6.1. Sulphur compounds

Mean: *) > 50 WHO Guidelines Max: *) >125 WHO Guidelines.

Trends

Figure 6.7-Figure 6.10 show, as examples, trends in the SO2 data on an annual basis for some selected stations in different countries. The figures show the maximum, 98 percentile and median of 24 h values from Norway, Denmark, the Netherlands, Spain and Greece. German data (from Bremen) show 98 percentile and mean values based on œ h values, while Swiss data show 95 percentile and mean values based on œ h data. Each of the trend figures covers a part of the period 1976-1994.

Most stations show a downward trend in SO2-concentrations. The downward trend is very clear in Norway, Denmark, the Netherlands and Switzerland. In Germany (Bremen) and Spain the downward trend is less pronounced. In Greece (Athens) top values have increased and median and mean values have been rather stable at a relatively high level compared to other countries.

For East Europe there are no trend data available to us.

Figure 6.7: SO2 trend in Norway 1977-1993 (µg/m3). Data from national reports.

Figure 6.8: SO2 trend in Aalborg, Denmark 1983-1994 and Amsterdam, the Netherlands, 1976-1994 (µg/m3), APIS data.

Figure 6.9: SO2 trend in Athens, Greece 1983-1993 and Madrid, Spain, 1986-1993 (µg/m3), APIS data.

Figure 6.10: SO2 trend in Bremen, Germany, 1987-1993 and from Switzerland, 1984-1993 (µg/m3). Data from Bremen State and Swiss reports. The figures are based on œ h values.

6.1.4. Regional sulphur oxides concentrations and deposition

The EMEP measurement programme started in 1978. There were 99 monitoring stations in operation in 1993 in 26 countries. 77 stations reported precipitation data and 92 stations reported air concentration data during 1993.

A short description of the EMEP programme, including station list, is given in Appendix C.

The following presentation is based directly upon the 1993 EMEP data report (EMEP, 1995).

Data presentation

Annual averages of the 1993 air and precipitation data are presented in maps. The yearly precipitation mean concentrations are calculated from the daily values as precipitation-weighted averages. Average air concentrations are arithmetic averages of the daily means.

Kriging procedure

The concentration fields have been calculated using kriging methods. The EMEP grid is divided in 9 subareas. During the kriging process each area consists of 11x11 grid elements. In addition, 3 grid elements were added on each side of the sub-area in order to get an overlap and thus obtain a smooth concentration field for the whole area. After calculation on each of the sub-areas, consisting of 17x17 grid-elements, the results for each of the original 11x11 elements were concatenated to one 33x33 element concentration field. One grid element is 150 km x 150 km. In the kriging process, the averages are expressed through random functions. Separate calculations on nine subgrids rather than one calculation over a large grid area reduces the effects of the systematic concentration differences present in Europe.

Concentrations

Sulphur dioxide

The EMEP sites have been located away from local sources and are as far as possible representative for a larger region. One consequence of this is that the sulphur dioxide concentrations in Europe is higher in industrialised regions than the concentrations shown in Figure 6.11.

The lowest concentrations of SO2 during 1993 are found on the Atlantic coast in Ireland and in northern Scandinavia, where the averages are lower than 0.5 µg S/m3. From the United Kingdom eastward the concentrations increase from mainly 1–2 µg S/m3 to the highest SO2 concentrations measured in the network, in the area of eastern Germany, southern Poland, the Czech Republic, Slovakia and the north eastern part of Austria. In this region the annual average of SO2 in rural areas is above 5 µg S/m3, and peaks around 10 µg S/m3 are seen at one German and one Czech station. In Germany and Poland the concentrations decrease towards the north.

The concentrations of SO2 over the western part of Russia, the Baltic and southern Scandinavia are in the range 1 to 2.5 µg S/m3. Romania and Bulgaria do not take part in the measurements, and there is only one site in Greece. For this reason the kriged concentrations in this area are very uncertain and are thus removed from Figure 6.11. The situation is similar in the southern half of Italy. Relatively low concentrations are seen over France and central Spain.

The regional concentrations in central Europe (>10 µg SO2/m3, annual average) are higher than in most cities of Scandinavia and parts of North-western Europe.

Sulphate in aerosol

Figure 6.12 presents the annual averages of sulphate in aerosols in 1993. Large parts of Ireland and Scandinavia have averages lower than 0.5 µg S/m3. Towards south and east the concentrations increase to 1–1.5 in southern England to Denmark, southern Sweden and the Baltic. Most parts of western Europe experience concentrations in the range 1–2 µg S/m3. The highest concentrations are observed over south-eastern Europe, more than 8 µg S/m3 at one station in Greece and around 4 µg S/m3 at one station in Hungary. However, the sites in this area are relatively few and the inaccuracies are thus large. In Portugal and Spain the averages increases from 1–1.5 µg S/m3 in the north to 1.5–2 µg S/m3 in the south.

