Atmospheric Dust and Acid Rain Ed Areas of Europe and North America

Home , Acid rain

or the past several decades, sci- Atmospheric Dust entists have been studying acid F rain and how it affects the environment. As the harmful consequences of acidic air pollutants became increas- and Acid Rain ingly clear, governments in North Amer- ica and Europe began to regulate emissions of these compounds. Countries in Emissions of acidic air pollutants have fallen the European Union enacted a variety of laws to control the release of sulfur dramatically. Why is acid rain still a problem? dioxide and nitrogen oxides; the Clean Air Act imposed similar regulations in Atmospheric dust may be part of the answer the U.S. Policymakers expected these reductions to rejuvenate forests, lakes and streams in many regions. In some by Lars O. Hedin and Gene E. Likens respects, the issue seemed wrapped up.

NEUTRALIZATION LOWERS ACIDIC EMISSIONS ACIDITY OF RAIN

DUST EMISSIONS

NEUTRALIZED PARTICLES OR CLOUD DROPLETS

88 Scientific American December 1996 Copyright 1996 Scientific American, Inc. But the problem of acid rain has not found that all the attention given to acid- juice and beer; ammonia, baking soda gone away. Why is the rain falling on ic compounds in the atmosphere has ob- and antacid tablets are all bases. Most parts of Europe and North America still scured the fact that emissions of bases of the bases in the atmosphere can be acidic, despite tighter controls on pollu- have also decreased. A number of fac- found in airborne particles referred to tion? And why do some natural ecosys- tors seem to be diminishing the level of as atmospheric dust. These dust particles tems—in particular, forests—show lev- these atmospheric bases and in the pro- are rich in minerals such as calcium car- els of damage from acid rain greater cess aggravating the ecological effects of bonate and magnesium carbonate, which than scientists originally predicted? acid rain. Ironically, among these factors act as bases when they dissolve in water. Recent findings suggest that acid rain are some of the very steps that govern- Atmospheric dust particles originate is a much more complex phenomenon ments have taken to improve air quality. from a combination of sources. Fossil- than previously thought. Results from Acids and bases are measured by what fuel combustion and industrial activi- several studies point to the unexpected is known as the pH scale: solutions with ties, such as cement manufacturing, but critical role of chemicals in the at- a pH of less than 7 are acidic; those with mining operations and metal process- mosphere known as bases, which can a pH greater than 7 are basic; those with ing, generate particles that contain bas- counteract the effects of acid rain by a pH of 7 are neutral. Common acids es. Construction sites, farms and traffic neutralizing acidic pollutants. We have around the home include vinegar, orange on unpaved roads also contribute. Sourc- es such as forest fires and erosion caused by wind blowing over arid soils with little vegetation are considered natural yet can still be linked to human activity.

A Natural Antacid

n the air, dust particles can neutralize Iacid rain in a manner similar to the way antacids counteract excess acid in an upset stomach. In a sense, when an acid and a base combine, they cancel each other out, producing a more neutral substance. Neutralization in the atmosphere takes place as dust particles dissolve into acidic cloud-water droplets or combine directly with acidic gases such as sulfur dioxide or nitrogen oxides. These reactions also generate so- called base cations—a term used to de- scribe the positively charged atoms of elements such as calcium and magnesium that arise when mineral bases dissolve in water. In addition to lowering the acidity of precipitation, atmospheric base cations also neutralize acid rain once they reach the ground—although the chemistry is a bit different than in the atmosphere. O OSTI T Small particles of clay and humus (de- OBER

R cayed organic matter) in soil bear nega-

ATMOSPHERIC DUST (brown arrows) contains chemicals known as bases, which neutralize the acidic air pollutants (yellow arrows) that cause acid rain. Industrial emissions, agricultural processes, such as plowing, and traffic on unpaved roads contribute to atmospheric dust. Natural sources include forest fires and erosion by wind. Acidic pollutants derive primarily from the burning of fossil fuels in facto- ries, cars and homes. An additional bene- fit of dust particles is that they deliver nutrients to forests; unfortunately, dust can cause health and environmental problems.

