Acid rain has been one of the first and longest running of the multi-national environmental issues. The phenomenon was described in the 1950s and 60s by noted scientists such as Dr Eville Gorham, Dr Svante Odén and Dr Harold Harvey. Scientific and public reaction to their work was generally negative.

Over the past two decades, the scientific understanding of the problem has dramatically improved. As this information became understood by the public, support for the control of atmospheric pollution increased to the point where virtually all industrialised countries are now implementing sulphur dioxide control programmes.

The industrial reaction to environmental control has become much more positive over the past few years with many industrial sectors now being equal partners in discussions of solutions to environmental problems. While pollution control programmes cost considerable amounts of money, it has been shown that these costs are manageable and that clean environments are conducive to economic development.

Opinion polls have shown that the public in Canada and the U.K., for example, places high priority on the need for environmental protection.

The positive industrial attitude, the increased public concern and the need to understand the science of complex environmental issues, in order to design the best possible control programme, together necessitate the involvement of environmental scientists in public debates if rational decisions are to be taken.

Like oxidation in a flame, oxidation in the atmosphere is mediated by free radicals. By far the most important radical in the troposphere is the hydroxyl radical, OH. It is generated photochemically, and reacts with most atmospheric trace gases, in many instances as the first, and rate-determining, step in the reaction chains leading to their oxidation. Usually these chains regenerate OH thus maintaining OH at relatively large day-time concentrations of the order of 106 cm−3.

The most important reactions controlling local concentrations of OH form the framework of a photochemical reaction model, the results of which are compared to OH concentrations measured in the field at locations with different concentrations of pollutant attributable to the activities of man. The model-predicted OH concentrations are based on the simultaneously measured concentrations and photolysis rates of those trace gases that dominate OH chemistry at the local level.

Predicted and measured OH concentrations correlate reasonably well where concentrations of atmospheric pollutants are small but not where they are large. Explanations are being sought.

The deposition of reactive gases on terrestrial surfaces is one of the primary mechanisms by which pollutant gases are removed from the atmosphere. The chemical properties of the gases (SO2, NO2, HNO3, HCl) and of the absorbing surfaces lead to differing rates of exchange and controlling processes. The most reactive gases, HNO3, HCl (and for many surfaces NH3) exhibit negligible surface resistances; deposition velocities (Vg) appropriate for short vegetation ranging from 2 to 5 cm s−1, for forests Vg may approach 10 cm s−1. The large rates of deposition for NH3 on moorland and forests lead to annual inputs, in areas with large atmospheric concentrations of NH3 (≥ 5 μg NH3 m−3), ranging from 20 to 60 kg N ha−1. The net exchange of NH3 over cropland, attributable to deposition during vegetative growth and emission of NH3 during senescence, is less well known but believed to be small.

The co-deposition of NH3 and SO2 on external surfaces of plant canopies is believed to enhance SO2deposition with reported deposition velocities over short vegetation of 2.0 cm s−1.

Rates of cloud droplet deposition to vegetation have been shown to be very similar to rates of momentum deposition (i.e. Vt ≈ ram−1). These findings provide the basis for estimates of cloud deposition inputs of major ions to upland Britain where they may contribute up to 30% of the wet deposited sulphur and nitrogen.

Regional atmospheric sulphur/nitrogen budgets have been used in several regions of the world to help build up an overview of fluxes, reservoir contents and turnover times, and as a basis for quantitative modelling. However, there are some deficiencies in the regional budget approach. The analysis is of necessity superficial, and can give a false impression of certainty. This paper reviews the regional budgets for sulphur/nitrogen which have been created in various regions of the world, analyses their common findings, addresses the issue of uncertainty, and recommends areas of future research.

Most regional sulphur/nitrogen budgets have been constructed for North America, northern Europe and a few in other parts of Europe. A common finding is that anthropogenic emissions are large relative to natural emissions and that there is substantial international transport. The results of the budget analysis, while having elements of uncertainty, have proved to be valuable both in the synthesis of knowledge, and in their contribution to the increased awareness among policy makers and the public at large about the long-range transport of sulphur/nitrogen emissions.

Unfortunately, there have been few regional budgets for the developing portions of the world. Given the projected increase in population – and per capita S and N emissions for these regions – we encourage a future focus on regional sulphur/nitrogen budgets for the less developed regions of the world.