Figure .11: Sulphur dioxide in rural areas 1993, annual average (µg S/m3).
Factor 2.0.

Figure .12: Sulphate in aerosols 1993, annual average (µg S/m3).
Factor 3.0.

Wet deposition

Sulphate in precipitation

Sulphate in precipitation, corrected for sea-salt contributions are presented in Figure 6.13. As for most other pollutants of anthropogenic origin, the lowest concentrations are seen at the Atlantic coasts of Portugal and Ireland and in northern Scandinavia. Most of the Continent is exposed to annual averages less than 1 mg S/l. As for sulphate in aerosol phase, the region with higher concentrations stretch over the eastern parts of Europe, around 1.5 mg S/l or higher at stations in Croatia, Hungary, Poland and Yugoslavia. There is also a station in Ireland with concentrations at the same level. In Portugal and Spain there is a west to east increasing gradient, with a concentration of about 1 mg/l in the eastern parts.

pH in precipitation

The annual averages of pH in precipitation are presented in Figure 6.14. Over most parts of Europe the pH is lower than 5. The lowest pH values are seen in Poland and in middle Sweden; below 4.4, equivalent to an acid concentration of 40 µeql. Towards southern Europe there is in general an increase of pH.

Calcium in precipitation

Both industrial emissions of fly ash and other mineral particles, and soil dust are sources for calcium in precipitation, Figure 6.15. The high concentrations in southern Europe are mainly derived from soil dust, from wind erosion in the Sahara desert, and from local sources. The calcium content in both soil and rocks is high in many places in southern Europe. In eastern Europe industrial emissions, e.g. from cement plants, steelworks and power plants, give rise to concentrations of 10-20 µg/l of Ca in precipitation, while the concentrations of calcium in precipitation is low in the northern part of Great Britain and in most of Scandinavia and Finland. Calcium and other base cations increase the pH of precipitation.

Figure 6.13: Sulphate in precipitation 1993 (mg S/l).

Figure 6.14: pH in precipitation 1993 (pH units).

Figure 6.15: Calcium in precipitation 1993 (µg Ca/l).

Trends for the years 1985-1993

The EMEP Meteorological Synthesizing Centre-West (The Norwegian Meteorological Institute) has calculated sulphur reduction trends with distance from major emission sources for the period 1985-1993.

The EMEP-monitoring network have been classified into three broad groups. Stations have been grouped geographically according to the average straight-line distances of transport of deposition. "Long distance" sites are those lying approximately 1000 km or more on average from deposition quantity weighted sources, "medium distance" sites are those at between 500 km and 1000 km, and "short distance" stations are those under 500 km on average from sources. For sulphur dioxide, airborne sulphate, and sulphate in precipitation average monitored and modelled concentrations are plotted. The selection criteria for sites is as standard, i.e. that model and monitored results must be available on 25% of common days for precipitation results, and that air quality results must be reported for 75% of days. The further constraint is that stations must satisfy the criteria during each of the years. The time period considered is from 1985 to the end of 1993 for SO2 and SO4= in air, with 1992 the final date for sulphate in precipitation due to a lower number of stations reporting results at time of analysis.

Figure 6.16-Figure 6.18 show observed and modelled levels and trends of SO2 in air, sulphate in air and sulphate in precipitation.

The main conclusions from this trend analysis, are:

• There is a downward trend for all the sulphur parameters.
• Totally, there has been a close to 50% reduction from 1985 to 1993.
• Both measured and modelled results show that the reduction has been largest for SO2 in air, about 60%.
• The reduction has been the smallest for SO4 in deposition at sites far away from source areas, about 35% reduction.

The analysis show some discrepancies between the measured and modelled results.

It should also be noticed that the trend analysis has been done on a selected and rather limited set of sites (as shown in the figures), selected based on the criteria mentioned above. No analysis has been published on possible differences in trend between various parts of Europe.

The reduced SO2 concentrations in regional air over Europe coincides well, on the macro scale, with the reduced SO2 emissions described earlier in this chapter.

Figure 6.16: Time trend comparison for SO2 in air across the EMEP network.
a) Short distances from sources (<~500 km).
b) Intermediate distance (~500-1000 km).
c) Longer distances from sources (>~1000 km).

Figure 6.17: Time trend comparison for SO4= in air across the EMEP network.
a) Short distances from sources (<~500 km).
b) Intermediate distance (~500-1000 km).
c) Longer distances from sources (>~1000 km).

Figure 6.18: Time trend comparison for SO4= in precipitation across the EMEP network.
a) Short distances from sources (<~500 km).
b) Intermediate distance (~500-1000 km).
c) Longer distances from sources (>~1000 km).