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On the relation between anthropogenic SO2 emissions and concentration of sulfate in air and precipitation
-s1/sv
Atmofghnic Entlironment Vol. 23, No. 5, pp. 959~!+56,19EV. Printed in Great Brit&.
s3m+aoo Pergatnon Press pk
ON THE RELATION BETWEEN ANTHROPOGENIC SO, EMISSIONS AND CONCENTRATION OF SULFATE IN AIR AND PRECIPITATION CAROLINE LECK and HENNING RODHE Department of Meteorology, University of Stockholm, S-106 91 Stockholm, Sweden (First received 28 January 1988 and receiwdfor indication
27 October 1988)
Abatraet-The relationship between anthropogenic SO, emissions and the concentration of sulfate in air and precipitation has been investigated by comparing variations over the years, during the year and during the week. A 50 per cent increase in sulfate concentration in Swedish pr~ipitation between the late 1950s and the early 1970~3, followed by a 20-25 per cent decrease in concentration until the early 1980s agree well with estimated changes in anthropogenic SO, emissions in the countries that contribute to the deposition in Sweden. A more detailed analysis of sulfate in precipitation during the period 19724986 shows a decrease in concentration of about 40% and in deposition of about 20%. This decrease compares reasonably well with estimates based on model calculations of long-range transport and reported reductions in emissions in northern Europe. Variation of SO, concentrations during the week exhibits a systematic pattern in approximate agreement with the expected variation in emission rates. A reasonable agreement also exists for sulfate concentration in air during the summer months in regions close to the major emission areas. We conclude that there seems to be a qualitative agreement between changes in anthro~~nic SO2 emissions and changes in the concentration and deposition of sulfate in northern Europe. Key word index: Sulfate, sulfur dioxide, concentration, deposition, proportionality.
INTRODUCTION
It has been suggested that the relationship between anthropogenic emissions of SO, and the deposition of sulfate in precipitation may be significantly different from proportionality (NRC, 1983). This could be due to complex nonlinear chemical and physical transformation processes occurring in the atmosphere, or to the existence of large natural emissions. A better understanding of the relationship between emissions and deposition would make possible quantitative estimates of the size of emission reduction needed to achieve a specified deposition limit, such as those defiwd by Nilsson (1986). Another potential benefit of such knowledge might be inferences about mechanisms of atmospheric transformation processes. There are chemical reasons for deviations from a proportional relation between emissions and deposition. For example, changes in the inanitions of the OH radical caused by changes in the emissions of oxides of nitrogen may influence the rate of transformation of SO2 to SOi- and thereby the geographical distribution of S deposition (Rodhe et al., 1981). Similarly, an increase in acidity of cloud and rain water, caused by increased emissions of S and N compounds, may slow down the liquid phase transformation of SO2 A further discussion of factors involved with proportionality can be found in Clark et al. (1987). 959
These questions have previously been addressed by Op~nh~mer (1983a,b, 1984), He used model calculations of the S balance over eastern North America to conclude that uniform regional reduction in annual SO2 emissions would produce nearly proportional reductions in wet S deposition within that particular region. In the present study we have analyzed data for SO, and SO:- in air and SO:- in precipitation in relation to SO2 emissions over the years, during a year and during a week. The data base We use two different sets of atmospheric chemistry data. Monthly data on SOi- in precipitation from nine Swedish stations operated by the International Meteorolo~~l Institute in Stockholm (Rodhe and Granat, 1984), hereafter referred to as the IMI-data. A few stations’ data are available since 1955. Daily data on aerosol SO:- and SO, in air from about 30 stations in northern Europe operated by national institutions as part of the EMEP program (EMEP, 1981). Data from the period 1977-1981 were used in this study. The nine stations belonging to the IMI network were selected from a total of about 30 long-term records and lack of known local contamination. The
960
CAROLINE LECK and HENNING RODHE
EMEP stations were chosen so as to represent welldefined geographical regions.