A review of the literature on forest soil change in North America, Central Europe. Sweden, U.K., and Australia reveals that changes are occurring in both polluted and unpolluted sites at a greater rate than previously suspected. Acid deposition has played a major role in recent acidification in some areas of Europe and, to a more limited extent, in Sweden and eastern North America. However, rapid rates of soil acidification are occurring in western North America and Australia due to internal processes such as tree uptake and nitrification associated with excessive nitrogen fixation. The presence of extremely acid soils is not necessarily an indicator of significant acidic deposition, as evidenced by their presence in unpolluted, even pristine forests of the north-western U.S.A. and Alaska. Numerous studies in Sweden, Australia, and North America show the important effects of tree uptake and harvesting upon soil acidification in managed forests. Furthermore, arguments can be presented that harvesting takes a greater toll upon the pools of potentially limiting cations than leaching.

The rate at which soils are changing in some instances calls for a re-evaluation of the budget analyses used to predict soil change. Specifically, inter-horizon changes due to uptake and recycling by vegetation, the interactions of such changes with naturally- and anthropogenically-produced acids, and the effects of aluminium uptake and recycling need further evaluation and study.

The physiological and biochemical features of plants that are responsible for determining susceptibility or tolerance to air pollutants have often proved hard to identify. In recent years, however, there has been new experimental evidence of responses which may be of critical importance. These include (a) changes in stomatal behaviour, affecting plant-water relations, (b) alterations in carbon and nitrogen assimilation and partitioning which can influence root growth, and (c) interference with the processes of winter hardening. Evidence of these changes to physiological processes, and the ways in which responses to pollutants may become more significant to plant survival in the presence of other environmental stresses, are discussed. The longer-term consequences, or secondary responses, such as effects on associations with other organisms, are also briefly reviewed. We have also examined the possibility that ecosystems may be overloaded with inputs of nitrogen from the atmosphere.

Unravelling the subtle effects of air pollution on vegetation requires adherence to the experimental method for testing hypotheses. Three experimental approaches are described. Field release of pollutants causes minimal disturbance of other aspects of the environment but is difficult to control and to operate continuously. Closed chambers, such as glasshouses, are furthest removed from field conditions but many aspects of their environments can be controlled. There is scope for the more sophisticated use of computer controlled glasshouses to investigate responses of stands of crop plants and natural/semi natural communities. Open-top chambers (OTCs) are a popular research tool, but results from major studies such as the U.S. National Crop Loss Assessment Network are of uncertain general value. Incursion of air into the tops of OTCs creates vertical pollution gradients. Evaporation, and the stomatal control of transpiration in OTCs may be very different from that in the field. Uptake of pollutant gases in OTCs may also differ from that in the field, directly because of differences in air movement, and/or indirectly through differences in the distribution of temperatures and moisture.

The development and design of a U.K. research programme on red spruce is used to illustrate (i) the need to develop hypotheses from a wide range of observations, (ii) the advantages of using a range of experimental approaches and (iii) the requirement to synthesise results before reaching general conclusions.

Forest decline in Europe is centred around areas where air pollution is heaviest. Although statistical relations are still debatable at the stand level, they are a basis for the discussion of mechanisms by which air pollutants affect forest health. The aetiologies of different syndromes of decline are discussed. Exposure to large concentrations of gaseous pollutants appears to have short-term rather than long-lasting effects, whereas pathogens seem to be of only secondary importance. The deposition of sulphur and nitrogen (nitrate and ammonium) pollutants has significantly modified soil chemistry and plant nutrition. In acidic low-pH soils spruce roots, instead of utilising nitrate, preferentially take up ammonium which interferes with the uptake of other cations, notably magnesium. The nitrate remaining in soil solution, as a result of the preferential uptake of ammonium, is leached together with sulphate to groundwater, accelerating soil acidification and further decreasing the calcium and/or magnesium to aluminium ratios in soil solution. Soil solution chemistry affects root development, and thus water and nutrient uptake. Canopy uptake of nitrogen, especially of ammonium, which is additional to root uptake, may occur and appears to stimulate growth inciting a nitrogen to cation imbalance with the consequential production of decline symptoms.

As empirical relationships between acidic deposition and its effects on soils and surface waters have become increasingly understood, the focus of attention has shifted to the reversal of acidification in response to decreased amounts of acidic deposition.

Recent decreases in acidic deposition, in both North America and parts of Europe, have initiated the reversal of acidification and the recovery of impacted ecosystems. Reversal has also been investigated in large-scale experiments with whole ecosystems. Predictive models can account for much of this empirical and experimental data, at least at the catchment scale, over periods of years.

Discrepancies between observed and predicted effects are attributable in part to the increasingly important role of nitrogen in soil and water acidification. ‘Nitrogen saturation’ threatens to offset the incipient recovery following decreases in SO2 emissions. Future climate change might further exacerbate the problem by increasing the rate of mineralisation of soil organic matter and releasing stored nitrogen.

