R.I. Smith

Regional estimation of pollutant gas dry deposition in the UK: model description, sensitivity analyses and outputs

A “big-leaf” resistance analogy model for dry deposition of sulphur dioxide, nitrogen dioxide, ammonia and nitric acid is described with a stomatal compensation point included to allow bi-directional exchange of ammonia. The model derivation is constrained by measurement data and it is parameterized for UK conditions. Monthly average dry deposition estimates are provided at the 5 km×5 km spatial scale. The model uses data available nationally at the appropriate spatial and temporal scales, such as gas concentration, land use, wind speed, temperature, rainfall and vapour pressure. A method is presented to overcome the lack of suitable solar radiation data. The effect of uncertainty in model inputs and in model parameterization is explored using sensitivity analyses. SO2 deposition is sensitive to variation in gas concentration, wind speed and wet surface uptake parameters. NO2 deposition is sensitive to parameters and inputs regulating stomatal behaviour, including solar radiation and temperature, as well as to gas concentration. The use of monthly or annual average NO2 concentrations may underestimate deposition substantially in some areas. HNO3 dry deposition is sensitive to wind speed and concentration. NH3 dry deposition to moorland and forest land uses, where the majority of deposition occurs, is sensitive to concentration, wind speed and choice of canopy resistance parameters. For arable and grassland areas, with both deposition and emission of NH3, the model is sensitive to all the model inputs and parameter choices. A full uncertainty analysis requires further work on the reliability of input variables and model parameter choices but these results quantitatively focus on the important areas of the model for each gas. Estimated dry deposition to the UK (excluding Northern Ireland) of SO2 is 135 Gg S yr−1 for 1996, for NH3 is 97 Gg N yr−1, for NO2 is 26 Gg N yr−1 and the preliminary estimate for HNO3 is 42 Gg N yr−1. For sulphur and reduced nitrogen, estimated dry deposition accounts for 40% of total deposition, including wet and cloud droplet deposition. NO2 dry deposition only accounts for 15% of total oxidised nitrogen deposition, but another 25% may come from the dry deposition of HNO3, giving a similar 40% overall by dry deposition. The sensitivity of the model to parameter values and the comparisons of modelled output with measurements show that parameter choices may be valid only at the scale of European countries rather than the whole continent.

Assessment of the magnitude of ammonia emissions in the United Kingdom

Abstract Estimates of ammonia emission in the U.K. have been critically reviewed with the aim of establishing the magnitude and uncertainty of each of the sources. European studies are also reviewed, with the U.K. providing a useful case study to highlight the uncertainties common to all ammonia emission inventories. This analysis of the emission factors and their application to U.K. sources supports an emission of 450 (231–715) Gg NH 3 yr −1 . Agricultural activities are confirmed as the major source, providing 406 (215–630) Gg NH 3 yr −1 (90% of the total), and therefore dominate uncertainties. Non-agricultural sources include sewage, pets, horses, humans, combustion and wild animals, though these contribute only 44 (16–85) Gg yr −1 . Cattle represent the largest single uncertainty, accounting for 245 (119–389) Gg yr −1 . The major uncertainties for cattle derive from estimation of the amount of nitrogen (N) excreted, the % N volatilized from land spreading of wastes, and the % N volatilized from stored farm-yard manure. Similar relative uncertainties apply to each of sheep, pigs and poultry, as well as fertilized crops, though these are quantitatively less important. Accounting; for regional differences in livestock demography, emission of 347, 63 and 40 Gg yr −1 are estimated for England & Wales, Scotland, and Northern Ireland, respectively. Though very uncertain, the total is in good agreement with estimates required to balance the U.K. atmospheric NH. budget.

