Trends in bulk deposition of acidity in the UK, 1988-2007, assessed using additive models

Previous analyses of deposition trends in the UK have used traditional linear methods, but recognised that neither emissions nor acid deposition had followed linear trends. Here we employ a non-linear technique, an additive model, to determine trends in both concentrations and bulk deposition loads (precipitation-weighted) at 12 Acid Deposition Monitoring Network (ADMN) sites most closely co-located with sites in the UK Acid Waters Monitoring Network (AWMN). Bulk deposition data were collected weekly or two-weekly over the 20 year period from 1988 to 2007 and all samples were analysed according to common protocols. Periods of significant increase or decrease over the period of monitoring were identified using the first derivative of the fitted trend, computed using finite differences. Results of the trend analysis show that:11 out of 12 bulk deposition sites show significant increasing trends in pH.Concentrations of non-marine sulphate (xSO₄^2-) show significant decreasing trends at ten out of 12 sites and appear to be the main driver of changes in bulk deposition pH since the late 1990s, though earlier trends in bulk deposition pH are so far unexplained.Trends in concentrations of nitrogen (N) species are mixed with many sites showing no significant trend. For NO₃^- three sites show a general decline while four sites show a short period of significant increase in the early 1990s which in three cases reverses to a significant declining trend in the late 1990s. For NH ₄⁺ five sites show some periods of significant decline in concentration but these are only prolonged at three sites.Precipitation- corrected deposition loads show very similar trends to concentrations for xSO₄^2- and N species, but for N species there are more sites with significant trends.For total Cl^- concentrations, seven sites show declining trends but only four of these remain significant with precipitation weighting. Non-marine chloride (xCl^-) concentrations decline significantly at nine sites, reducing to six sites with precipitation weighting. Hence reductions in xCl^- are acting alongside xSO ₄^2- as drivers of declining acidity in bulk deposition.Small declining trends in precipitation measured in the bulk deposition collectors may reflect changes in sampling methodology as sampling frequency changed from weekly to 2-weekly during the ADMN monitoring period. These non-linear trends explain the lack of significant trends using linear methods on shorter data series in previous analyses and may help to explain non-linear patterns in chemical recovery in surface waters.