In experiments lakes 223 (L223) and 302 South (L302S) in the Experimental Lakes Area in north-western Ontario, and Little Rock Lake (LRL) in northern Wisconsin, were progressively acidified with sulphuric acid from original pH values of 6.1–6.8 to 4.7–5.1. Although the lakes were at different locations with different physical settings and assemblages of plants and animals including fish, there were remarkable similarities in their responses, particularly in regard to biogeochemical processes and effects on biota at lower trophic levels.

All three lakes generated an important part of their buffering capacity internally b\ the reduction of sulphate, and to a lesser extent by the reduction of nitrate. Alkalinity production increased as concentrations of biologically-active strong acid anions increased. Models relating the residence times of sulphate and nitrate to water renewal, or first-order kinetics, effectively predicted events.

Acidification disrupted nitrogen cycling in all three lakes. Nitrification was inhibited in L223 and L302S, while in LRL, nitrogen fixation was greatly decreased at low pH.

The phytoplankton communities of all three lakes were originally dominated by chrysophyceans and cryptophyceans. However acidification changed the dominant species and decreased diversity. Acidification tended to increase phytoplankton production and standing crop slightly, probably because light penetration was increased.

Littoral zones of all three lakes became increasingly dominated by a few species of filamentous green algae, which created nuisance blooms by pH 5.6. Mats or clouds of algae changed the entire character of the littoral zone.

Acidification of L223 and L302S caused the loss of several species of large benthic crustaceans as pH changed from 6 to 5.6. Large, acid-sensitive littoral crustaceans were absent from LRL before acidification, probably because the lake was already too acidic.

As acidity increased, the dominance of cladocerans within zooplankton communities increased. Daphnia catawba appeared at pH values near 5.6 and became more abundant at lower pHs as the lakes were acidified. Its appearance coincided with a decline in other Daphnia species: another cladoceran, Bosmina longirostris, increased in the experimentally-acidified lakes as did Keratella taurocephala: they became the dominant rotifers. Several sensitive zooplankton species declined or disappeared as the lakes were acidified, most notably Daphnia galeata mendotae, Epischura lacustris, Diaptomus sicilis and Keratella cochlearis.

The responses of different fish varied; they appeared to depend on the sensitivity of key organisms in the food chain. The ability of key fish species to reproduce was impaired as early as pH 5.8; their reproduction, except for yellow perch in LRL, had ceased at pH 5.0 in all the three lakes.

Acidification consistently reduced the diversity and richness of species in taxonomic groups studied, these effects resulting from losses of species and the increased dominance of a few acidophilic taxa.

Responses of experimentally-acidified lakes in north-western Ontario and atmospherically-acidified lakes in eastern Ontario were similar in most respects where records allowed comparisons to be made, notably in relation to biogeochemical processes and the disappearance of acid-sensitive biota.

When the acidification of L223 was reversed, several biotic components recovered quickly. Fish resumed reproduction at pHs similar to those at which it ceased when the lake was being acidified. The condition of lake trout improved as a result of greatly increased populations of small fish, their prey. Many species of insects and crustaceans that had been extirpated by acidification returned. Assemblages of phytoplankton and chironomids have retained an acidophilic character, although their diversity during recovery is similar to that at comparable pHs during progressive acidification. As their chemistry recovered, atmospherically-acidified lakes in the Sudbury area were able to sustain recruitment by species offish, including lake trout and white sucker, with rapid increases in the diversity of invertebrate taxa. Results from both L223 and lakes near Sudbury suggest a rapid partial recovery of lacustrine communities when acidification is reversed.

It is concluded that the experimental lakes responded similarly to acidification, and that experimental acidification can reliably indicate the effects of acidification attributable to acidic precipitation.

Acidification in geologically-sensitive areas subject to appreciable acidic deposition has affected the activities of microbes, plants and animals in poorly-buffered freshwater ecosystems in Europe and N. America. Biota can be influenced directly by changes in water quality during both short acidic episodes and longer-term sustained periods of acidification: they are affected indirectly by alterations to the balance of acid-sensitive and acid-tolerant species at different trophic levels sometimes leading to a lessening of grazing pressures. Together the chemical and biological alterations result in adverse effects on some biogeochemical processes including the increased mobilisation and leaching of biologically active elements such as mercury, copper and zinc. Many field observations have now been corroborated by field (whole-lake) and laboratory experiments.

