The influence of long term trends in pollutant emissions on deposition of sulphur and nitrogen and exceedance of critical loads in the United Kingdom

Abstract

In the United Kingdom, as with other European countries, land-based emissions of NO_x and SO₂ have fallen significantly over the last few decades. SO₂ emissions fell from a peak of 3185 Gg S in 1970 to 344 Gg S in 2005 and are forecast by business-as-usual emissions scenarios to fall to 172 Gg by 2020. NO_x emissions were at a maximum of 951 Gg N in 1970 and fell to 378 by 2005 with a further decrease to 243 Gg N forecast by 2020. These large changes in emissions have not been matched by emissions changes for NH₃ which decreased from 315 Gg N in 1990 to 259 in 2005 and are forecast to fall to 222 by 2020. The Fine Resolution Atmospheric Multi-pollutant Exchange model (FRAME) has been applied to model the spatial distribution of sulphur and nitrogen deposition over the United Kingdom during a 15-year time period (1990–2005) and compared with measured deposition of sulphate, nitrate and ammonium from the national monitoring network. Wet deposition of nitrogen and sulphur was found to decrease more slowly than the emissions reductions rate. This is attributed to a number of factors including increases in emissions from international shipping and changing rates of atmospheric oxidation. The modelled time series was extended to a 50-year period from 1970 to 2020. The modelled deposition of SO_x, NO_y and NH_x to the UK was found to fall by 87%, 52% and 25% during this period. The percentage area of sensitive habitats in the United Kingdom for which critical loads are exceeded is estimated to fall from 85% in 1970 to 37% in 2020 for acidic deposition and from 73% to 49% for nutrient nitrogen deposition. The significant reduction in land emissions of SO₂ and NO_x focuses further attention in controlling emissions from international shipping. Future policies to control emissions of ammonia from agriculture will be required to effect further significant reductions in nitrogen deposition.

Introduction

Acid deposition, originating mainly from man made emissions of SO₂, NO_x 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 NO_x 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, NO_x 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.