Temporal trends in sulphate concentrations at European sites and relationships to sulphur dioxide

Temporal trends in sulphate data taken from UK networks from the period 2001–2008 have been examined, together with trends in relevant precursor gases. In general, trends in sulphate are small, and the data sets are not of sufficient length to determine the direction of trend with confidence. Since relatively short periods of high or low concentration near to the start or finish of the period have a disproportionate influence, the choice of period over which the trend is calculated is crucial to the outcome. All six sites showed a significant reducing trend in sulphur dioxide, while ammonia data appear to be affected by sampling problems and site relocations and clear trends are not apparent. Data relating annual mean airborne concentrations of sulphur dioxide and sulphate from several countries can be related through a relationship of the form: χ [SO42−] = a·χ [SO2]b + c in which a, b and c are constants and χ represents concentrations. While constant b remains the same for different countries, a and c can change in ways that appear to relate to either the distance from major SO2 sources, or the oxidising capacity of the atmosphere. Using the relationship between SO42− and SO2 derived from UK sites allows estimation of the reduction in sulphur dioxide emissions affecting UK sites needed to reduce sulphate concentrations by 1 μg m−3. This is 55% and 49% for Harwell and North Kensington respectively.

Research highlights
► Trends in sulphate from 2001–2008 are small and variable between sites.
► Trends in sulphur dioxide are consistently downward.
► The SO42−:SO2 relationship is curvilinear.
► Around 50% reduction in SO2 emissions will deliver 1 μg m−3 reduction in SO42− in the southern UK.