Decomposition of organic matter, attributable to microbes, has been found to be affected adversely at some locations. Conspicuously, acidification favours the prolific growth of filamentous algae particularly species of Mougeolia: it also shifts the balance of diatoms, minimising the occurrence of circumneutral species and favouring that of acidophilous and acidobiontic types. Because of their ‘resistant’ siliceous skeletons, it has been possible to trace the historical progression of acidification by examining the remains of diatoms in sediment accumulations. While their species diversity is significantly decreased, the biomass and productivity of dinoflagellates and chrysophytes are only slightly affected.

Like that of phytoplankton (dinoflagellates, chrysophytes and some diatoms), the species diversity of zooplankton is significantly decreased by acidification which has variable effects on the biomass and productivity of the group as a whole: the loss of daphnids is particularly conspicuous. Of the benthic macroinvertebrates, snails, crayfish, clams and freshwater shrimps (amphipods) have been found to be acid-sensitive whereas dragonflies and water boatmen (corixids) increase, particularly where populations of predetory fish have decreased.

Acidification has been responsible for the loss of fish from significant parts of Norway and Sweden: losses, partial or complete, have also been documented in geologically-sensitive areas of the U.K. and other parts of Europe, also Canada and the U.S.A. Eels seem to be the least sensitive to acidic conditions, followed by pike, trout, minnow and roach — the latter being the most sensitive. Prior to being eliminated, populations of fish become unbalanced because of recruitment failure and the mean age, and sometimes size, of the survivors increases. Accumulations of manganese and mercury have been detected in fish taken from acidified freshwaters. Short-term events (episodes) of extreme acidity and/or large concentrations of water-soluble aluminium have led to major fish kills.

The development of embryonic and larval stages of amphibians, including the common frog, toad and natterjack toad, is acid-sensitive. The distribution of fish-eating birds may be influenced by the effects of acidification on the performance of their prey. That of the dipper, a riverine bird, as well as its breeding success, has been restricted where acidification has decreased the availability of its macroinvertebrate prey. Deleterious effects on fish-eating mammals of reduced supplies of prey, sometimes with accumulations of mercury and cadmium and attributable directly and indirectly to acidification, have not, as yet, been confirmed.

The main pollutants affecting materials are sulphur dioxide and sulphates, nitrogen oxides and nitrates, chlorides, carbon dioxide and ozone. In marine environments it is important to separate the effects of anthropogenic pollutants from marine sources of the same substances. The effects of paniculate matter, especially from diesel vehicle emissions is of increasing significance.

In buildings containing books, historical artefacts or other works of art indoor pollutants may cause significant degradation. These pollutants may either be lower concentrations of pollutants originating outdoors or other substances generated from synthetic materials, paints, varnishes and display cases as well as from combustion for heating.

The materials most sensitive to pollutants are calcareous building stones and ferrous metals. Manifestations of damage include losses of mass, changes in porosity, discoloration and embrittlement. In the case of stone it is difficult to (1) dissociate the effects of historical concentrations of pollution from current ones and (2) to establish background rates of erosion or decay in the absence of pollutants.

Damage to materials in polluted atmospheres can be attributed to dry or wet deposition of pollutants, or dissolution in rainfall in the case of stone. Estimates of the contributions of the different mechanisms of decay have been made in programmes in the U.S.A. and the U.K.

To obtain estimates of the economic cost of damage from acid deposition it is necessary to know how decay rates are related quantitatively to pollutants and meteorological parameters (damage functions), and the distribution of materials exposed in buildings and in geographical areas. There are few damage functions available and those in existence lack general applicability. There is. therefore, much importance attached to national and international exposure programmes that have been established to assess the responses of a range of materials to different environments. Such work is also of interest in predicting the potential effects of climatic changes on materials.

Methodologies have been developed to enable estimates of economic costs of damage to be made. These are being refined with better data on rates of decay obtained from mathematical models and from exposure programmes. The concept of target loads for damage, originally devised for the natural environment, is being introduced to help assess the action needed to reduce degradation of sensitive materials and buildings.

The first part of the fifth and final day of Acidic Deposition: Its Nature and Impacts was devoted to a discussion of three questions:

(1) What is now known of the nature and impacts of acidic deposition (recognising that effects on materials and buildings were being discussed for the first time in the series of international conferences of which Acidic Deposition was the latest)?

(2) Where are the gaps in knowledge perceived to be?

(3) Is it necessary to fill some or all of the gaps/are the gaps worth filling?

To a significant extent the answers to these questions influence the reactions of the lay public and their political representatives, but it has to be recognised that the politics of acidic deposition have, on occasion, outpaced scientific comprehension largely because of the inability/unwillingness of scientists to communicate. This is a serious weakness, particularly in relation to pollution and other environmental issues where the problems are complex and cannot be conceived in terms of black and white.