Modelling long-term trends in UK sulphur deposition

On the UK-scale, sulphur dioxide (SO2) dry deposition has declined at a fas ter 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 rele ase height of emissions; oxidation of SO2 may have been oxidant limited in previous years; the canopy resistance Of SO2 has changed in response to cha nging ammonia concentrations. Of these three hypotheses, the first has been tested here using a 10-layer Lagrangian acid deposition model. The model r eproduced the rate of decline of observed total S deposition but not the ma gnitude. The mismatch in magnitude between modelled and observed UK S depos ition is consistent with the model framework and its limitations. More impo rtantly, 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 se ts of UK emission data. The UK-derived emissions data show a small increase in the proportion of lowlevel SO2 sources to the total and a small decreas e in proportion of the high-level SO2 sources over the period 1989-1994. Th ese changes are not apparently consistent with observed changes in SO2 conc entrations nor changes in wet and dry S deposition. Thus, it is unsurprisin g that the modelling does not simulate the observed changes in fractions of wet and dry deposition. The current constraints of the modelling and the e missions data have the consequence that future scenarios of rates of change of total S deposition may be optimistic for locations that are dominated b y wet deposition, remote from sources. Crown Copyright (C) 2001 Published b y Elsevier Science Ltd. All rights reserved.