1O’E
15’E
ZO’E
ZS’E
Trends over the latest 30 years Rodhe and Granat (1984) analyzed data on SOi- in precipitation from 1955 to 1982 for a number of stations in northern Europe. Stations in Scandinavia generally exhibit an upward trend from the late 1950s to the early 1970s. This trend is in approximate agreement with the estimated increase in anthropogenie SO2 emissions in Europe during the same period (Fisher, 1984; EMEP, 1986), cf Fig. 1. From the early 1970s to 1982 the analysis by Rodhe and Granat (1984) shows a decreasing trend in SOicon~ntrat~on in pr~ipitation at most ~andinavian stations. The decrease is on average 20-2.5 per cent, cJ Fig. 1. Such a decrease is consistent with a reduction in SO, emissions that took place during the same period in several of those countries that contribute to the deposition of SOi- in Scandinavia, i.e. Belgium, Czechoslovakia, Denmark, G.D.R., Finland, France, The Netherlands, Norway, Poland, Sweden, U.K. and West Germany (Fisher, 1984; EMEP, 1986). During the period 1955-1982 the precipitation amounts in Scandinavia remained essentially constant. Thus, the deposition of SO:- exhibits a pattern similar to that of the concentration shown in Fig. 1. in order to investigate in more detail the changes in SOi- deposition that have taken place during the past 15 years, i.e. the period from which we have the most complete precipitation chemistry data (Granat, 1987) we have analyzed SO:- data from nine Swedish IMI stations. The locations of the stations are shown in Fig. 2. In Fig. 3 we show exampIes of temporal variations of the annual average concentration (votume weighted averages) of SO:- in precipitation over
&lfotc plf
Suif ur dioxide emissions ~O*t~nn~~/y~arl -x-
cont. 1)
--*-100
LO
30
20 50
10
0
I
1950-54
I
55-59
I
al-64
1
65-69
I
70-74
I
75-79
I
ao-84
Fig. 1. Long-term changes in the concentration of excess (non-sea&t) sulfate in precipitation in Scaudinavia (Rodhe and Granat, 1984j aud ~th~~~ SO, emissions in Europe (see text for details).
0
j0’N
i5’N
Fig. 2. Precipitation sampiing stations in Sweden used in the analysis shown in Table 1.
the period 1972-1986. In the early 1980s the precipitation collectors were changed from bulk to wet-only at these stations, Parallel sampling was carried out at most stations for 5 years. No systematic diBe=ces were observed (Granat, 1988). Linear regression analysis was performed on annual average values of SOi- concentration as a function of time from each of the nine stations. A summary of the results is shown in Table 1. It is seen that the concentration values show a negative trend at al1 nine stations. The median value of the slopes is - 1.6 peq e- 1a- ’ (or in relative units about - 2.4% a- I). It would seem logical to focus on deposition values, as opposed to concentration when comparing with the emission trends (Farmer et al., 1987). However, if the deposition values are based on water volumes measured in precipitation chemistry gauges they are not always representative of the true precipitation volumes. Even if the correct precipitation volume were measured at each site, there is the additional complication that this quantity exhibits spatial variations that are considerably larger than those of the ~n~a~on values (Granat, 1988). In order to avoid these difficulties we have estimated the trends in deposition by first calculating an area1 average annual precipitation for an area in south central Sweden including the sites HA, Fo, RK and Sn. These prer;ipitUion amounts, plotted in Fig. Care based on data from the Swedish Meteorol~~l and Hydrological Institute.
961
Anthropogenic SOs emissions and concentration of sulfate
1970
1975
1980
1985
1990
50
1970
1975
1980
1985
1990
Fig. 3. Annual average concentration of sulfate @q d-r) in precipitation at stations Ap, Sn, Brand Ra over the period 1972-1986.
Table 1. Linear regression analysis of the concentration 1972-1986
Station LK Rtl :A Fo RK
Sn Gn AP
Number of years 11 15 15 15* 15 15 15 15 15
Average cont. (peq d-‘) 57 61 40 68 64 69 65
of sulfate in precipitation,
Changes in cont. (peq d-r a-‘) (% a-‘) -1.4 -1.9 -1.3 -2.3 -2.3 -1.4 -1.6 -1.1 - 1.8
-2.5 -3.1 -3.3 - 3.4 -3.6 -2.0 -2.5 -1.4 -2.0
Correlation (R’) 0.21 0.50 0.49 0.50 0.64
0.23 0.53 0.11 0.40
* One year missing.