Copyright 1997 Scientific American, Inc. tive charges and thus attract positively With the exception of early work in the were enacted because, as scientists have charged cations, such as calcium and 1950s by Hans Egnér of Uppsala Agri- known for some time, microscopic par- magnesium; as a result, soils contain a cultural University in Sweden and Eville ticles suspended in the air can cause a natural store of base cations attached Gorham of the Freshwater Biological range of health problems when inhaled; to these particles. As acidic rainwater Association laboratory in England, sci- they also degrade visibility and contrib- drains into the ground, the base cations entists have not paid much attention to ute to a host of other environmental give up their places to the positively the idea that the atmosphere can be a problems. Governments in North Amer- charged hydrogen ions found in acids, major source of base cations found in ica and Europe have for over 20 years which bind more tightly to the soil par- soils. Scientists have traditionally thought designated acceptable air-quality stan- ticles. Because these particles sequester that the slow dissolution of minerals dards for particulate matter; these regu- hydrogen ions, the acidity of the water and rocks in deeper parts of the soil re- lations were quite distinct from those that flows through the soil stays low. In plenished base cations, in a natural pro- focusing on acidic pollution. (Atmo- some soils the process becomes more cess called chemical weathering. spheric dust from other sources appears complex: acid rain triggers the dissolu- But recent findings, including our own to have dropped off as well: Gary J. tion of toxic aluminum ions that also studies, are now revising the general view Stensland and Donald F. Gatz of the Il- displace the base cations. of how bases enter soils and how forests linois State Water Survey have found As long as the soil has an abundant depend on atmospheric inputs of min- that emissions of particles containing supply of base cations, this buffering erals and nutrients. In some forests the bases have fallen in response to less system, known as cation exchange, pro- atmosphere actually appears to be the traffic on unpaved roads.) tects forests from the harmful effects of main source of base cations. These new Working together with European sci- acid rain. But the natural reserves of results suggest that many forests are entists, we began by evaluating the base cations can become depleted if soils more sensitive to changes in atmospher- longest records of precipitation chem- that are naturally poor in bases are ex- ic chemistry than scientists once believed. istry that can be found in eastern North posed to acid rain over decades, as has America and western Europe. By mea- been the case in regions of Europe and Less Dust, More Damage suring base cations dissolved in snow North America. In these areas, hydro- and rainwater, we can keep track of the gen ions and aluminum ions have dis- fforts to reduce emissions of acidic levels of mineral bases in the atmosphere placed a large part of the available base Eair pollutants offered encouraging and monitor the input of these base cat- cations in soils, allowing levels of alu- results at first: levels of atmospheric sul- ions into forest ecosystems. Our find- minum to rise and leaving the soil high- fur, for instance, have dropped dramati- ings were startling: we discovered that ly acidic. Furthermore, such acidified cally over the past three decades in much atmospheric bases have declined at un- soils can no longer protect downstream of Europe and eastern North America. expectedly steep rates during the past 10 ecosystems from acid rain: waters that The two of us became concerned, how- to 30 years. The longest existing North drain these forests carry both acids and ever, that policymakers and scientists American record, collected at the Hub- aluminum into streams, lakes and rivers. alike might be neglecting the role of at- bard Brook Experimental Forest in New Dust particles may serve one other im- mospheric bases in their attempts to eval- Hampshire, showed a 49 percent drop portant role. Elements such as calcium uate whether these reductions in sulfur in atmospheric base cations since 1965. and magnesium, as well as sodium and compounds have benefited the environ- On the other side of the Atlantic we potassium—all of which can be found ment. Considering the significance of found that the longest-running high- in mineral dust—are essential nutrients basic chemicals to both forest growth quality European record, from the for- for most plants. Acid rain not only dis- and the prevention of acid rain, we de- ested area of Sjöängen in southern Swe- lodges these elements from clay and hu- cided to investigate whether levels of at- den, showed a 74 percent decrease in mus particles, from which plants get mospheric dust have also changed over base cations since 1971. Our analyses of most of their nutrients, it also washes time in response to lower emissions im- several other records confirmed with few them into rivers and streams, depleting posed by new regulations. exceptions that atmospheric bases have the ecosystem of its store of minerals. Regulations to limit emissions of dust declined precipitously across extend-

SWEDEN NORTHEASTERN U.S. 40 80 80

60 60 30 SULFUR POLLUTANTS 40 40 JENNIFER C. CHRISTIANSEN 20 20

CONCENTRATION 20

BASE CATIONS IN TREE RINGS CALCIUM 0 0 10 (MICROEQUIVALENTS PER LITER) 1960 1970 1980 1990 1960 1970 1980 1990 (MICROEQUIVALENTS PER GRAM) 1960 1970 1980 1990 YEAR YEAR YEAR Note: microequivalents are used to measure quantities of acids and bases. SOURCE: E. A. Bondietti et al., 1990 PARALLEL DECREASES in acidic sulfur pollutants and the dition, other studies have shown that levels of the base cation base cations that neutralize them cancel out much of the expect- calcium have decreased in the trees of a New Hampshire forest ed beneﬁt from reducing acidic pollutants. The authors’ studies over the past several decades; such decreases in essential nutri- in Sweden and the U.S. provide evidence for these trends. In ad- ents further weaken forests.

