atmospheric deposition in fenno-scandia: characteristics and trends

ATMOSPHERIC DEPOSITION IN FENNO-SCANDIA:

CHARACTERISTICS AND TRENDS

Arne Semb and Harald Dovland

Norwegian Institute for Air Research P.O. Box 130, N-2001~Lillestr~m, Norway

ABSTRACT. Chemical analyses of daily precipitation samples from "back-

ground" stations in Europe are discussed together with measurements of airborne SO2 and sulphate aerosol, and trends in energy usage and SO2 emissions° Emission sources contributing to the major part of the con-

centrations of sulphate and nitrate in precipitation are mostly 500 to

i000 km from the receptor area. Although there are no general statisti- cally significant trends in the precipitation chemistry data, minor changes point to an effect of reduced SO2 emissions in some areas. The daily data can be used to infer general conclusions with respect to precipitation scavenging efficiency.

1. INTRODUCTION

The purpose of this presentation is to give an overview of the present situation with respect to long-range transport and deposition of S and

N compounds in Northern Europe. The discussion will be based on known emission trends and changes in fossil fuel consumption patterns, and a

record of precipitation chemistry and air quality data for background sampling stations since the early 1970s.

Most of the data have been obtained through the OECD Programme on Long Range Transport of Air Pollutants (OECD, 1977), the Norwegian SNSF- project (Overrein et a~.j1980) and the present Co-operative programme on monitoring and evaluation of long-range transmission of air pollutants in Europe (EMEP) organized under the United Economic Commission for Europe.

2. EMISSIONS

The SO2 emissions inventory developed for the OECD Programme was updated in 1978 and is currently being revised. No major change in energy con- sumptions has occurred in this period. The energy crisis in 1973 initi- ally seems to have stabilized the energy consumption. Since about 1978,

however, use of liquid fuel has declined, particularly the use of resi-

Water, Air and Soil Pollution 30 (1986) 5-16. ~) 1986 by D. Reidel Publishing Company.

6 ARNE SEMB and HARALD DOVLAND

dual fuel oil for electricity generation and industrial heat production.

Use of fuel has declined generally in the industrial sector, while consumption of fuel for motor vehicles has increased slightly. It is seen from Figure 1 that only minor changes occurred between 1970 and 1979,

after which the consumption of liquid fuel, particularly residual fuel

oil, has decreased markedly.

Figure i: European consumption of solid and liquid fossil fuel, in

energy equivalents, constructed from available international statistics.

ATMOSPHERIC DEPOSITION IN FENNO-SCANDIA 7

It has been estimated that this changing pattern of oil consumption

resulted in a reduction of S02 emissions from oil combustion of 38%, or

3.8 Mt as S, between 1979 and 1982 (CONCAWE, 1984).

Table I gives figures for total SO2 emissions from some European countries, received from authorities in the different countries.

TABLE I. Sulphur dioxide emissions in 12 countries in north-

western Europe (kg/yr as S).

OECD/ EMEP EMEP A 1973 - 83 Country LRTAP (~ 1978) (~ 1983) (%)

(~ 1973)

Austria 221 215 147 Belgium 449 380 305 Denmark 250 228 152 Finland 274 270 179 France 1616 1800 1945 Federal Rep.

of Germany 1964 1815 1500 Ireland 80 87 70

Netherlands 290 240 170

Norway 91 75 51 Sweden 415 275 156 Switzerland 76 58 45 United Kingdom 2803 2490 1845

12 countries 8579 7933 6565

- 33

- 39

- 39

- 35 + 20

- 24

- 13

- 41

- 44

- 62

- 41

- 34

- 23

Few countries have systematic inventories of NO x emissions, and re- liable estimates are difficult to make in the absence of adequate emis-

sion factor~. This applies both for fossil fueled thermoelectric power

plants and for motor vehicles. A preliminary survey by Semb and Amble

(1981) indicated that the total emission of NO x Mtonne as NO2 as compared with 20 Mtonne of SO2-S. This was based on 1975 fuel consumption figures, and did not include emissions of NO from agricultural areas. Extrapolating from field measurements (Slemr and Seiler, 1984) Johansson and Granat, 1984), indicates that this emission source may be approach 1 Mtonne/yr for Europe as a whole. An important point in this respect is the constnnption of N fertilizer in Europe, which was doubled between 1970 and 1980 (United Nations, 1983). This increase in N fertilizer consumption, and the resulting agricultural surplus, has probably had an even larger influence on the emissions of ammonia.