Dispersion, deposition and impacts of atmospheric ammonia: quantifying local budgets and spatial variability

Ammonia is a reactive pollutant emitted primarily by agricultural sources near ground level in the rural environment. The consequence of these factors is that, in addition to the effects of long-range pollutant transport, ammonia has major effects at a local scale, with emission and receptor areas often closely located in the rural landscape. There is a substantial local spatial variability that needs to be considered in effects assessments, while variations in local deposition may affect the amount of ammonia available for impacts further afield. The wide-ranging UK programme ADEPT (Ammonia Distribution and Effects ProjecT) has addressed these issues through a combination of measurement and modelling activities concerning the distribution of emissions, atmospheric transport, deposition and effects assessment. The results are illustrated here by summarizing the findings of a joint experiment at Burrington Moor, Devon, and wider modelling contrasting the variability at a field scale with 5 km resolution estimates for the UK. The fraction of emitted NH3 deposited locally is shown to depend critically on the downwind land-cover, with fluxes being dependent on interactions with the ammonia compensation point. This will restrict deposition back to agricultural land, but may mean that non-conservation woodlands could be of benefit to recapture a significant fraction of emissions. The generalized models demonstrate the high spatial variability of ammonia impacts, with a case study being used to show the consequences at a field scale. In source regions substantial variability occurs at sub-1 km levels and this will have major consequences for the emission reduction targets needed to protect ecosystems.

Modelling photochemical oxidant formation, transport, deposition and exposure of terrestrial ecosystems.

The chemical processes responsible for production of photochemical oxidants within the troposphere have been the subject of laboratory and field study throughout the last three decades. During the same period, models to simulate the atmospheric chemistry, transport and deposition of ozone (O(3)) from individual urban sources and from regions have been developed. The models differ greatly in the complexity of chemical schemes, in the underlying meteorology and in spatial and temporal resolution. Input information from land use, spatial and temporally disaggregated emission inventories and meteorology have all improved considerably in recent years and are not fully implemented in current models. The development of control strategies in both North America and Europe to close the gaps between current exceedances of environmental limits, guide values, critical levels or loads and full compliance with these limits provides the focus for policy makers and the support agencies for the research. The models represent the only method of testing a range of control options in advance of implementation. This paper describes currently applied models of photochemical oxidant production and transport at global and regional scales and their ability to simulate individual episodes as well as photochemical oxidant climatology. The success of current models in quantifying the exposure of terrestrial surfaces and the population to potentially damaging O(3) concentrations (and dose) is examined. The analysis shows the degree to which the underlying processes and their application within the models limit the quality of the model products.

Regional mass budgets of oxidized and reduced nitrogen and their relative contribution to the nitrogen inputs of sensitive ecosystems

Wet deposition of nitrogen is reasonably well monitored throughout Europe, whereas the dry deposition inputs are provided largely by models. Recent long-term measurements of NO2 and NH3 fluxes to semi-natural vegetation have shown that rates of NH3 deposition exceed those of NO2, typically by an order of magnitude. Incorporating the results of these dry deposition measurements in regional deposition budgets shows that the inputs of reduced nitrogen contribute the dominant fraction of the total nitrogen inputs in most regions of the UK. The results are illustrated by comparing the atmospheric mass-budget for oxidized nitrogen over the UK. Of the annual UK emissions of NOx, amounting to 780 kt N (Salway et al., 1997), only 5% is dry deposited to terrestrial surfaces within the country while 15% is wet deposited, whereas for the reduced nitrogen, 42% of emissions (of the 260 kt N, Salway et al., 1997) are dry deposited and 46% are wet deposited. Even more striking are the relative contributions of oxidized and reduced nitrogen to semi-natural vegetation, which is a particularly efficient sink for NH3. The species composition of semi-natural vegetation is also regarded as very sensitive to nitrogen inputs. The distribution of nitrogen deposition among different land uses shows that the average input to forest in the UK is 33 kg N ha−1 annually of which 78% is reduced nitrogen. The other land uses receive about 15 kg N ha−1 of nitrogen of which between 55% and 65% is NHx. Critical loads for nutrient nitrogen are exceeded primarily in forested and moorland areas as a consequence of NH3 dry deposition and wet NH4+ deposition. For forests the area in exceedance of 20 kg N ha−1 year−1 critical load represents 70% of the forest area (1.4 × 106 ha−1) while for moorland the area in exceedance is 13% of the moorland area and occupies 1.04 × 106 ha−1).

A Spatial Analysis of Atmospheric Ammonia and Ammonium in the U.K.