The SO:- deposition in this area is then calculated by multiplying the precipitation amounts with the average of the concentration values at the four sites (Fig 4). It is seen from Fig 4 that because of an upward trend in precipitation during this period the decrease in SOi- deposition is less pronounced than the decrease
in SO:- concentration. The annual sulfate deposition decreases by about 0.56meqm-2a-’ or 1.4% a-‘, We have used the statistics collected within the EMEP program on anthropogenic emissions of S in the various European countries (Eliassen et al., 1988; OECD, 1977) and a country to country matrix
962
CAROLINELECKand
‘I Y-Q-&__
HENNING RODHE
The result of this calculation is that the Swedish wet deposition of SO:- should have decreased by ap-
proximately 20% during this 13 year period. The largest fraction of this decrease-about halfshould be due to SO, emission reductions in Sweden. Significant positive effects are due also to reductions in Denmark, U.K. and F.R.G. This result compares favorably with the observed change in annual deposition of 1.4% per year or 18% over a 13 year period. Seasonal variations
1970
1975
1980
1985
1990
i970
1975
19.80
1985
19so
Since anthropogenic S emissions over Europe vary systematically with the season it is tempting to use observed seasonal changes in the concentration and deposition of SO, and SOi- to investigate how changes in emissions influence the deposition rates. The analysis of daily observations of SO, in air and SOi- in aerosol particles obtained from the EMEP network (EMEP, 1981) is shown in Fig. 5. Three groups of stations were considered based on distance
[ HAFORKS”)
__ 10
0so;
5i
Fig. 4. Area1 average values of sulfate concentration @eq C-r) in precipitation, precipitation amounts (mm) and wet deposition of sulfate (meq m-’ a- ‘) at stations HA, Fo, RK and Sn over the period 1972-1986.
on wet SOi- deposition, kindly provided by the Norwegian Meteorological Institute, to estimate the change in SOi- wet deposition that would have been expected from 1973 to 1985, if the relation between emission and wet deposition had been proportional.
Fig. 5. Monthly mean concentrations of daily observations of SO, (IreSme3) and SOi- OlgSmA3) in air obtained from the EMEP-network, 1977-1981. Ranges represent standard de&ions divided by tire square root of the number of observations.
Anthropogenic SO, emissions and concentration of sulfate from the main source area on the European continent,
c$ Fig. 6. The concentration of SO, in air exhibits a pronounced seasonal pattern in qualitative agreement with variations in anthropogenic emissions of SO, in Europe (OECD, 1977). A niarked decrease in concentration away from the European Continent is also evident. The reason for the pronohnced peak in SO2 concentration in March at the northernmost stations (group 3) is probably a maximum in the frequency of occurrence of air flows from the European continent during this season (Heintzenberg and Larsson, 1983). The concentration of SO:- exhibits a less pronounced seasonal cycle, especially at the southernmost stations (group 1). This probably reflects a slower oxidation rate of SO, during winter. A comparison between observed seasonal changes such as in Fig. 5, changes in the concentration of SO:in precipitation, and changes in SOi- deposition is complicated by the changes that occur in temperature, mixing height, precipitation type and intensity, transport patterns etc. It is interesting to note that whereas the anthro~genic SO, emissions exhibit a pronounced maximum in winter, caused by the demand of energy for heating, the wet deposition within Europe is smaller in winter than in summer (Rodhe and Granat, 1984). This can be explained by lower precipitation amounts or lower scavenging efficiencies (or both) in winter. The total deposition, including dry deposition, is also likely to be smaller in winter than in summer (Rodhe and Granat, 1983). This is mainly because of less efficient deposition to dormant vegetation and to snow covered ground. As a result, a larger fraction of the S is transported away from the continent in winter (Rodhe and Granat, 1983). EIecause of these complications, it is not possible to draw any firm conclusions
Fig.6. Location of three groups (l-3) of stations (EMEPnetwork) based on distance from the main source area on tbe European continent.
963
from the observed seasonal variations about the relation between long-term changes in emission and in &position. Variations duriw the week Variations during the week represent yet another potential source of information regarding the relationship between emissions and deposition. The main problem in investigating such variations is the weak input signal. Anthropogenic emissions of S vary by only lO-20% between weekends and other days of the week, cf: Fig. 7. For this analysis, we used a set of daily observations of SOz in air and particulate SOi- obtained from the EMEP network (EMEP, 1981) as described above. Average values were calculated for each day of the week for winter (October-March) and summer (April-September), separately. The results are shown in Fig. 8. As expected, the variations arwgenerally small, most of the values being within 10% of the average. However, certain systematic variations do seem to occur. The SO, conoentration in group 1 during the summer season is lower on Saturdays and Sundays, in fair agreement with the emission pattern in Fig. I. In group 2, only Saturdays have concentrations significantly lower than average. The SO, pattern for the winter season is also quite well defined in groups 1 and 2. In group 3, the variation is less systematic, The tendency for low concentrations on Sudays and Mondays may be associated with a delay due to transport from the European Continent and southern Scandinavia. It should also be remembered that the data set for espeeiaily group 3 consists of only very few stations. The aerosol SO:- concentrations during summer, at least in groups 1 and 2, are also lowest during the weekends. This may indicate a rather rapid transformation of SOz to SOi- during this season. In winter the situation is less clear. Most of the variations during the week are within the standard deviation of the individual days divided by the square root of the
Fig. 7. Annual averages of normalized anthropogenic emission rates of SO, during a week. Afinogenova and Cal&n (1981) (-A-), Fisher (personal communication 1985) (-O-), Levander (personal communication 1.985)(x --).
