Long-term Trends in Surface Water Quality of Five Lakes in Japan

Since 1983, the Ministry of the Environment of Japan has conducted nation-wide acid deposition surveys. To investigate the effects of acid deposition on surface water, we used the nonparametric Mann–Kendall test to find temporal trends in pH, alkalinity, and electrical conductivity (EC) in more than 10 years of data collected from five lakes and their catchments (Lake Kuttara: northernmost; Lake Kamakita: near Tokyo; Lake Ijira: central; Lake Banryu: western; and Lake Unagiike: southernmost). The pH of Lake Ijira water has declined slightly since the mid-1990s, corresponding with the downward trends seen in the pH and alkalinity of the river water flowing into the lake. There were significant upward trends in the EC of both the lake and stream water; the same trends were also found for concentrations. These trends show evidence of acidification due to atmospheric deposition, and this is the first such finding in Japan based on significant long-term trends. Lake Ijira is located about 40 km north of the Chukyo industrial area near Nagoya. The annual depositions of H⁺, nss-, and in Lake Ijira were among the highest of all deposition monitoring sites, suggesting that this is the main cause of the significant acidification observed in Lake Ijira. No significant trends suggesting acidification were observed in any of the other lake catchments in spite of the significant upward trends in EC. Upward trends in pH and alkalinity at Lake Banryu and upward trends in alkalinity at Lake Kamakita were detected, but no change in pH or alkalinity at Lake Kuttara and Lake Unagiike was observed.     

A chronology of nitrogen deposition in the UK between 1900 and 2000

Measurements of the concentrations of nitrogen compounds in air and precipitation in the UK have been made since the mid-19th century, but no networks operating to common protocols and having traceable analytical procedures were established until the 1950s. From 1986 onwards, a high-quality network of sampling stations for precipitation chemistry was established across the UK. In the following decade, monitoring networks provided measurement of NO₂, NH₃, HNO₃ and a satisfactory understanding of the dry deposition process for these gases allowed dry deposition to be quantified. Maps of N deposition for oxidized and reduced compounds at a spatial scale of 5 km × 5 km are available from 1986 to 2000. Between 1950 and 1985, the more limited measurements, beginning with those of the European Air Chemistry Network (EACN) provide a reasonable basis to estimate wet deposition of NO^? ₃?N and NH⁺ ₄?N. For the first half of the century, estimates of deposition were scaled with emissions assuming a constant relationship between emission and deposition for each of the components of the wet and dry deposition budget at the country scale. Emissions of oxidized N, which dominated total nitrogen emissions throughout the century, increased from 312 kt N annually in 1900 to a peak of 787 kt for the decade 1980–1990 and then declined to 460 kt in 2000. Emissions of reduced N, largely from coal combustion were about 168 kt N in 1900, increasing to a peak of 263 kt N in 2000 and by now dominated by agricultural sources. Reduced N dominated the deposition budget at the country scale, increasing from 163 kt N in 1900 to 211 kt N in 2000, while deposition of oxidized N was 66 kt N in 1900 and 191 kt N in 2000. Over the century, 68 Mt (Tg) of fixed N was emitted within the UK, 78% as NO _x , while 29 Mt of nitrogen was deposited (43% of emissions), equivalent to 1.2 t N ha^?1, on average, with 60% in the reduced form. Deposition to semi-natural ecosystems is approximately 15 Tg N, equivalent to between 1 and 5 t N ha^?1, over the century and appears to be accumulating in soil. The N deposition over the century is similar in magnitude to the total soil N inventory in surface horizons.     