As measures are implemented internationally to reduce SO2 and NOx emissions, attention is falling on the contribution of NH3 emissions to acidification, nitrogen eutrophication, and aerosol formation. In the U.K., a monitoring network has been established to measure the spatial distribution and long-term trends in atmospheric gaseous NH3 and aerosol NH4. At the same time, an atmospheric chemistry and transport model, FRAME, has been developed with a focus on reduced nitrogen (NHx). The monitoring data are important to evaluate the model, while the model is essential for a more detailed spatial assessment. The national network is established with over 80 sampling locations. Measurements of NH3 and NH4 (at up to 50 sites) have been made using a new low-cost denuder-filterpack system. Additionally, improved passive sampling methods for NH3 have been applied to explore local variability. The measurements confirm the high spatial variability of NH3 (annual means 0.06 to 11 mg NH3 m), consistent with its nature as a primary pollutant emitted from ground-level sources, while NH4, being a slowly formed secondary product, shows much less spatial variability (0.14 to 2.4 mg NH4 m). These features are reproduced in the FRAME model, which provides estimates at a 5-km level. Analysis of the underlying NH3 emission inventory shows that sheep emissions may have been underestimated and nonagricultural sources overestimated relative to emissions from cattle. The combination of model and measurements is applied to estimate spatial patterns of dry deposition to different vegetation types. The combined approach provides the basis to assess NHx responses across the U.K. to international emission controls.

Atmospheric inputs and catchment solute fluxes for major ions in five Welsh upland catchments

The hydrochemical budgets of major inorganic ions in three grassland and two forest catchments (ranging from 72 to 376 ha) in the uplands of Wales have been measured over a 2 year period. The catchment outputs of major ions have been compared with the total atmospheric inputs, the latter comprising dry deposition of NO2, SO2 and NH3, cloud droplet (occult) deposition of SO42−, NO3−, Cl−, Na+, Ca2+, Mg2+ and NH4+, and wet deposition of the same ions. The enhancement in wet deposition by orographic effects was modelled to provide wet deposition. The catchment output over 2 years is in good agreement with atmospheric inputs for the marine ions Cl− (±5%) and, to a lesser extent, Na+ (±10%). For sulphur, good agreement between inputs and outputs is also generally obtained (±10%), demonstrating the need to quantify all of the major atmospheric deposition pathways. The majority of the sulphur (approximately 70%) is wet deposited, and these studies provide strong support for the procedures to modify orographic enhancement of wet deposition in the UK uplands. The cloud droplet deposition pathway is generally a minor component of the budget but may approach 25% for afforested catchments. Dry deposition of SO2 and NO2 is a relatively small component (20–30%) of the S and N budgets, whereas dry deposition of NH3 is a major input (approximately 50%), although current estimates of annual NH3 inputs are subject to large uncertainties. If chloride is assumed to behave as a conservative ion in the catchments, then the mass flux estimates show that the annual base cation weathering within catchments unaffected by bedrock mineralisation and agricultural liming may be estimated directly from the mass budget, and should range from 1–11 kg ha−1 year−1 (Ca2+) and from 2–6 kg ha−1 year−1 (Mg2+).

Trends in wet and dry deposition of sulphur in the United Kingdom

UK data on sulphur deposition trends between the 1960s and 1990s are presented. Long term data sets of sulphur dioxide (SO2) concentrations at two sites have been analysed and dry deposition determined using a resistance model. Wet deposition has been calculated from non-marine sulphate concentration and rainfall fields for 1978–80 and 1989–93. These maps have been interpolated and corrected for seeder feeder enhancement. The wet deposition of sulphur declined by about 43 % between 1979 and 1993 whereas emissions of sulphur declined by about 32 %. An indication of the trends in sulphur dry deposition is provided by data from Eskdalemuir, a site in southern Scotland where wet deposition and SO2 concentration have been measured since 1979. Dry deposition at Eskdalemuir has decreased by 70 % and wet deposition by 48 %. Hence, while wet deposition has responded approximately linearly with the decline in UK emissions of sulphur dioxide, dry deposition has declined at twice the rate of decline in UK emissions.

