transport of pollution.* 1.3 Transformations The major acid precursors emitted as a result of man’s activities are sulphur dioxide (SO2) and nitric oxide (NO). These materials undergo oxidation to sulphate aerosol and gaseous nitric acid respectively. Molecular oxygen, although abundant, reacts relatively slowly with SO2 and nitrogen oxides and, hence, more reactive oxidising species present at much lower concentrations in the troposphere are responsible for most of the production of acidic species. *

Throughout this Report, the Federal Republic of Germany (‘West Germany’) and the German Democratic Republic (‘East Germany’) are referred to, in abbreviated form, as F.R.G. and G.D.R. respectively.

8

INTRODUCTION

Figure 1.3 EMEP emission data in the 150 km×150 km grid network: thousands of tonnes of sulphur per year.

The major homogeneous gas-phase oxidation route for SO2 is through a reaction with the hydroxyl radical, OH: OH is produced by a complex chemical mechanism involving ozone, nitrogen oxides, carbon monoxide, hydrocarbons, water vapour and sunlight. Reaction with SO2 is not a major loss process for OH and it is possible that some OH is reformed during the aerosol production step. Hence, the rate of the homogeneous production of acidic aerosol from SO2 is probably approximately proportional to SO2 concentration. Under typical conditions in the tropospheric boundary layer, maximum average homogeneous oxidation rate constants of around 1 % per hour are predicted. SO2 is soluble in water and the dissolved species can undergo oxidation in cloud and rain drops:

*See

list of abbreviations, Appendix 2 (page 54).

THE FATE OF AIRBORNE POLLUTION

9

SO2 solubility is inversely related to H+ concentration, and, as dissolution of SO2 produces an increase in H +, SO uptake in droplets is self-inhibiting. 2 Oxidation leads to a further increase in acidity: Although catalysed oxidation by molecular oxygen may in certain special circumstances produce efficient aqueous oxidation of dissolved SO2, ozone and hydrogen peroxide are probably the most important stable oxidants. Some ozone in the troposphere is the result of stratospheric incursion and both ozone and hydrogen peroxide are produced by complex chemical processes similar to those involved in OH production. Ozone is generally more abundant than SO2 in the boundary layer and oxidises it rapidly in aqueous solution under neutral and slightly acid conditions, but the rate is inversely related to H + concentration and is relatively low at the pH values of acid rain and clouds. The rate of reaction of hydrogen peroxide with dissolved SO2 is proportional to H+ concentration, however, and this cancels the inverse H+ concentration effect on SO2 solubility when sufficient hydrogen peroxide is available; thus a pH-independent rate of sulphate production is produced over the range of likely atmospheric circumstances. However, hydrogen peroxide is probably appreciably less abundant than SO2 under most conditions in the boundary layer and, hence, as its formation rate is low, the oxidation is limited by peroxide availability and the amount of sulphate formed may be virtually independent of SO2 in many circumstances. Model calculations indicate that aqueous oxidation in non-precipitating clouds involving ozone or hydrogen peroxide is unlikely to produce an overall rate constant for the oxidation of SO2 in the boundary layer that would be substantially greater than 1 % per hour. It has been postulated recently that aqueous droplets may scavenge oxidising radicals from the gas phase. Models, involving many unvalidated assumptions, indicate that such processes could lead to rapid oxidation of dissolved SO2. The effect of SO2 concentration on oxidant concentrations in such systems is not well defined at present. NO is initially oxidised to nitrogen dioxide in the troposphere, mainly by reaction with ozone: NO2 is oxidised homogeneously to nitric acid in the daytime by reaction with OH: The rate constant for this reaction is about 9 times that for the given corresponding reaction for SO2. As nitrogen oxides play a crucial role in determining OH concentrations, nitric acid production is unlikely to be simply related to nitrogen oxide concentrations. Heterogeneous processes in aqueous droplets are unlikely to provide a substantial pathway for nitric acid production. Overnight, NO2 can be further oxidised in the gas phase to nitrogen trioxide: This in turn can react with NO2 and water vapour to produce nitric acid: This process may be the major production route for nitric acid in some circumstances and the dependence of the reaction rate on nitrogen oxide concentrations is unlikely to be simple. Concentrations of oxidants within an expanding combustion-generated plume from a point source are generally lower than those in the ambient atmosphere because of the higher nitric oxide concentrations. Thus, rate constants for acid production would be lower although the produced acid concentrations within the plume boundaries could be higher because of the higher concentrations of primary species.