Long Term NH3 Flux Measurements above Grasslands in The Netherlands

Ammonia concentration gradients were measuredabove a grassland in an agricultural region fromJuly 1998 to July 2000 at the locationSchagerbrug in the Netherlands. They were used tocalculate the surface-atmosphere exchange ofammonia by means of the aerodynamic gradienttechnique. Measurements of the ammonia exchangewere also performed at a grass field that is partof a major wetland reserve in the centre of theNetherlands (Oostvaardersplassen), during theperiod June 1994–September 1995. At Schagerbrug,low net emissions or small depositions weremeasured during the winter months, and a netemission up to 5 kg NH₃ ha^-1 month^-1 in summer. The net annual emission was 26 kgN ha^-1, with manure application contributingabout 50% of this emission. AtOostvaardersplassen mainly deposition occurred,but large emissions were measured in autumn. Themeasured exchange fluxes are compared to modelevaluations that incorporate plant physiologicaldata, such as apoplastic NH₄ ⁺concentrations and pH. Results show a goodagreement between field observations and modelsimulations, although some improvements arenecessary during nighttime periods. Themeasurements are conducted within the GRAMINAEproject, a second tranche project of EC TERI.     

Long Term Trends in Concentration of Major Pollutants (SO<Subscript>2</Subscript>, CO,NO, NO<Subscript>2</Subscript>, O<Subscript>3</Subscript> and PM<Subscript>10</Subscript>) in Prague – Czech Republic (Analysis of Data Between 1992 and 2005)

To assess the effect of changes in traffic density and fuels used for heating at the beginning of the 1990s, 1992–2005 monthly averages of PM₁₀, SO₂, NO₂, NO, CO and O₃ from Prague, the Czech capital, were analyzed together with long term trends in emissions of major pollutants, fuel consumption and number of vehicles registered in Prague. The data from all monitoring stations were retrieved from the database of the state automated monitoring system. Correlation coefficients between ambient monthly averaged temperature and all pollutants of concern showed distinct seasonal trends. The results showed that while SO₂ and to some extent also CO concentrations dropped namely in the first half of the analyzed period (1992–1997) as a result decreased fossil fuel consumption for local heating, the behaviour of other pollutant concentrations followed a different pattern. PM₁₀ concentrations decreased during the beginning of the 1990s but showed a sign of increase after 2000. Concentrations of ozone and NO₂ did not reveal any significant change throughout the whole studied period. It can be concluded that during the studied period traditional urban sources of pollution, such as coal and oil combustion, lost their importance but were simultaneously substituted by pollutants from automotive transport (namely PM and NO₂) making the problem of air quality even worse.     

A Chronology of Nitrogen Deposition in the UK Between 1900 and 2000

Measurements of the concentrations of nitrogen compounds in air and precipitation in the UK have been made since the mid-19th century, but no networks operating to common protocols and having traceable analytical procedures were established until the 1950s. From 1986 onwards, a high-quality network of sampling stations for precipitation chemistry was established across the UK. In the following decade, monitoring networks provided measurement of NO₂, NH₃, HNO₃ and a satisfactory understanding of the dry deposition process for these gases allowed dry deposition to be quantified. Maps of N deposition for oxidized and reduced compounds at a spatial scale of 5 km × 5 km are available from 1986 to 2000. Between 1950 and 1985, the more limited measurements, beginning with those of the European Air Chemistry Network (EACN) provide a reasonable basis to estimate wet deposition of NO ₃ ^– –N and NH ₄ ⁺ –N. For the first half of the century, estimates of deposition were scaled with emissions assuming a constant relationship between emission and deposition for each of the components of the wet and dry deposition budget at the country scale. Emissions of oxidized N, which dominated total nitrogen emissions throughout the century, increased from 312 kt N annually in 1900 to a peak of 787 kt for the decade 1980–1990 and then declined to 460 kt in 2000. Emissions of reduced N, largely from coal combustion were about 168 kt N in 1900, increasing to a peak of 263 kt N in 2000 and by now dominated by agricultural sources. Reduced N dominated the deposition budget at the country scale, increasing from 163 kt N in 1900 to 211 kt N in 2000, while deposition of oxidized N was 66 kt N in 1900 and 191 kt N in 2000. Over the century, 68 Mt (Tg) of fixed N was emitted within the UK, 78% as NO _x , while 29 Mt of nitrogen was deposited (43% of emissions), equivalent to 1.2 t N ha^–1, on average, with 60% in the reduced form. Deposition to semi-natural ecosystems is approximately 15 Tg N, equivalent to between 1 and 5 t N ha^–1, over the century and appears to be accumulating in soil. The N deposition over the century is similar in magnitude to the total soil N inventory in surface horizons.     