CAROLINE LECK and HENNING RODHE
964
P
I
z ;1 g 0.90 -
aeo-
summer group
1
W
wTF~SMTW
1.20 ,
F
S
S
M
T
W
1
4 8
T
502 1.107
-L‘q-L&
/
-i-u:.!
$y,y
;
1
z
L
winter group 2
summer group 2
WTFSSMTW
0.90
I WTFSSMTW
WTFSSt-tT’i
Fig 8. Normalized mean concentrations of SO1 and SOi- in air during a week for summer (April-September) and winter (October-March) obtained from the EMEP-network, 1977-1981. Ranges represent standard deviations divided by the square root of the number of observations.
CONCLUSIONS
tion and deposition of SOi- in northern Europe. It is still an open question whether such approximate pro~rtion~~ty will be sustained in the future if the chemical climate of the region is substantially modified through large changes (decreases or increases) in the emissions of S compounds, N compounds or hy~~rbons.
observed variations of SOi- in air and in precipitation with variations in anthropogenic emissions of SO,, we conclude that during the past 10-15 years there has been a reasonable agreement between anthropogenic emission and the concentra-
Acknowledgements-We are grateful to Lennart Granat for providing data and useful information and to Mark Rood and Paui Schlyter for carrying out some of the statistical analyses. We also thank CoNeaguesin the Department for comments on an early version of the manuscript. The figures have been drafted by Ulla Jonsson.
of observations. The slower transformation rate in winter probably obscures the signal defined by variations in SO, emissions.
number
By comparing
965
Anthropogenic SO, emissions and concentration of sulfate
W
T
F
S
S
M
T
W
summer group 2
=
0.80 ‘iJTFS;MTW
W
T
F
S
S
M
T
W
1.20 ‘ii 51.10,
summer qroup 3
SO;
cl -I-
T
-r-
I
winter group 3 W
T
F
S
S
M
T
W
Fig. 8 (Contd).
REFERENCES
Ahogenova 0. G. and Galpzrin M. V. (1981) Temporal nonuniformity of SO, emissions to the atmosphere in European countries with moderate climate. EMEP/MSC-E Technical Report i/81. Co-operative Programme for Monitoring and Evaluation of Long-Range Transmission of Air Pollution in Europe. Meteorological Synthesizing Centre-East (MSC-E). BetteIheim I. and Littler A. (1979) Historical trends of sulfur dioxide emissions in Europe since 1865. Report PLCXf/1/79. Central Electricity Generating Board, EngClark Ib. A., Fisher B. R. A. and Striven R. A. (1987) The wet deposition of sulphste and its relationship to sulphur
dioxide emissions. Atmmpherie ~nv~ro~~at 21, 11254131. Eliassen A., Pressman A., Saltbones J. and Galperin M. (1988) Summarized calculation results of transboundary deposition for 1980 and 1983-1986. Report Co-operative Programme for Monitoring and Evaluation of the longrange Transmission of Air Pollutants in Europa (EMEP). EMEP (1981) Co-operative program for monitoring and evaluationof the long-rangetr~s~~on of air pollutants in Europe. EMEP sampling stations site descriptions. EMEP/CCC Report l/81. Norwegian Institute for Air Research, Lillestrm Norway. EMEP (1986) Co-operative programme for monitoring and evaluation oflong-range transmission of air pollution in Europe. Sulphur budgets for Europe for 1979,1980,1983
966
CAROLINELECK and HENNING RODHE
and 1984. EMEP/MSC-W Note. The Norwegian Meteorological Institute, OsIo. Farmer G., agile R. J., Davies T. D., B~b~orn~ P. and Kellv P. M. f19871 RelationliDs between conctmtration andhepositi~n ok nitrate andsulphate in precipitation. Nature 328, 787-789. Fisher B. E. A. (1984) The long-range transport of air pollutants-some thoughts on the state of modeling. Atmosphereric Enoirmment 1% 553-562. Granat L. (1987) Deposition measurements in Sweden. Commission of the European communities. In P~~sieo-C~em~cal Behaviour of Atmospheric Pollutants (edited by G. Angeletti and G. Restelli), pp. 4W69. D. Reidel, Dordrecht. Granat L. (1988) Concentration gradients in atmospheric precipitation in areas of high a&ma1 precipitation. Prooeedinas of NATO Advanced Workshou on Acid Demosition Processes at High Elevated Sites,.~inbur~, 8112 September 1986. Granat L. (1988) The air and precipitation chemistry network within PMK. Report from activities during 1987. The Swedish National Environment Protection Board. PMK Report 3475. Heintxenberg J. and Larsson S. (1983) SO* and SOi- in the Arctic interpretation of observations at three Norwegian A~ti~u~~tie stations. defer 35B, 255-265. Nilsson J. (ed.) (1986) Critical loads for sulphur and nitrogen.
