Modelling long-term trends in UK sulphur deposition

On the UK-scale, sulphur dioxide (SO₂) dry deposition has declined at a faster rate than SO₂ emissions, but S wet deposition has declined at a slower rate than emissions. Three hypotheses have been put forward to explain this non-linear response to changes in emissions: changes in the effective release height of emissions; oxidation of SO₂ may have been oxidant limited in previous years; the canopy resistance of SO₂ has changed in response to changing ammonia concentrations. Of these three hypotheses, the first has been tested here using a 10-layer Lagrangian acid deposition model. The model reproduced the rate of decline of observed total S deposition but not the magnitude. The mismatch in magnitude between modelled and observed UK S deposition is consistent with the model framework and its limitations. More importantly, the model did not reproduce the changes in the proportions of wet and dry deposition to the UK that have been observed using two different sets of UK emission data. The UK-derived emissions data show a small increase in the proportion of low-level SO₂ sources to the total and a small decrease in proportion of the high-level SO₂ sources over the period 1989–1994. These changes are not apparently consistent with observed changes in SO₂ concentrations nor changes in wet and dry S deposition. Thus, it is unsurprising that the modelling does not simulate the observed changes in fractions of wet and dry deposition. The current constraints of the modelling and the emissions data have the consequence that future scenarios of rates of change of total S deposition may be optimistic for locations that are dominated by wet deposition, remote from sources.