90 Scientific American December 1996 Copyright 1996 Scientific American, Inc. Atmospheric Dust and Acid Rain ed areas of Europe and North America. But have these cuts in atmospheric bases been strong enough to counteract—or even nullify—the expected environmental beneﬁts of reductions in acidic emissions? Our research indicates that this indeed has been the case. We found that the decline in bases has of- HYDROGEN ION ten mirrored the downturn in atmospheric sulfur, at rates sharp enough to offset a large part of the drop in sulfur compounds. For example, we found SOIL PARTICLE that the decrease in base cations can- BASE celed out between 54 and 68 percent of CATION the reductions in atmospheric sulfur in Sweden and up to 100 percent at some locations in eastern North America [see ALUMINUM ION illustration on opposite page]. These trends mean that declines in bases have kept the atmosphere sensitive to acidic compounds despite reduced emissions of these chemicals. When we began this SOIL PARTICLE work, we certainly did not anticipate that reductions in one form of pollutants—dust particles—would be found O OSTI to decrease the success of reductions of T OBER

another pollutant, sulfur dioxide. R The numerous sources of dust particles and the often sketchy information BASE CATIONS (green) in soil provide nutrients for plants, which absorb the chemicals through their roots. Typically, base cations attach themselves to particles of humus on emissions of particulates make it dif- or clay (left). But when acid rain falls on the soil, hydrogen ions (red) from the rain dis- ficult to determine why these sharp re- place the base cations, which are then washed away. Over time, the hydrogen ions, to- ductions in atmospheric bases have oc- gether with aluminum ions (blue) released from the soil as a result of acid rain, can build curred. We do know that new and clean- up on particles (right). Not only do hydrogen and aluminum displace essential nutrients, er industrial techniques, developed in but they interfere with the plant’s biochemistry; aluminum in particular can be toxic. accordance with regulations on the release of particulate matter, have been an important factor. For example, im- many and the U.S. Several recent stud- acid rain but also because they do not proved combustion efficiency and the ies of ailing forests show that the pre- receive enough base cations from the at- practice of scrubbing particles from cipitous loss of base cations can be a mosphere. Scientists can now pinpoint smokestacks have curtailed particulate key factor in the phenomenon of forest the origin of base cations and trace their pollution associated with the burning decline. Ernst-Detlef Schulze and his movements through forest ecosystems of fossil fuels. Evaluating the contribu- colleagues at the University of Bayreuth by looking at the natural isotopes of the tion of more diffuse sources of dust— have argued that depletion of magne- element strontium (determined by eval- traffic, agricultural methods and wind sium in soils has played a significant role uating the number of neutrons in the erosion, for instance—has been more in the dwindling of spruce forests in the nucleus of a strontium atom), which can difficult. But our studies suggest that Fichtelgebirge of Germany. Although be used as a tracer for calcium. Stron- the decline in dust particles mainly re- their evidence is less clear, researchers at tium atoms that derive from the bedrock flects changes in human behavior as op- Oak Ridge National Laboratory in Ten- and those that come from the atmo- posed to natural variations. nessee, led by Samuel B. McLaughlin, sphere tend to exist as different mixtures have found that the slowdown in growth of isotopes. This technique has illustrat- A Major Source of Nutrients of red spruce trees in the southern Ap- ed that atmospheric dust is in fact a palachian Mountains correlates with critical source of mineral ions in many cientists have watched for years as lower availability of calcium in soils. In- forest ecosystems. Scalcium, magnesium and potassium terestingly, small-scale experiments in- Moreover, in certain regions, where levels have dropped in forest soils around volving fertilization of some forests with soils tend to be damaged by acid rain or the world. For example, Leif Hallbäck- base cations, particularly calcium and naturally low in base cations, most of en and Carl Olof Tamm, both at Upp- magnesium, have ameliorated damage— the calcium appears to come from the sala Agricultural University in Sweden, in the sugar maple forests of Quebec, for atmosphere rather than the bedrock. have documented losses of 56 to 74 per- instance, and in Norway spruce and sil- For instance, we have determined that cent of the available cations in Norway ver fir forests of Germany and France. in unpolluted forests of Chile, the dom- spruce forests over the past 60 years. Reports such as these made us wonder inant tree species, the southern beech, Other reports show similarly dramatic whether certain soils are suffering not feeds on calcium that originates almost losses of base cations in England, Ger- only because of continued exposure to exclusively in the atmosphere.