Long Term Trends in Sulphur and Nitrogen Deposition in Europe and the Cause of Non-linearities

Emissions of sulphur and oxidized nitrogen compounds in Europe have been reduced following a series of control measures during the last two decades. These changes have taken place during a period in which the primary gases and the wet deposition throughout Europe were extensively monitored. Since the end of the 1970s, for example land based sulphur emissions declined by between 90 and 70% depending on the region. Over the same period the total deposition of sulphur and its partitioning into wet and dry deposition have declined, but the spatial pattern in the reduction in deposition differs from that of emission and has changed with time. Such non-linearities in the emission-deposition relationship are important to understand as they complicate the process of assessing the effects of emission reduction strategies. Observed non-linearities in terrestrial sulphur emission-deposition patterns have been identified in north west Europe due to increases in marine emissions, and are currently slowing the recovery of freshwater ecosystems. Changes in the relative amounts of SO₂ and NH₃ in air over the last two decades have also changed the affinity of terrestrial surfaces for SO₂ and have therefore changed the deposition velocity of SO₂ over substantial areas. The consequence of this effect has been the very rapid reduction in ambient SO₂ concentration in some of the major source areas of Europe, where NH₃ did not change much. Interactions between the different pollutants, generating non-linearities are now being incorporated in long-range transport models to simulate the effects of historical emission trends and to provide projections into the future. This paper identifies non-linearities in emission deposition relationships for sulphur and nitrogen compounds in Europe using data from the EMEP long-rang transport model and measured concentration fields of the major ions in precipitation and of SO₂ and NO₂ in surface air.     

Modelling Seasonal Dynamics from Temporal Variation in Agricultural Practices in the UK Ammonia Emission Inventory

Most ammonia (NH₃) emission inventories have been calculated on an annual basis and do not take into account the seasonal variability of emissions that occur as a consequence of climate and agricultural practices that change throughout the year. When used as input to atmospheric transport models to simulate concentration fields, these models therefore fail to capture seasonal variations in ammonia concentration and dry and wet deposition. In this study, seasonal NH₃ emissions from agriculture were modelled on a monthly basis for the year 2000, by incorporating temporal aspects of farming practice. These monthly emissions were then spatially distributed using the AENEID model (Atmospheric Emissions for National Environmental Impacts Determination). The monthly model took the temporal variation in the magnitude of the ammonia emissions, as well as the fine scale (1-km) spatial variation of those temporal changes into account to provide improved outputs at 5-km resolution. The resulting NH₃ emission maps showed a strong seasonal emission pattern, with the highest emissions during springtime (March and April) and the lowest emissions during summer (May to July). This emission pattern was mainly influenced by whether cattle were outside grazing or housed and by the application of manures and fertilizers to the land. When the modelled emissions were compared with measured NH₃ concentrations, the comparison suggested that the modelled emission trend corresponds fairly well with the seasonal trend in the measurements. The remaining discrepancies point to the need to develop functional parametrisations of the interactions with climatic seasonal variation.     