Nordic Council of Ministers, Environmental Report 1986: II. NRC (1983) Acid deposition; atmosphe~c processes in North America. U.S. National Research Council. National Academy Press, Washington, DC. OECD (1977) The OECD program on long-range transport of air pollutants, measurements and findings. OECD, Paris. Oppenheimer M. (1983a) The relationship of sulfur emissions to sulfate in precipitation. Atmospheric ~~uiro~nt 17, 451-460. Oppenheimer M. (1983b) The relationship of sulfuremissions to sulfate in precipitation-II. Gasphase processes. Atmospheric Environment 17, 1489-1495. Oppenheimer M. (1984) The relationship of sulfur emissions to sulfate in precipitation-III. Subregional budget analysis. Atmospheric Environment 1% 403-408. Rodhe H., Crutzen P. and Vanderpol A. (1981) Formation of sulfuric and nitric acid in the atmosphere during longrange transport. Telius 33, 132-141. Rodhe H. and Granat L. (1983) Summer and winter budgets for sulfur over Europe: an indication of large seasonal variations of residence time. ldiijaras 87, 1-6. Rodhe H. and Granat L. (1984) A~Ievaluation of sulfate in European precipitation 1955-1982. Atmospheric Enuironment l&2619-2627.
Atmofghnic Entlironment Vol. 23, No. 5, pp. 959~!+56,19EV. Printed in Great Brit&.
s3m+aoo Pergatnon Press pk
ON THE RELATION BETWEEN ANTHROPOGENIC SO, EMISSIONS AND CONCENTRATION OF SULFATE IN AIR AND PRECIPITATION CAROLINE LECK and HENNING RODHE Department of Meteorology, University of Stockholm, S-106 91 Stockholm, Sweden (First received 28 January 1988 and receiwdfor indication
27 October 1988)
Abatraet-The relationship between anthropogenic SO, emissions and the concentration of sulfate in air and precipitation has been investigated by comparing variations over the years, during the year and during the week. A 50 per cent increase in sulfate concentration in Swedish pr~ipitation between the late 1950s and the early 1970~3, followed by a 20-25 per cent decrease in concentration until the early 1980s agree well with estimated changes in anthropogenic SO, emissions in the countries that contribute to the deposition in Sweden. A more detailed analysis of sulfate in precipitation during the period 19724986 shows a decrease in concentration of about 40% and in deposition of about 20%. This decrease compares reasonably well with estimates based on model calculations of long-range transport and reported reductions in emissions in northern Europe. Variation of SO, concentrations during the week exhibits a systematic pattern in approximate agreement with the expected variation in emission rates. A reasonable agreement also exists for sulfate concentration in air during the summer months in regions close to the major emission areas. We conclude that there seems to be a qualitative agreement between changes in anthro~~nic SO2 emissions and changes in the concentration and deposition of sulfate in northern Europe. Key word index: Sulfate, sulfur dioxide, concentration, deposition, proportionality.