8 ARNE SEMB and H A R A L D DOVLAND

3. OBSERVED CONCENTRATIONS AND DEPOSITIONS

3.1 Deposition of acidic substances

The EMEP provides data on precipitation chemistry and/or ambient con-

centrations of SO2 and sulphate aerosol from more than 80 "background stations" in 24 European countries. The objective of EMEP is to provide

governments with information on the deposition and concentration of air pollutants and on the fluxes across boundaries. Thus, the measurements are also important in order to validate the model calculations which provide the integrated information that is required for policy deci- sions.

The results from the first 5 yr of the EMEP (1979.1982) have been used to give isolines for mean concentrations of sulphate, nitrate, and ammonium ions in precipitation in Europe. The main features of these maps (Figure 2 are essentially the same, with the highest concentrations

near the largest emissions. The pH in precipitation is largely determined by the balance

between sulphate and nitrate on one side, and ammonium and other soil-

derived components on the other. In general, however, the areas with

the highest emissions of SO2 and NO x also have intensive agricultural production, so that there are only small anomalies in the pH-pattern as compared with the concentration of sulphate or nitrate in precipi-

tation. The deposition of S and N occur as dry deposition or with snow

and rain. The wet deposition processes are the most important in Scandinavia, particularly in areas withheavy precipitation~ where the amounts brought down approach 15 haUl of SO4-S and 7 kg ha- of both NO3-N and NH4-N. This can be rather variable, because of the varia- tions in precipitation amounts with topographic elements. It is our experience, however, that the chemical composition of precipitation is

less variable than the amount of precipitation. Knowledge of the pre-

cipitation pattern can therefore be used to extrapolate from a limited

number of precipitation sampling stations. The wet deposition pattern for sulphate in Scandinavia has been

constructed using data for a number of stations operated outside the EMEP system to obtain adequate deposition data (Figure 3). Daily, weekly, and even monthly sample collection periods are used. It has been found that by selecting proper sampling sites, the chemical composition is not critical dependent on sampling period. However, evaporation from the gauges is a problem in the summer months.

There is still not a satisfactory method available for determining the dry deposition to an actual, complex area. For a discussion of the

present state of the art, reference is made to Pruppacher et ~Z.j (1983) and a workshop proceeding edited by Grennfelt (1983). One result of these reconsiderations appears that the annual average dry deposition velocity for SO2 in Scandinavia may be considerably less than 0.8 cm s -l, chiefly because the highest concentrations occur in the winter period,

with little vegetation activity and snow covered ground. Dry deposition of NO2 is also small, partly for the same reason, but

may be more significant for HNO3, judging from the very limited ambient


NI

!

/

Figure 2. Mean weighted concentrations of non.marine sulphate, nitrate, and a[~onium in precipitation at background stations in Europe 1978-1982.


804-8 kg/ha.a

i

<,

/

I

I L

\

X 1

7 / , , > ' ,.

2

\ \\

I

J L

/ / /

/

12

Figure 3. Deposition of sulphate in Scandinavia by precipitation. Based on EMEP data and other observations for 1983 (Joranger, Granat and J~rvinen, priv. commun.).


air concentration data.

The deposition of aerosols has aroused much controversy because of

the difficulty in determining important parameters such as size distri-

bution, surface area, and the complicated topography of coniferous forest canopies.

Recent studies (Wiman and ~gren, 1985; Wiman, 1985), emphasize the inhomogenity in aerosol deposition. Over-all deposition rates of

sulphate aerosols are still difficult to quantify, but the deposition -I

velocity for a forested area may be in the range 0.i to 0.5 cm s Adding together the dry depositions result in a contribution of S

and N up to 30% of the deposition by precipitation in Scandinavia. Closer to the source areas, dry deposition is much more important, and will be the major deposition process over most of Europe.

3.2 Trends

The record of measurements at EMEP sites are from 1978 to 1984. In time, these are expected to give information of changing ambient concentra-

tions and precipitation chemistry, which may be compared with emission data. For sites in Scandinavia, where weather and atmospheric transport

conditions vary strongly from one year to the next, clear trends cannot be expected from short data series. For most of the stations there is a clear trend for lower concentrations of SO2 in air (Figure 4). Sulphur dioxide concentration is, of course, more influenced by "near-by" indi- genous sources than the more long-lived transformation product.