Temporal Trends in Water Chemistry in the Turkey Lakes Watershed Ontario,Canada, 1982-1999

The Turkey Lakes Watershed (TLW) was established in 1980 as asite for study of the ecosystem effects of acidic deposition, andsince then there has been 40% reduction in North AmericanSO₂ emissions. Monitoring records for bulk deposition,shallow and deep ground water, two headwater streams and two lakeoutflows have been tested to identify statistically significantmonotonic trends. The TLW appears to be responding to decliningacidifying emissions because the most prevalent chemical trendacross sample types/stations was decreasing SO₄ ^2-. Increasing pH was detected in four of the seven data sets, butonly the H⁺ decrease in bulk deposition was of a magnitudeto be an important ionic compensation for the SO₄ ^2-decline. There is little evidence of acidification recovery inTLW waters however. Increasing alkalinity was found only in theoutflow of the penultimate lake of the basin, and in fact, deepground water and the other lake outflow had decreasing alkalinitytrends (i.e., continuing acidification). For the surface waterstations, the greater part of the ionic compensation fordeclining SO₄ ^2- was decreasing base cations, and as aresult, these waters are probably becoming more dilute with time,although only the headwater streams exhibited decliningconductivity. Five of seven data sets had increasing dissolvedorganic carbon concentrations. Increasing NO₃ ^- wasimportant in ground waters. Drought has strongly influencedtrends and delayed recovery by mobilizing S stored in catchmentwetlands and/or soils.     

Atmospheric Budget of Non-Seasalt Chlorine in the Kanto District,Japan

To evaluate the influence of anthropogenic emission of HCl on the air quality in the Kanto district of Japan, the atmospheric budgetof non-seasalt Cl (nssCl) was analyzed. The Kanto district, which consists of the Tokyo metropolis and the six surrounding prefectures, is the most densely populated region in Japan. The emission intensity of HCl is extremely high compared with those in other regions and most western countries. In this study, the annual wet and dry depositions of nssClwere estimated on a 0.25 × 0.25° grid over the Kanto district based on the concentration monitoring resultsand meteorological data. The budget analysis was conducted by comparing the estimated deposition with the emission of HCl. As a result, the annual total (wet + dry) deposition of nssCl was estimated to be 32 kt yr^-1, which closely agreed with HCl emission (36 kt yr^-1) from waste incineration in this area. The result suggested that the anthropogenic emission of HCl had a significant influence on the air quality and the deposition of acidity in this area.     

Trends in Chemical Composition of Wet-only Precipitation at Rural French Monitoring Stations Over the 1990–2003 Period

The long-term monitoring of precipitation and its chemical composition are important for identifying trends in rain quality and for assessing the effectiveness of pollution control strategies. A statistical test has been used to the atmospheric concentrations measured in the French rural monitoring network (MERA) in order to bring out spatio-temporal trends in precipitation quality in France over the period 1990–2003. The non-parametric Mann–Kendall test which has been developed for detecting and estimating monotonic trends in the time series was used and applied in our study at annual values of wet-only precipitation concentrations. The emission data suggest that SO₂ and NO _x emissions decreased (−3.3 and −2.0% year⁻¹, respectively) contrary to NH₃ emissions that increased slightly (+0.2% year⁻¹) over the period 1990–2002 in France. On the national scale, the pH values have a significant decreasing trend of −0.025 ± 0.02 unit pH year⁻¹. and concentrations in precipitation have a significant decreasing trend, −3.0 ± 1.6 and −3.3 ± 0.6% year⁻¹, respectively, corresponding with the downward trends in SO₂ emissions in France (−3.3% year⁻¹). A good correlation (R ² = 0.84) between SO₂ emissions and concentrations was obtained. The decreasing trend of was more significant (−5.4 ± 5.2% year⁻¹) than that of (−1.3 ± 2.4% year⁻¹). Globally, the concentration of the major ions showed a clear downward trend including marine and alkaline ions. In addition, the relative contribution of HNO₃ to acidity precipitation increased by 51% over the studied period.     