INTRODUCTION
It has been suggested that the relationship between anthropogenic emissions of SO, and the deposition of sulfate in precipitation may be significantly different from proportionality (NRC, 1983). This could be due to complex nonlinear chemical and physical transformation processes occurring in the atmosphere, or to the existence of large natural emissions. A better understanding of the relationship between emissions and deposition would make possible quantitative estimates of the size of emission reduction needed to achieve a specified deposition limit, such as those defiwd by Nilsson (1986). Another potential benefit of such knowledge might be inferences about mechanisms of atmospheric transformation processes. There are chemical reasons for deviations from a proportional relation between emissions and deposition. For example, changes in the inanitions of the OH radical caused by changes in the emissions of oxides of nitrogen may influence the rate of transformation of SO2 to SOi- and thereby the geographical distribution of S deposition (Rodhe et al., 1981). Similarly, an increase in acidity of cloud and rain water, caused by increased emissions of S and N compounds, may slow down the liquid phase transformation of SO2 A further discussion of factors involved with proportionality can be found in Clark et al. (1987). 959
These questions have previously been addressed by Op~nh~mer (1983a,b, 1984), He used model calculations of the S balance over eastern North America to conclude that uniform regional reduction in annual SO2 emissions would produce nearly proportional reductions in wet S deposition within that particular region. In the present study we have analyzed data for SO, and SO:- in air and SO:- in precipitation in relation to SO2 emissions over the years, during a year and during a week. The data base We use two different sets of atmospheric chemistry data. Monthly data on SOi- in precipitation from nine Swedish stations operated by the International Meteorolo~~l Institute in Stockholm (Rodhe and Granat, 1984), hereafter referred to as the IMI-data. A few stations’ data are available since 1955. Daily data on aerosol SO:- and SO, in air from about 30 stations in northern Europe operated by national institutions as part of the EMEP program (EMEP, 1981). Data from the period 1977-1981 were used in this study. The nine stations belonging to the IMI network were selected from a total of about 30 long-term records and lack of known local contamination. The
960
CAROLINE LECK and HENNING RODHE
EMEP stations were chosen so as to represent welldefined geographical regions.
1O’E
15’E
ZO’E
ZS’E
Trends over the latest 30 years Rodhe and Granat (1984) analyzed data on SOi- in precipitation from 1955 to 1982 for a number of stations in northern Europe. Stations in Scandinavia generally exhibit an upward trend from the late 1950s to the early 1970s. This trend is in approximate agreement with the estimated increase in anthropogenie SO2 emissions in Europe during the same period (Fisher, 1984; EMEP, 1986), cf Fig. 1. From the early 1970s to 1982 the analysis by Rodhe and Granat (1984) shows a decreasing trend in SOicon~ntrat~on in pr~ipitation at most ~andinavian stations. The decrease is on average 20-2.5 per cent, cJ Fig. 1. Such a decrease is consistent with a reduction in SO, emissions that took place during the same period in several of those countries that contribute to the deposition of SOi- in Scandinavia, i.e. Belgium, Czechoslovakia, Denmark, G.D.R., Finland, France, The Netherlands, Norway, Poland, Sweden, U.K. and West Germany (Fisher, 1984; EMEP, 1986). During the period 1955-1982 the precipitation amounts in Scandinavia remained essentially constant. Thus, the deposition of SO:- exhibits a pattern similar to that of the concentration shown in Fig. 1. in order to investigate in more detail the changes in SOi- deposition that have taken place during the past 15 years, i.e. the period from which we have the most complete precipitation chemistry data (Granat, 1987) we have analyzed SO:- data from nine Swedish IMI stations. The locations of the stations are shown in Fig. 2. In Fig. 3 we show exampIes of temporal variations of the annual average concentration (votume weighted averages) of SO:- in precipitation over
&lfotc plf
Suif ur dioxide emissions ~O*t~nn~~/y~arl -x-
cont. 1)
--*-100
LO
30
20 50
10
0
I
1950-54
I
55-59
I
al-64
1
65-69
I
70-74
I
75-79
I
ao-84
Fig. 1. Long-term changes in the concentration of excess (non-sea&t) sulfate in precipitation in Scaudinavia (Rodhe and Granat, 1984j aud ~th~~~ SO, emissions in Europe (see text for details).
0
j0’N
i5’N
Fig. 2. Precipitation sampiing stations in Sweden used in the analysis shown in Table 1.
the period 1972-1986. In the early 1980s the precipitation collectors were changed from bulk to wet-only at these stations, Parallel sampling was carried out at most stations for 5 years. No systematic diBe=ces were observed (Granat, 1988). Linear regression analysis was performed on annual average values of SOi- concentration as a function of time from each of the nine stations. A summary of the results is shown in Table 1. It is seen that the concentration values show a negative trend at al1 nine stations. The median value of the slopes is - 1.6 peq e- 1a- ’ (or in relative units about - 2.4% a- I). It would seem logical to focus on deposition values, as opposed to concentration when comparing with the emission trends (Farmer et al., 1987). However, if the deposition values are based on water volumes measured in precipitation chemistry gauges they are not always representative of the true precipitation volumes. Even if the correct precipitation volume were measured at each site, there is the additional complication that this quantity exhibits spatial variations that are considerably larger than those of the ~n~a~on values (Granat, 1988). In order to avoid these difficulties we have estimated the trends in deposition by first calculating an area1 average annual precipitation for an area in south central Sweden including the sites HA, Fo, RK and Sn. These prer;ipitUion amounts, plotted in Fig. Care based on data from the Swedish Meteorol~~l and Hydrological Institute.