Acid Deposition in Wales: the results of the 1995 Welsh Acid Waters Survey

In 1995, bulk precipitation was sampled at weekly intervals from 25 collectors located across Wales. The precipitation chemistry was dilute, acid (overall mean pH 4.88), and dominated by seasalts with a sodium to chloride ratio close to that of seawater. Seventy percent of the sulphate was from non-seasalt sources and the equivalent ratio of sulphate to nitrate was 1.8. Precipitation chemistry varied spatially with a decrease in the proportion of seasalts but an increasing proportion of excess sulphate with distance eastwards. Seasonal maximum concentrations of seasalts occurred in the first 3 months of the year whilst maximum excess sulphate, nitrate and ammonium concentrations were observed during the summer months. Maps of modelled wet plus dry deposition at 20-km square resolution showed that the acid-sensitive uplands of Snowdonia, the Cambrian mountains and the Brecon Beacons received large inputs of excess sulphur (15–25 kg S ha−1 year−1). Total inorganic nitrogen deposition to much of Wales was between 20 and 25 kg N ha−1 year−1. In 1995, Wales received 32 ktonnes of excess sulphur and 42 ktonnes of inorganic nitrogen equivalent to 9 and 11%, respectively, of the UK total deposition (based on 1992–94 figures). Nitrogen deposition contributed 60% of the total acidifying input to Wales. Since the first survey in 1984, the overall mean total sulphate concentration in rainfall has declined by about 28% to 2 mg litre−1; comparable to changes at long-term monitoring sites. Nitrate concentrations also declined by 18% to 0.23 mg N litre−1 whilst there was no statistically significant change in ammonium concentrations. Between 1984 and 1995, wet deposition of total sulphur to Wales decreased by 22% from 36 to 28 ktonnes, whilst wet deposited inorganic nitrogen declined from 27 to 22 ktonnes. These intensive regional data from the far west of Europe provide valuable insight into current deposition trends and the relative importance of sulphur and nitrogen deposition to acidification.

Nonlinearities in source receptor relationships for sulfur and nitrogen compounds.

Abstract The relationship between emissions and deposition of air pollutants, both spatially and in time forms an important focus for science and for policy makers. In practice, this relationship may become nonlinear if the underlying processes change with time, or in space. Nonlinearities may also appear due to errors in emission or deposition data, and careful scrutiny of both data sources and their relationship provides a means of picking up such deficiencies. Nonlinearities in source receptor relationships for sulfur and nitrogen compounds in Europe have been identified in measurement data for the UK. In the case of sulfur, the dry deposition process has been shown to be strongly influenced by ambient concentrations of NH3, leading to substantial increases in deposition rate as SO2 concentrations decline and the ratio SO2/NH3 decreases. The field evidence extends to measurements over three different surfaces in three countries across Europe. A mechanistic understanding of the cause of this nonlinearity has been provided. Apparent nonlinearities also exist in the sulfur deposition field through the influence of shipping emissions. The effect is clear at west coast locations, where during a period in which land-based sulfur emissions declined by 50%, no significant decline in concentrations of SO4 2− in precipitation were observed. The sites affected are primarily the coastal regions of southwestern UK, where shipping sources contribute a substantial fraction of the deposited sulfur, but the effect is not detectable elsewhere. Full quantification of the spatially disaggregated emission and their changes in time will eliminate this apparent nonlinearity in the source—receptor data. For oxidized nitrogen emission and deposition in the UK, there is strong evidence of nonlinearity in the source—receptor relationship. The concentrations and deposition of NO3− in precipitation have declined little following a reduction in emissions of 45% during the period 1987 to 2001. The data imply a significant decrease in the average transport distance for oxidized nitrogen and most probably an increase in the average oxidation rate. However, the net effect of changes in aerosol chemistry due to changes in sulfur emissions and less competition for the main oxidants as a consequence of reductions in sulfur emission have not been separated. A quantitative explanation of the cause of this nonlinearity is lacking and the effects are therefore identified as an important uncertainty for the development of further protocols to control acidification, eutrophication and photochemical oxidants in Europe.