Sulphate and Nitrate in Precipitation and Soil Water in Pine Forests in Latvia

The SO₄–S and NO₃–N concentrations and pH in bulk precipitation, throughfall, stemflow and soil water for the 1994–2004 period were studied in pine forests in Latvia (Rucava and Taurene Integrated Monitoring stations). The SO₄–S and NO₃–N concentrations decreased over the study period, simultaneously with a decrease of acidity in precipitation. The changes were more evident in the western part of Latvia, probably due to declining long-range air pollution from West Europe. The trend of decreasing sulphate concentrations and increasing pH in precipitation were not followed by respective changes in soil water. In the upper soil horizon sulphate ion concentrations and acidity increased in soil water. Over the observation period, nitrate concentrations also showed an increasing trend in soil water at Rucava and Taurene, but these changes were not statistically significant.     

The spatial distribution of ammonia emitted from seabirds and its contribution to atmospheric nitrogen deposition in the UK

Simple bioenergetics models were used to derive annual nitrogen excretion rates of each seabird species occurring at colonies in the UK. These were combined with population distribution data and an estimated fraction of nitrogen volatilized to estimate the spatial distribution of NH₃ emissions from seabird colonies at a 1 km resolution. The effect of these emissions on atmospheric NH₃ concentrations and nitrogen deposition in the UK was assessed using the FRAME atmospheric chemistry and transport model. The total emission of NH₃ from the UK seabird colonies is estimated at 2.7 kt yr^?1. Emissions from seabirds are largely concentrated in remote parts of Britain, where agricultural and other anthropogenic emissions are minimal. Although seabirds account for less than 1% of total UK NH₃ emissions (～370 kt yr^?1), their occurrence in remote areas and frequently large colony sizes results in seabirds providing a major fraction of the atmospheric nitrogen deposition for many remote ecosystems.     

Atmospheric Deposition of Reactive Nitrogen on Turf Grassland in Central Japan: Comparison of the Contribution of Wet and Dry Deposition

The atmospheric deposition of reactive nitrogen on turf grassland in Tsukuba, central Japan, was investigated from July 2003 to December 2004. The target components were ammonium, nitrate, and nitrite ions for wet deposition and gaseous ammonia, nitric and nitrous acids, and particulate ammonium, nitrate, and nitrite for dry deposition. Organic nitrogen was also evaluated by subtracting the amount of inorganic nitrogen from total nitrogen. A wet-only sampler and filter holders were used to collect precipitation and the atmospheric components, respectively. An inferential method was applied to calculate the dry deposition velocity of gases and particles, which involved the effects of surface wetness and ammonia volatilization through stomata on the dry deposition velocity. The mean fraction of the monthly wet to total deposition was different among chemical species; 37, 77, and 1% for ammoniacal, nitrate-, and nitrite-nitrogen, respectively. The annual deposition of inorganic nitrogen in 2004 was 47 and 48 mmol m⁻² yr⁻¹ for wet and dry deposition, respectively; 51% of atmospheric deposition was contributed by dry deposition. The annual wet deposition in 2004 was 20, 27, and 0.07 mmol m⁻² yr⁻¹, and the annual dry deposition in 2004 was 35, 7.4, and 5.4 mmol m⁻² yr⁻¹ for ammoniacal, nitrate-, and nitrite-nitrogen, respectively. Ammoniacal nitrogen was the most important reactive nitrogen because of its remarkable contribution to both wet and dry deposition. The median ratio of the organic nitrogen concentration to total nitrogen was 9.8, 17, and 15% for precipitation, gases, and particles, respectively.     

Trends in the Water Chemistry of High Altitude Lakes in Europe

Here we present the chemical trends of seven high altitude lakes, analysed within the AL:PE and MOLAR Projects of the EU (1999) and selected on the basis of the availability of complete and reliable data for the period 1984–1999. The lakes are representative of the Scandinavian Alps, the Cairngorm Mountains in Scotland, the Alps and the Pyrenees. Significant trends were identified for some indicators of acidification, for instance pH and alkalinity, but not all lakes reacted similarly to decreasing depositions of sulphate and base cations. Differences in lake response are discussed in relation to recent variations of atmospheric deposition chemistry and associated changes in climatic conditions. Beside individual variations of the studied lakes, depending, among other things, on altitude and morphology, catchment characteristics and climate trends play a major role for the reaction of high altitude lakes on changes in atmospheric depositions.     