961
Anthropogenic SOs emissions and concentration of sulfate
1970
1975
1980
1985
1990
50
1970
1975
1980
1985
1990
Fig. 3. Annual average concentration of sulfate @q d-r) in precipitation at stations Ap, Sn, Brand Ra over the period 1972-1986.
Table 1. Linear regression analysis of the concentration 1972-1986
Station LK Rtl :A Fo RK
Sn Gn AP
Number of years 11 15 15 15* 15 15 15 15 15
Average cont. (peq d-‘) 57 61 40 68 64 69 65
of sulfate in precipitation,
Changes in cont. (peq d-r a-‘) (% a-‘) -1.4 -1.9 -1.3 -2.3 -2.3 -1.4 -1.6 -1.1 - 1.8
-2.5 -3.1 -3.3 - 3.4 -3.6 -2.0 -2.5 -1.4 -2.0
Correlation (R’) 0.21 0.50 0.49 0.50 0.64
0.23 0.53 0.11 0.40
* One year missing.
The SO:- deposition in this area is then calculated by multiplying the precipitation amounts with the average of the concentration values at the four sites (Fig 4). It is seen from Fig 4 that because of an upward trend in precipitation during this period the decrease in SOi- deposition is less pronounced than the decrease
in SO:- concentration. The annual sulfate deposition decreases by about 0.56meqm-2a-’ or 1.4% a-‘, We have used the statistics collected within the EMEP program on anthropogenic emissions of S in the various European countries (Eliassen et al., 1988; OECD, 1977) and a country to country matrix
962
CAROLINELECKand
‘I Y-Q-&__
HENNING RODHE
The result of this calculation is that the Swedish wet deposition of SO:- should have decreased by ap-
proximately 20% during this 13 year period. The largest fraction of this decrease-about halfshould be due to SO, emission reductions in Sweden. Significant positive effects are due also to reductions in Denmark, U.K. and F.R.G. This result compares favorably with the observed change in annual deposition of 1.4% per year or 18% over a 13 year period. Seasonal variations
1970
1975
1980
1985
1990
i970
1975
19.80
1985
19so
Since anthropogenic S emissions over Europe vary systematically with the season it is tempting to use observed seasonal changes in the concentration and deposition of SO, and SOi- to investigate how changes in emissions influence the deposition rates. The analysis of daily observations of SO, in air and SOi- in aerosol particles obtained from the EMEP network (EMEP, 1981) is shown in Fig. 5. Three groups of stations were considered based on distance
[ HAFORKS”)
__ 10
0so;
5i
Fig. 4. Area1 average values of sulfate concentration @eq C-r) in precipitation, precipitation amounts (mm) and wet deposition of sulfate (meq m-’ a- ‘) at stations HA, Fo, RK and Sn over the period 1972-1986.
on wet SOi- deposition, kindly provided by the Norwegian Meteorological Institute, to estimate the change in SOi- wet deposition that would have been expected from 1973 to 1985, if the relation between emission and wet deposition had been proportional.
Fig. 5. Monthly mean concentrations of daily observations of SO, (IreSme3) and SOi- OlgSmA3) in air obtained from the EMEP-network, 1977-1981. Ranges represent standard de&ions divided by tire square root of the number of observations.
Anthropogenic SO, emissions and concentration of sulfate from the main source area on the European continent,
c$ Fig. 6. The concentration of SO, in air exhibits a pronounced seasonal pattern in qualitative agreement with variations in anthropogenic emissions of SO, in Europe (OECD, 1977). A niarked decrease in concentration away from the European Continent is also evident. The reason for the pronohnced peak in SO2 concentration in March at the northernmost stations (group 3) is probably a maximum in the frequency of occurrence of air flows from the European continent during this season (Heintzenberg and Larsson, 1983). The concentration of SO:- exhibits a less pronounced seasonal cycle, especially at the southernmost stations (group 1). This probably reflects a slower oxidation rate of SO, during winter. A comparison between observed seasonal changes such as in Fig. 5, changes in the concentration of SO:in precipitation, and changes in SOi- deposition is complicated by the changes that occur in temperature, mixing height, precipitation type and intensity, transport patterns etc. It is interesting to note that whereas the anthro~genic SO, emissions exhibit a pronounced maximum in winter, caused by the demand of energy for heating, the wet deposition within Europe is smaller in winter than in summer (Rodhe and Granat, 1984). This can be explained by lower precipitation amounts or lower scavenging efficiencies (or both) in winter. The total deposition, including dry deposition, is also likely to be smaller in winter than in summer (Rodhe and Granat, 1983). This is mainly because of less efficient deposition to dormant vegetation and to snow covered ground. As a result, a larger fraction of the S is transported away from the continent in winter (Rodhe and Granat, 1983). EIecause of these complications, it is not possible to draw any firm conclusions
Fig.6. Location of three groups (l-3) of stations (EMEPnetwork) based on distance from the main source area on tbe European continent.
