The influence of long term trends in pollutant emissions on deposition of sulphur and nitrogen and exceedance of critical loads in the United Kingdom
Introduction
Acid deposition, originating mainly from man made emissions of SO2, NOx and NH3 has had significant ecological and economic consequences, especially during the last two decades of the 20th century, affecting forests, soil and freshwater ecosystems in large areas of Europe (Posch et al., 1997, EEA, 1998, Berge et al., 1999, Davies et al., 2004, Vuorenmaa, 2004, Błaś et al., 2008, Fagerli and Aas, 2008). The total area affected by exceedance of acidifying sulphur and nitrogen on a European scale was about 20% in the mid eighties (Posch et al., 1997, Berge et al., 1999, Mill et al., 2003).
In many countries depositions and concentrations to a large extent originate from sources outside the countries themselves and it is necessary to assess the changes on a larger scale, taking into account the transboundary fluxes (Fowler et al., 2007a, Fowler et al., 2007b). Hence the problems of acidification have been addressed internationally by the 1979 Geneva Convention on Long Range Trans-boundary Air Pollution (CLRTAP) and two protocols targeting sulphur emission reductions have been signed in Europe (Berge et al., 1999). As a result of international pollutant abatement policy and structural changes in industrial sector, substantial reductions in gaseous emissions have been observed, with SO2 being reduced most significantly. Land-based sulphur emissions from 1970 to 2005 declined by between 90% and 70% depending on the region of Europe (Fowler et al., 2007a, Fowler et al., 2007b). Larger reductions were found in the area of the former Soviet Union and western Europe than in central eastern Europe (Berge et al., 1999, Mitosek et al., 2004). For oxidized and reduced nitrogen the overall European reductions from 1980 to 2003 were between 20% and 50%. Both oxidized and reduced nitrogen emissions fell more in eastern Europe than in western Europe (Fowler et al., 2005, Fagerli and Aas, 2008).
Air pollution emissions of SO2 and NOx have decreased in the UK significantly during recent decades due to active control measures and economic changes. There were large changes in the power industry sector where coal was changed for gas and abatement facilities were installed. The deposition of sulphur and nitrogen in the UK has declined since the peak in emission in 1970 by 50% and 16%, respectively (NEGTAP, 2001, Fowler et al., 2005). Emissions of NH3 have changed little since the peak emissions in the mid 1980s, but a decline of 12% relative to 1990 is expected by 2010 (EMEP, 2006).
Here attention is focused on non-linearities in the relationship between the emission and deposition changes. The national reduction in deposition is clearly smaller than the reduction in emission and it also leads to slower reductions in critical loads exceedance in remote areas than may be expected (NEGTAP, 2001, Irwin et al., 2002, Fournier et al., 2004). Non-linearities for sulphur emission–deposition patterns in the UK can be explained by increases in emissions from international shipping, changes in atmospheric oxidation rates and complex interactions between the different pollutants which can influence deposition rates (i.e. the co-deposition of NH3 and SO2; Fowler et al., 2007a, Fowler et al., 2007b). Mayerhofer et al. (2002) concluded that for regional air pollution the development of the air pollutant emissions is more important than the effect of climate change on the dispersion and chemical transformation of air pollutants. Fagerli and Aas (2008) investigated the role of sulphur in the emission–deposition relationship of oxidized and reduced nitrogen and to what extent the reductions of SO2 emissions have influenced the trends of the nitrogen compounds.
The UK Lagrangian trajectory models such as HARM (Metcalfe et al., 2001), TRACK (Lee et al., 2000) and FRAME (the Fine Resolution Atmospheric Multi-pollutants Exchange model; Singles et al., 1998) have been developed to assess acid deposition to sensitive areas. These models use a spatial emissions inventory and give deposition at grid squares throughout the UK at a 5 km × 5 km resolution.