Air Quality Management in Izmir Region of Turkey as Required by Clean Air Plans

As a first step to work out an abatement plan against air pollution, a local emission inventory with 1 hr temporal and 1 km spatial resolution in the city of Izmir and its surroundings was prepared. The study area consisted of a 200 × 170 km² rectangle having the city of Izmir at the centre. The studied pollutants were total particulate matter (PM), sulfur oxides (SO_x), nitrogen oxides (NO_x), volatile organic compounds (VOC) and carbon monoxide (CO). Emissions of these pollutants were determined by estimation methods making use of suitable emission factors. Emission sources were evaluated in three categories; point, area and line sources. For year 2000 total emissions in the study area on an average day were estimated as 173 tons PM, 299 tons SOx, 136 tons NOx, 68 tons VOC and 320 tons CO. At the second part of the study, calculated emissions were transformed into air quality predictions in the area by using the Industrial Source Complex – Short Term (ISCST3) dispersion model. Model results were tested with monitoring data from urban air quality stations obtained during the year 2000. Results of the past, present and future air quality estimates in the region were discussed. In order to do so, future scenarios including various control technology applications were formulated and tested to see their effect on the future air quality.     

The Control of SO2 Dry Deposition on to Natural Surfaces by NH3 and its Effects on Regional Deposition

Two years of continuous measurements of SO₂deposition fluxes to moorland vegetation are reported. The mean flux of 2.8 ng SO₂ m^-2 s^-1 is regulated predominantly by surface resistance (r _c) which, even for wet surfaces, was seldom smaller than 100 s m^-1. The control of surface resistance is shown to be regulated by the ratio of NH₃SO₂ concentrations with an excess of NH₃ generating the small surface resistances for SO₂. A dynamic surface chemistry model is used to simulate the effects of NH₃ on SO₂ deposition flux and is able to capture responses to short-term changes in ambient concentrations of SO₂, NH₃ and meteorological conditions. The coupling between surface resistance and NH₃/SO₂ concentration ratios shows that the deposition velocity for SO₂ is regulated by the regional pollution climate. Recent long-term SO₂ flux measurements in a transect over Europe demonstrate the close link between NH₃/SO₂ concentrations and rc (SO₂). The deposition velocity for SO₂ is predicted to have increased with time since the 1970s and imply a 40% increase in v _d at a site at which the annual mean ambient SO₂ concentrations declined from 47 to 3 g m^-3 between 1973 and 1998.     

Critical Loads of Acidity for Surface Waters in South-Central Ontario, Canada: Regional Application of the First-Order Acidity Balance (FAB) Model

Major sulphur emission control programs have been implemented in North America, resulting in current emissions being ～30% less than those in 1980. However, the level of acidic deposition remaining is still unlikely to promote widespread recovery of aquatic ecosystems. The First-order Acidity Balance (FAB) model has been applied to south-central Ontario (285 lakes in the Muskoka River Catchment) to evaluate the need for further reductions in emissions. As a result of the past decline in deposition, the proportion of lakes with critical loads exceedance has dropped substantially; however, further reductions in sulphur and nitrogen emissions are required to eliminate critical loads exceedance. Based on bulk deposition of sulphate and nitrogen (41.1 mmol_c m^-2 yr^-1 and 62.5 mmol_c m^-2 yr^-1, respectively) for the period 1995–1999, 166 lakes (58.3%) exceedcritical loads. Even with full implementation of SO₂ abatementprograms in Canada (achieved in 1994) and the United States (legislated for 2010), critical loads will be exceeded in a large proportion (46.6%) of the study lakes.     

