Fourth Report of the Review Group on Acid Rain
June 1997
Prepared at the request of the Department of the Environment, Transport and the Regions
Index of reports
Executive Summary
Introduction
Emissions
Atmospheric chemistry
Wet deposition
Gases and particles
Dry deposition
Cloud droplet interception
Total deposition
Models
Trends over time
Future deposition
Introduction
This report is the fourth in a series examining the nature and extent of acid deposition throughout the United Kingdom. In it we present the current acidic pollution climate and consider whether the benefits of policies to reduce emissions are now detectable in trends in air and precipitation quality. Detailed assessment of the environmental impacts of acid deposition lies outside our remit. We have, however, compared current acidic inputs with critical loads and estimated the likely improvement due to further planned reductions in sulphur dioxide emissions.
Emissions
Our understanding of emissions of sulphur dioxide (SO2) and oxides of nitrogen (NOx) is generally sound, but ammonia (NH3) emissions remain poorly characterised. In part this arises from the biological nature of the sources, the wide range of relevant agricultural practices and the influence of soil and climatic conditions. The flux of ammonia to land surfaces is bi-directional with both emission and deposition occurring at different times; this further complicates the estimation of net emissions.
National emissions of sulphur dioxide have halved since 1970 whereas emissions of nitrogen oxides have remained relatively constant over that period. In the case of SO2 this decrease originally occurred in the domestic and industrial sectors with power station emissions, the largest single source, starting to decrease only towards the end of the period. The comparative lack of trend in total NOx emissions conceals a significant change in the contributions of source categories; emissions from transport have increased while emissions from industry have decreased. Since 1992 the benefits of fitting catalytic converters to motor vehicles are becoming evident.
Forecast of emissions are inevitably uncertain but implementation of the Second Sulphur Protocol and continuing application of Integrated Pollution Control will result in further reductions with SO2 emissions in 2010 likely to be only 15% of the peak emissions in the late 1960s. NOx emissions will also be around 55% of the peak (1989-91) total by 2010.
Atmospheric Chemistry
There has been significant progress in understanding the detailed mechanisms of sulphur dioxide oxidation in clouds. Mechanisms for nitrate production, identified in laboratory studies, have now been observed in field conditions. As yet heterogeneous reactions of both sulphur and nitrogen oxides on the surface of cloud droplets are only poorly quantified. In general, chemical processes are sufficiently well understood for the purposes of modelling acid deposition at the regional scale, but some of the detailed physics and chemistry of the processes are far from being fully elucidated.
Wet Deposition
A reduction in the number of wet deposition monitoring sites (59 to 32) has degraded the precision of interpolated estimates in some areas. Current sites meet the network's aim of giving the broad pattern of acid deposition over the country but are barely adequate to map concentrations at the 20 km x 20 km scale. Extrapolation from low altitude sites underestimates the increased wet deposition in upland areas generated by seeder-feeder enhancement. Since our last report intensive monitoring and modelling studies have increased our confidence in the procedure used to correct interpolated maps for this effect. However, there is clearly a finer scale variability not resolved by these procedures which is limiting the resolution of the maps.
Gases and Particles
Annual average rural sulphur dioxide concentrations are small with maximum concentrations of about 6 ppb in the east Midlands. Concentrations of nitrogen dioxide show a pronounced winter maximum and exceed SO2 concentrations almost everywhere. Aerosol is now a substantial fraction of total airborne sulphur and nitrogen over the UK. A new network is providing improved measurements of SO2 in rural areas and a network to measure NH3 has recently been established.
Dry Deposition
Dry deposition of sulphur dioxide accounts for around 40% of sulphur deposition over the country, with the largest inputs in central England. The national dry deposition maps are calculated by combining a measured concentration field for SO2 with a process based model. The procedure is underpinned by two stations which continuously monitor rates of dry deposition by micrometeorological methods over representative landscapes in the countryside (moorland/grassland and arable land). The UK and the Netherlands are the only countries to have developed this technology, which helps validate the dry deposition mapping.
The main route for surface uptake of nitrogen dioxide appears to be the stomata of vegetation. The overall rate of removal by dry deposition is small and represents only a few percent of emissions. Nitric acid is rapidly deposited on surfaces of all types. The deposition of nitric acid, for which we have very few measurements, is therefore a significant component of the dry deposited oxidised N budget and, as yet, poorly quantified.
The processes controlling surfaced removal of ammonia are more complex since the surface can act as both a source of, and a sink for, the gas. However, there has been significant progress in the development of models of these processes. The net dry deposition to the UK is a larger proportion of emission than for sulphur or oxidised nitrogen.
Nevertheless, our understanding of the behaviour of reduced N species remains poor compared to S and oxidised N species. This should be borne in mind when assessing emissions reduction scenarios for NH3, either in isolation or as part of a multi-pollutant/multi-effect approach.