In this paper we consider recent trends in emissions of SO2, NOx and NH3 and compare the model with measurements from the UK national monitoring network for wet deposition of sulphate, nitrate and ammonium during the period 1990–2005. A longer time series covering a 50-year period from 1970, the earliest year for which a detailed emissions inventory for the UK is available, to year 2020, based on detailed emissions projections, is considered and an assessment of the associated changes in nitrogen and sulphur deposition and exceedance of acid and nitrogen critical loads for acid deposition and nutrient nitrogen deposition.
Section snippets
FRAME model description
A detailed description of the Fine Resolution Atmospheric Multi-pollutant Exchange model is provided by Singles et al. (1998), Fournier et al. (2005) and Dore et al. (2007). FRAME is a statistical atmospheric transport model that can be used to estimate the spatial distribution of sulphur and nitrogen deposited to the United Kingdom. FRAME simulates the main atmospheric processes (emission, diffusion, chemical transformations and deposition) taking place in a column of air moving along
Results and discussion
Modelled wet deposition results were compared against measured values. All the correlations are statistically significant (p-value < 0.05) as well as slopes and intercept value for linear regression are reasonable. The results for SO42−, give a correlation coefficient in the range of 0.60–0.83. Similar correlation coefficients are obtained for NH4+ and NO3− but with better results for intercept values.
For SO42− and NO3− mean bias and mean absolute error are higher at the beginning of the analyzed
Conclusion
An atmospheric transport model has been applied to assess the influence of reductions in emissions of pollutant gases (SO2, NOx, NH3) on sulphur and nitrogen deposition in the UK during a 50-year time period (1970–2020). The results of the model were compared with measurements of wet deposition (SO42−, NO3−, NH4+) from the UK national monitoring network covering the period 1990–2005. Both the model and the measurements showed that reductions in deposition during this 15-year period were less
Acknowledgements
This work was funded by the UK Department of the Environment, Food and Rural Affairs and the Natural Environment Research Council.
Małgorzata Matejko is a PhD student in the Department of Climatology and Atmosphere Protection at the University of Wroclaw. Her research interests include modelling the long-range transport, concentration and deposition of atmospheric pollutants.
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Małgorzata Matejko is a PhD student in the Department of Climatology and Atmosphere Protection at the University of Wroclaw. Her research interests include modelling the long-range transport, concentration and deposition of atmospheric pollutants.
Anthony Dore is an air pollution modeller at the Centre for Ecology in Edinburgh. He specialises in national scale computer simulation of pollutant deposition and has a particular interest in nitrogen and sulphur deposition.
Jane Hall is a senior scientific officer at CEH Bangor. Her main area of research is the development and application of critical loads and dynamic models to study the impacts of acidification, eutrophication and heavy metals on sensitive terrestrial and freshwater habitats.
Chris Dore is an independent consultant on air quality and pollutant emissions, and previously led the UK National Atmospheric Emissions Inventory. He chairs the UNECE Task Force on Emissions Inventories, and provides environmental policy support to Governments.
Marek Blas is a lecturer and scientist in the Department of Climatology and Atmosphere Protection at the University of Wroclaw. He studied meteorology and climatology science at Wroclaw University and is currently involved in research on mountain climatology, wet deposition of atmospheric pollutants and fog.
Maciej Kryza is a lecturer and scientist in the Department of Climatology and Atmosphere Protection at the University of Wroclaw. His current research interest concerns modelling of emission, transport and deposition of atmospheric pollutants at regional scale.
Ron Smith is a senior statistician at CEH Edinburgh, with an interest in application of statistical techniques for use in environmental sciences, in particular the land/atmosphere exchange of sulphur and nitrogen compounds. He chairs the Royal Statistical Society's Panel on Statistics for Ecosystem Change.
David Fowler is the former director of the biogeochemistry programme at the Centre for Ecology and Hydrology. His research interests include micrometeorological techniques for measurement of land-atmosphere exchange of trace gases and atmospheric processes influencing acid deposition in upland areas.