The spatial distribution of ammonia emitted from seabirds and its contribution to atmospheric nitrogen deposition in the UK

Simple bioenergetics models were used to derive annual nitrogen excretion rates of each seabird species occurring at colonies in the UK. These were combined with population distribution data and an estimated fraction of nitrogen volatilized to estimate the spatial distribution of NH₃ emissions from seabird colonies at a 1 km resolution. The effect of these emissions on atmospheric NH₃ concentrations and nitrogen deposition in the UK was assessed using the FRAME atmospheric chemistry and transport model. The total emission of NH₃ from the UK seabird colonies is estimated at 2.7 kt yr^–1. Emissions from seabirds are largely concentrated in remote parts of Britain, where agricultural and other anthropogenic emissions are minimal. Although seabirds account for less than 1% of total UK NH₃ emissions (370 kt yr^–1), their occurrence in remote areas and frequently large colony sizes results in seabirds providing a major fraction of the atmospheric nitrogen deposition for many remote ecosystems.     

Characteristics of air pollution in Beijing during sand-dust storm periods

In the Beijing area, March and April have the highest frequency of sand-dust weather. Floating dust, blowing sand, and dust storms, primarily from Mongolia, account for 71%, 20%, and 9% of sand-dust weather, respectively. Ambient air monitoring and analysis of recent meteorological data from Beijing sand-dust storm periods revealed that PM₁₀ mass concentrations during dust storm events remained at 1500 μg m⁻³, which is five to ten times higher than during non-dust storm periods, for fourteen hours on both April 6 and 25, 2000. During the same period, the concentrations in urban areas were comparable to those in suburban areas, while the concentrations of gaseous pollutants, such as SO₂, NO _x , NO₂, and O₃, remained at low levels, owing to strong winds. Furthermore, during sand-dust storm periods, aerosols were created that consisted not only of many coarse particles, but also of a large quantity of fine particles. The PM_2.5 concentration was approximately 230 μg m⁻³, accounting for 28% of the total PM₁₀ mass concentration. Crustal elements accounted for 60–70% of the chemical composition of PM_2.5, and sulfate and nitrate for much less, unlike the chemical composition of PM_2.5 on pollution days, which was primarily composed of sulfates, nitrates, and organic material. Although the very large particle specific surface area provided by dust storms would normally be conducive to heterogeneous reactions, the conversion rate from SO₂ to SO₄ ²⁻ was very low, because the relative humidity, less than 30%, was not high enough.     

Natural and Anthropogenic Changes in The Chemistry of Six UK Mountain Lakes, 1988 to 2000

Trends have been analysed for 12 years ofchemical data from six mountain lakes in the UK AcidWaters Monitoring Network (AWMN). With minimal localanthropogenic impacts, these sites offer the bestavailable opportunity for clear identification of surfacewater chemical response to external factors, whethernatural or anthropogenic. Results indicate that naturalclimatic variations have had a major impact on lakechemistry, through fluctuations in (i) intensity ofstorms, which cause dilution of weathering-derived basecations, and/or displacement of hydrogen and aluminiumions on soil exchange sites by deposited marine basecations; and (ii) winter temperature, which is thought tobe inversely related to spring nitrate (NO₃) maxima.Both climatic factors can be linked to the North AtlanticOscillation. For the first decade of AWMN monitoringthese natural `confounding factors' to a significantextent obscured any recovery from acidification due todeclining anthropogenic sulphur deposition. However, theadditional data presented here provide strengtheningevidence for chemical recovery at a number of sites, atwhich decreases in sulphate (SO₄), acidity andlabile aluminium can now be identified. It is believedthat changes at these sensitive mountain lake sites mayherald more widespread recovery in UK surface waters aspollutant emissions decline further. However, largeincreases in dissolved organic carbon, and hence inorganic acidity, may have partially offset reductions inmineral acidity. The cause of these increases remainsuncertain, but may be linked to climatic change.