Atmospheric Dust and Acid Rain Copyright 1996 Scientific American, Inc. Scientific American December 1996 91 These observations suggest that many expected and that the damage caused by forests that have been heavily damaged forests depend quite heavily on the at- the pollution is more serious than pre- by acid rain and have low stores of cal- mosphere for a supply of mineral bases; dicted. For instance, the widely quoted cium in the soil. the drops in atmospheric base cations conclusion from the 1990 National Acid What can we do about acid rain and have therefore led to a slower replenish- Precipitation Assessment Program (the atmospheric dust? Suggestions range ment of critical bases and nutrients in most recent evaluation of the problem of from the improbable to the feasible. Af- forest soils. Of course, natural levels of acid rain by the U.S. government), that ter the publication of one of our recent atmospheric dust have always varied, there was no clear evidence linking acid papers, a reader wrote proposing that but usually across centuries or millen- rain to forest damage, no longer seems forests might be saved by a hot-air bal- nia. Studies conducted by Paul A. May- tenable. loon campaign to drop calcium-rich ewski and his co-workers at the Univer- It is entirely feasible that continuing particles from the skies—a costly and sity of New Hampshire on impractical solution. De- ice cores from Greenland liberate increases in the indicate that the amounts release of particulates are of dust and calcium in the also unrealistic and would atmosphere have been set back progress in air- strongly affected by cli- pollution control by de- mate variations over the cades. One reasonable past 20,000 years. In the suggestion, however, is to coldest and driest global reduce emissions of acidic climates, high levels of cal- pollutants to levels that cium and dust prevailed, can be buffered by natu- whereas wetter and warm- wide ral quantities of basic ld er periods saw low con- or compounds in the atmo- one W centrations. Analysis of t sphere; such a goal would ony S modern trends, from T mean continued reduc- Y T around 700 A.D. to the T tions in sulfur dioxide and present, suggests that cur- nitrogen oxides, perhaps rent quantities of dust are JOHN BEA even greater than those relatively low compared SANDSTORM in the Sahara Desert can scatter dust particles around prescribed in the 1990 with conditions during the the globe. Studies in Amazon forests have turned up dust that origi- Amendments to the Clean past 20,000 years. One nated in the Sahara, more than 3,000 miles away. Air Act in the U.S. notable exception was the The ecological dilemma Dust Bowl, the extended drought of the acid rain, in combination with limited of atmospheric dust will very likely be mid-1930s in the western U.S. supplies of base cations, could produce with us for some time: base cations environmental conditions to which many take years to build up in soils, and it Remaining Questions plant species, particularly in sensitive may take decades or more for forests to ecosystems, have never been exposed in recover their depleted pools of nutri- s scientists have discovered the im- the course of their evolution. Conse- ents, even if levels of acidic air pollution A portance of bases in the atmo- quently, predicting how they will respond continue to fall. In the meantime, re- sphere and, more recently, the link be- over the next several decades will be ex- searchers and governments must devel- tween emissions of atmospheric dust tremely difﬁcult. And effects may not be op careful strategies not only for moni- and nutrients in the soil, they have be- limited to plants. Jaap Graveland and toring the current health of forests but gun to paint a new picture of how for- his colleagues at the University of Gro- also for predicting their stability in the ests respond to atmospheric pollution. ningen, have noted that certain birds, next century and beyond. Simple solu- This emerging view suggests that the ef- such as the great tits of the Netherlands tions do not always work in complex fects of acid rain are more complex than produce thinner, more fragile eggs in ecosystems. SA

The Authors Further Reading

LARS O. HEDIN and GENE E. LIKENS have Ecology and Decline of Red Spruce in the Eastern United States. Edited by worked together for more than a decade, examin- C. Eagar and M. B. Adams. Springer-Verlag, 1992. ing how acid rain affects forest and aquatic ecosys- Poor Reproduction in Forest Passerines from Decline in Snail Abundance tems. Hedin is an assistant professor in terrestrial of Acidiﬁed Soils. J. Graveland, R. van der Wal, J. H. van Balen and A. J. van biogeochemistry in the section of ecology and sys- Noordwijk in Nature, Vol. 368, pages 446–448; March 31, 1994. tematics at Cornell University. In addition to his re- Steep Declines in Atmospheric Base Cations in Regions of Europe and search on base cations, Hedin studies nutrient cy- North America. Lars O. Hedin, Lennart Granat, Gene E. Likens, T. Adri Buis- cles in unpolluted temperate and tropical forests. hand, James N. Galloway, Thomas J. Butler and Henning Rodhe in Nature, Vol. Likens is director of the Institute of Ecosystem 367, pages 351–354; January 27, 1994. Studies in Millbrook, N.Y. He has published exten- Long-Term Effects of Acid Rain: Response and Recovery of a Forest sively on the topic of acid rain and on how human Ecosystem. Gene E. Likens, Charles T. Driscoll and Donald C. Buso in Science, Vol. activities impact the environment. 272, pages 244–246; April 12, 1996.