The precipitation chemistry data presented in Figure 5 are precipitation-weighted half-yearly mean concentrations. In general, summer half-years have higher concentrations of both sulphate and ammonium ions, while nitrate is not so well correlated. Declining trends in the concentration of sulphate in precipitation is most pronounced at

Eskedalemuir in Scotland, which fit well with the reduction of SO2 in

the United Kingdom. A similar trend was observed for precipitation chemistry data from Pitlochry by Harriman and Wells (1984).

Apparent reductions of the concentration of sulphate in precipitation seem also to have occurred at Tange in Denmark and Aht~ri in

Finland. There is no clear trend at Birkenes, for the 10-yr periode 1974-1984. It should be mentioned that Rodhe and Granat (1984), in examining data from the European Air Chemistry Network, found increasing concentrations of sulphate in precipitation in "Central

Scandinavia" for successive 5-yr periods up to 1970-1974, then decreasing.

3.3 A precipitation event

Episodes with large deposition of sulphate, nitrate and ammonium ions are generally associated with heavy rainfall. It is not unusual to find that i0 to 20% of the annual deposition occurred on a single day. Figure 6 shows the deposition in Southern Norway during a 5-day period in September 1981. The very large amounts of precipitation and the precipitation pattern is typical of orographic precipitation, which occurred in conjunction with a front moving slowly from the West.


0

- S O 2 - S

/Jg/m3 - - - SO 4- S . . . . . . . . NO 2 - N

ESKDALEMUIR $ C O T L A N ~

i i I I I I I I i I

jug/m 3

BIRKENES\ . A N O R W A Y ~

i i i I I I i i i I !

/Jg/m 3

AHTARI FINLAND

l , i ~ - 7 9 1 - 8 0 I 1975 -76 -77 -7~ -~II-~2'-~3'-8~I-85'

Figure 4. Mean half-yearly concentrations of sulphur dioxide and sul-

phate in air at 3 EMEP stations.

It is apparent from the precipitation chemistry data that concen-

trations of water-soluble ions are considerably reduced as the air passes through the precipitation field. This is more pronounced for ni-

trate than for the other components, possibly indicating that below- cloud scavenging of gaseous HNO3 may be of some importance. However,

sulphate and ammonium ion concentrations are also considerably reduced, which is not surprising in view of the precipitation intensity and per-

sistence.

ATMOSPHERIC DEPOSITION IN ~NNO-SCANDIA 13

2. rag/{.

TANGE DENMARK

I " I " 1' ' I 1 I I ' I I . . . . I

mg/t

ESKDALEM SCOTLAND

I ! I ' ~ I I I | . . . . . . . . 1 | 1975 -76 -77 -78 -79 -80 -BI -~2-~3' -8L, -85

mg/t

BIRKENES NORWAY

I 1 I " l I I

mglt

- S04-S - NH4-N

....... _ N03-N

I I I I I

, , . . . . , , i i ~ t974' -751 -761 -77 -78 -79 I_80 i -BI -52 -53 -84 -85

Figure 5. Mean precipitation-weighted half-yearly concentrations of

sulphate, nitrate, and ammonium ions in precipitation at 4 EMEP

stations.

-I The rate of deposition on 18 September, in mg m -3 d , was about

one order of magnitutde higher than the average emission rate per unit area in Central Europe. Air parcels passed over areas with high rates of emission for i000 km transport distance, then for 500 km over areas with small or negligible emission rates. Typically, over half of the emitted SO2 is deposited en rou~Gj and most of the remaining converted to sulphates before entering the orographically enhanced precipitation area, where virtually all of the pollutants become deposited within 50 to i00 km. Convergence undoubtedly increase the amounts of water and pollutants processed by the precipitation system. Intensive field studies are needed to resolve all the complex meteorological and chemical processes taking place during such events.

LEGEND: F lOO

\

. j o s p ..

VATNEDALE '~":f.~i:

i o ~o 1ook~ LISTA

Figure 6. Amount of precipitation and deposition of sulphate, nitrate, and am/nonium in S. Norway, 16-20 September 1981. In the upper corner, 96 h, 850 mb back trajectories for the sampling period 18-19 September at Birkenes. The hatched area has a mean SO2-emission intensity exceed-

ing i0 mg S -2 • 24 hr -].