Cloud Droplet Interception
On a national scale cloud droplet interception is a minor deposition pathway contributing only about 5% and 7% of the total S and N deposition respectively. But in forested upland areas (>250 m asl) local pollutant deposition can be very substantially enhanced in this way. The process is highly variable with peak 1 km x 1 km values being an order of magnitude greater than 20 km x 20 km mean values. Concentrations of major ions in cloudwater are generally a factor of 5 greater than in rain and, in the case of sulphate, observed peak concentrations are in the range where damage to sensitive vegetation may occur.
Total Deposition
Annual mean total non-seasalt sulphur deposition to the UK over the period 1992-94 was 350 ktonnes S, of which about three-fifths was wet deposited and two-fifths dry deposited. Over the same period oxidised N deposition was 150 ktonnes N of which about three-quarters was wet deposited and one-quarter dry deposited. In the case of reduced N, an estimate of 230 ktonnes N, approximately equally divided between wet and dry, is much less certain. This deposition represents about one-fifth of national S and oxidised N emissions and more than four-fifths of reduced N emissions. So the UK exports most of its S and oxidised N.
Sulphur deposition not directly attributable to European sources constitutes only 5 to 10% of total deposition. This 'background' arises largely from marine biogenic sources, transatlantic transport from North America and re-circulated European emissions. In summer marine biogenic emissions can account for around one third of the 'background' but make a negligible contribution in winter.
Uncertainties in mapping total deposition are large and estimated as ±40% for central England and up to 80% in west Scotland for 20 km x 20 km squared. The 1 km x 1 km scale variability in wet deposition in complex terrain and in dry deposition close to major source regions, is likely to be a factor of 2 or 3 greater than 20 km x 20 km grid square averages. This currently precludes the estimation of critical loads exceedances at the 1 km x 1 km scale, but on the 20 km x 20 km scale current sulphur inputs result in critical loads being exceeded for 36% of the UK. If oxidised nitrogen and base cation deposition are taken into account this increases to 42%.
Models
Models of acid deposition have developed considerably since our last report. The source-receptor relationships derived from these models are more reliable for sulphur and oxidised nitrogen where they have a valuable role in assisting policy development. As yet there is much less confidence in such relationships derived from models for reduced nitrogen. Models cannot, as yet, reproduce all the features in the observed data and are not able to provide estimates of future deposition at the scale of interest to those studying the effects of acid deposition.
Trends Over Time
We now have sufficiently long, high-quality data sets to show that concentrations of SO2, aerosol sulphate and sulphate in precipitation have decreased throughout the UK in response to reductions in emissions. However, not all have decreased at a similar rate. Rural SO2 concentrations have decreased more rapidly than total national SO2 emissions while concentrations of aerosol sulphate and sulphate in precipitation have decreased at a rate similar to, or slightly less than, the rate of decrease in emissions. While it is likely that decreases in domestic emissions, and other low-level sources, have influenced the observed decline in rural SO2 concentrations, the magnitude of the decrease is not consistent with the decrease in the domestic sector reported in the national atmospheric emissions inventory. There is, therefore, evidence of a non-linearity of emissions and deposition of sulphate over regional scales. Possible causes include oxidant limited transformation of SO2 to sulphate aerosol, or a change in canopy resistance to SO2 deposition as a consequence of changes in ambient NH3 concentrations. The former explanation is broadly consistent with observed oxidation in orographic cloud at Great Dun Fell.
Trends in the concentration and deposition of oxidised and reduced nitrogen over the last decade are small and lie within the uncertainty of the calculated values. Hence, there is a clear trend of an increasing nitrogen contribution, relative to sulphur, to total acidifying inputs.
Future Deposition
Modelling studies suggest that implementation of the second Sulphur Protocol will result in greatly reduced sulphur deposition, wet and dry deposition being only about a quarter and a fifth of the maximum values of the late sixties respectively. Even then sulphur deposition alone will continue to exceed empirical critical loads for soils in some highly sensitive areas of Britain, comprising about 8% of the land area. Moreover, the possible non-linearities described above may mean that the rate of decrease will not be even over the country.
A practical consequence of the potential non-linearity between sulphur emissions and deposition is that the benefits of the reduction in sulphur emissions are mainly taking place close to major sources, over urban area and arable land. The upland areas of north and west of the UK are benefiting least, and they may be protected less quickly than envisaged. The evidence for non-linearity is new, and not yet robust nor fully understood, but serves as an important reminder that even in this extensively researched field important issues remain to be addressed.
When additional inputs from nitrogen species and HCl are taken into account the level of potential damage is greater. Over much of the country nitrogen deposition will become a more important source of acidification than sulphur.
The growing importance of nitrogen compounds among the acidifying components and its links with other major issues (eutrophication, photochemical oxidants and climate change) provide a new focus for scientific and political interest in the pollution climate.
Other reports prepared for DETR
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