963
from the observed seasonal variations about the relation between long-term changes in emission and in &position. Variations duriw the week Variations during the week represent yet another potential source of information regarding the relationship between emissions and deposition. The main problem in investigating such variations is the weak input signal. Anthropogenic emissions of S vary by only lO-20% between weekends and other days of the week, cf: Fig. 7. For this analysis, we used a set of daily observations of SOz in air and particulate SOi- obtained from the EMEP network (EMEP, 1981) as described above. Average values were calculated for each day of the week for winter (October-March) and summer (April-September), separately. The results are shown in Fig. 8. As expected, the variations arwgenerally small, most of the values being within 10% of the average. However, certain systematic variations do seem to occur. The SO, conoentration in group 1 during the summer season is lower on Saturdays and Sundays, in fair agreement with the emission pattern in Fig. I. In group 2, only Saturdays have concentrations significantly lower than average. The SO, pattern for the winter season is also quite well defined in groups 1 and 2. In group 3, the variation is less systematic, The tendency for low concentrations on Sudays and Mondays may be associated with a delay due to transport from the European Continent and southern Scandinavia. It should also be remembered that the data set for espeeiaily group 3 consists of only very few stations. The aerosol SO:- concentrations during summer, at least in groups 1 and 2, are also lowest during the weekends. This may indicate a rather rapid transformation of SOz to SOi- during this season. In winter the situation is less clear. Most of the variations during the week are within the standard deviation of the individual days divided by the square root of the
Fig. 7. Annual averages of normalized anthropogenic emission rates of SO, during a week. Afinogenova and Cal&n (1981) (-A-), Fisher (personal communication 1985) (-O-), Levander (personal communication 1.985)(x --).
CAROLINE LECK and HENNING RODHE
964
P
I
z ;1 g 0.90 -
aeo-
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1
W
wTF~SMTW
1.20 ,
F
S
S
M
T
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502 1.107
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Fig 8. Normalized mean concentrations of SO1 and SOi- in air during a week for summer (April-September) and winter (October-March) obtained from the EMEP-network, 1977-1981. Ranges represent standard deviations divided by the square root of the number of observations.
CONCLUSIONS
tion and deposition of SOi- in northern Europe. It is still an open question whether such approximate pro~rtion~~ty will be sustained in the future if the chemical climate of the region is substantially modified through large changes (decreases or increases) in the emissions of S compounds, N compounds or hy~~rbons.
observed variations of SOi- in air and in precipitation with variations in anthropogenic emissions of SO,, we conclude that during the past 10-15 years there has been a reasonable agreement between anthropogenic emission and the concentra-
Acknowledgements-We are grateful to Lennart Granat for providing data and useful information and to Mark Rood and Paui Schlyter for carrying out some of the statistical analyses. We also thank CoNeaguesin the Department for comments on an early version of the manuscript. The figures have been drafted by Ulla Jonsson.
of observations. The slower transformation rate in winter probably obscures the signal defined by variations in SO, emissions.
number
By comparing
965
Anthropogenic SO, emissions and concentration of sulfate
W
T
F
S
S
M
T
W
summer group 2
=
0.80 ‘iJTFS;MTW
W
T
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S
M
T
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1.20 ‘ii 51.10,
summer qroup 3
SO;
cl -I-
T
-r-
I
winter group 3 W
T
F
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S
M
T
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Fig. 8 (Contd).
REFERENCES
Ahogenova 0. G. and Galpzrin M. V. (1981) Temporal nonuniformity of SO, emissions to the atmosphere in European countries with moderate climate. EMEP/MSC-E Technical Report i/81. Co-operative Programme for Monitoring and Evaluation of Long-Range Transmission of Air Pollution in Europe. Meteorological Synthesizing Centre-East (MSC-E). BetteIheim I. and Littler A. (1979) Historical trends of sulfur dioxide emissions in Europe since 1865. Report PLCXf/1/79. Central Electricity Generating Board, EngClark Ib. A., Fisher B. R. A. and Striven R. A. (1987) The wet deposition of sulphste and its relationship to sulphur
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CAROLINELECK and HENNING RODHE
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