3.4 Photochemical oxidants

During the last decade, photochemical oxidants and their precursors

have received increasing attention, mainly because of their potential

effects on crops and forest (e.g., Grennfelt and Schjoldager, 1984} Eliassen e9 a~.j 1982). The formation of 03 and other photochemical oxidants in the troposphere, involves emissions of NO x and volatile

organic compounds. Although research on photochemical oxidants seems to be at an earlier stage of development than research on acid deposition, there is evidence that large-scale formation and transport of

03 is important. Thus, photochemical oxidants is another environmental problem of international dimensions.

Available 03 measurements show that higher concentrations occur episodically in southern Scandinavia. Due to the variable summer climate in northern Europe, the frequency of episodes vary substantially. This

may be illustrated by the occurrence of O3 episodes in southern Norway during the last 8 yr (Table II). An 03 episode has been tentatively defined as a day with 03 concentrations above 120 ~g m -3 over a large

area (i.e. measured at several stations simultaneously) or concentrations above 200 ~g m -3 at one station.

TABLE II. Occurrence of 0s episodes in southern Norway (see

text for further explanation[.

Year 1977 1978 1979 1980 1981 1982 1983 1984

Number of

episode 2 3 5 75 17 27 9 5 days

The most pronounced oxidant episodes in southern Scandinavia are assosicated with sunny weather with light winds over northern Europe,

and transport of the polluted air masses towards Scandinavia from the

major source areas in northern Europe.

4. CONCLUDING REMARKS

Measurements of air and precipitation quality at rural sites in northern Europe may vary significantly from one year to another due to meteorological factors. Trend evaluations based on relatively short data series are therefore uncertain unless meteorological data are taken into account. A simple evaluation of available measurements indicate, however, decreasing concentrations of S in air and precipitation at some stations. Less data are available on N compounds, and no conclusion can be drawn, con- tinued measurements are important in order to facilitate future trend anaiysis. A better understanding an dquantitative knowledge of deposition processes is an important task for future research. Such studies will most likely require extensive field studies.

At present, photochemical oxidants seem to play an increasing role


in relation to large-scale effects on the environment. This should be reflected in our national and international monitoring programmes. As a first approximation, acidic deposition and photochemical oxidants may be considered independently. However, in future scientific studies, it may be of importance to consider both these envionmental problems as an entity.

5. REFERENCES

CONCAWE: 1984, Sulphur dioxide emissions from oil refineries and combustion of oil products in western Europe in 1979 and 1982. Report No. 10/84, CONCAWE, den Haag, Netherlands.

Eliassen, A., Hov, @., Isaksen, I.S.A., Saltbones, J. and Stordal, F.: 1982, Journal of Applied Meteorology, 2__1, 1645.

Grennfelt, P. (ed.): 1983, Torrdeposition av svavel- och kv~vef~ro- reningar. SNV PM 1973 Swedish Environment Protection Board, Solna, Sweden.

Grennfelt, P. and Schjoldager, J.: 1984, Ambio, I_3, 61.

Harriman, B. and Wells, D.E.: 1984, Causes and effects of surface water acidification in Scotland. The Institute of Water Pollution Control, Maidstone, Kent, England.

Johansson, C. and Granat, L.: 1984, Tellus, 36B, 26.

OECD: 1977, The OECD Programme on long range transport of air pollutants. Measurements and Findings. OECD, Paris, France.

Overrein, L.N., Seip, H.M. and Tollan, A.: 1980, Acid Precipitation - effects on forest and fish. Final report on the SNSF-project. SNSF- project FR 19/80. Norwegian Institute for Water Research, Oslo.

Pruppacher, H.R., Semonin, R.G. and Slinn, W.G.N.: 1983, Precipitation Scavenging, Dry Deposition and Resuspension. Proceedings of the Fourth International Conference, Santa Monica, California. Elsevier, New York.

Rohde, H. and Granat, L.: 1984, Atmospheric Environment, ~, 2627.

Semb, A. and Amble, E.: 1981, Emission of nitrogen oxides from fossil combustion in Europe. NILU TR 13/81. Norwegian Institute for Air

Research, Lillest~m.

Slemr, F. and Seiler, W.: 1984, J. Atmospheric Chemistry, ~, i.

United Nations: 1983, 1981 Statistical Yearbook. New York.

Wiman, B.L.B.: 1985, Aerosol dynamics in coniferous forests. Department

of Plant Ecology, University of Lund, Sweden.

Wiman, B.L.B. and Agren, G.I.: 1985, Atmospheric Environment, !~, 335.

atmospheric deposition in fenno-scandia: characteristics and trends

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