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INTRODUCTION

1.4 Contribution of motor vehicles Motor vehicles can contribute to the acidification of rain in two ways. First, roughly a third of the NOx emissions in the U.K. are derived from motor vehicle exhausts. As stated above, nitric oxide is oxidised in the atmosphere to nitrogen dioxide and thence to nitric acid, the latter being fairly efficiently scavenged in rain systems and observed as the nitrate ion in collected precipitation. Second, motor vehicles contribute both NOx and hydrocarbons to the ambient air on a widely distributed basis. These are the essential precursors to oxidants formed photochemically in the atmosphere. The oxidation rates of both SO2 and NO diluting into the ambient air from point sources are strongly dependent upon the concentrations of these precursors. Ambient air quality is similarly thought to influence the progress of oxidation in clouds because the oxidant precursors are again hydrocarbons and NOx in the ambient air. Because of these complex interactions between pollutants from different sources, it is not possible to make a realistic quantitative assessment of the contribution of motor vehicles to acidic deposition or to the acidification of rain. 1.5 Deposition Deposition processes clean the atmosphere of pollutants and deposit them on to land or sea. Wet deposition —the removal by rain and snow—has long been studied, using funnels to collect the precipitation for chemical analysis. However, removal by rain is not the only significant deposition process. There is also dry deposition, which means dust fall and various processes for direct absorption of the pollutant by the vegetation, soil, water or buildings that form the surface. Thirdly there is the collection of wind-blown fog or cloud water on vegetation and other surfaces. These three processes ultimately remove all the acidifying materials from the atmosphere. Provided sufficient care is taken to expose the collector correctly, to avoid contamination and to analyse the samples by suitable techniques, wet deposition can be measured reliably by use of rain collectors. The contamination of the collector by dry deposition may have a small effect on the result but this effect may be largely avoided by using ‘wet-only’ collectors which place a lid over the funnel whenever no precipitation is falling. Networks of rain gauges have been used to measure the variation of wet deposition across Europe. There must be quite large uncertainties in the deposition at individual locations between the sampling points, but to some extent these average out and the total annual deposition over the area is probably known to within about 30%. Although the networks give a direct measure of the amount of deposit in precipitation, they do not tell us anything about the mechanism for removal. Sulphate and nitrate in rain could result from incorporation of particles containing these compounds or from absorption and reaction of the gaseous sulphur and nitrogen oxides. Several mechanisms have been proposed for the incorporation of the particles, and one of these, involving condensation of water on to the particles during cloud formation, seems particularly significant and capable of accounting for a large proportion of the material in rain. There is also evidence from field data that a large fraction of the gaseous compounds is incorporated into rain but the mechanisms are difficult to quantify. If sulphur dioxide is to be removed in rain, it requires an oxidation process that can operate sufficiently rapidly in the cloud droplets; although hydrogen peroxide and ozone and some free radicals are possibly capable of doing this, the factors that control the amount of sulphate formed in this fashion are not known. Uncertainty in the mechanism for removal by rain makes it difficult to be certain of the detailed effects of a reduction of emissions from a particular source area on rain chemistry.

THE FATE OF AIRBORNE POLLUTION

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Dry deposition processes are different for particles and gases. Very large particles* (diameter d>10µm) fall slowly, like fine rain; intermediate particles (1