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1.What is Acid Rain?

What is Acid Rain? ??Water can dissolve many different substances. We use water for laundry to get rid of soil. Thus soil is dissolved into water. In nature, precipitation is a part of natural water circulation as explained in Figure 1, and all substances in water are transported from the atmosphere to the ground, rivers, and lakes. When this occurs, sulfur oxide and nitrogen oxide, which are air pollutants, are incorporated into precipitation as acid and fall onto the ground. This phenomenon is called acid rain. Generally this phenomenon is known as rainfall but recently it is also called "acid deposition", and this includes forms of precipitation such as fog (wet deposition process), gases, and aerosol (particles), and forms which do not dissolve into water (dry deposition process).

Figure 1: Water circulation and broad overview of acid rain

2.The effects of Acid Rain

   When acid (hydrogen ions) from acid rain flows into the ground, it is neutralized and elutes calcium ions and magnesium ions which are necessary nutrients for plants.  When it penetrates deeper into the ground, it decomposes minerals and elutes aluminum ions, which are harmful to aquatic life and plants.  Acid eventually flows into groundwater, lakes, and rivers and acidifies the water.  The elution of aluminum ions is considered to be a breaking point.

   Once acidification of soil progresses, it affects terrestrial water.  In Europe and North America , acidification of lakes is a serious problem.  In Northern Europe , one of the measures currently implemented is to disperse lime into lakes but still none of them are decisive in resolving the problem. 

   The effects of acidification of lakes for the ecosystem have been confirmed by reports and experiments.  As pH values decrease in lakes, young fish die and only large fish can survive in those lakes.

   In the case of terrestrial water acidification, damage starts to appear once pH values fall below 6.5.  Most insects, if not eaten by fish, will die below pH4.5 and white moss will accumulate at the bottom of lakes. 

   No significant impact on ecosystems by acidification has been reported in Japan for a long time.  However, a trend of stream water acidification was recently observed in the watershed of Lake Ijira in the middle of Japan , where the magnitude of wet deposition of acidifying substances was remarkably large. 

   Soil acidification affects not only terrestrial water but also plants.  There’s a concern that plants also get affected by acid fog and acid gases.  In 1973, we saw cucumbers and tobacco plants become brown and die in the Kanto region.  Acid and hydrogen peroxide in drizzle have thought be the cause for this.

   Also damage to fir trees in the Black Forest in Germany and the spruce forest in the Whiteface Mountains of New York in the U.S. are well known as possible examples of damage done by acidic precipitation and fog (clouds).

Photo 1 ※Please click to each photograph

   Photographs 1 were taken in 1995 and they show the region called the Black Triangle surrounded by Germany , Czech Republic , and Poland .  We can see clearly that most trees in the region have died.

   The natural environment is not the only thing affected by acid rain.  Damage done to buildings, particularly to sculptures and historical buildings (churches) in Europe is well known.  

Photo 2  ※Please click to each photograph

   Photographs 2” show the exterior wall of Cologne Dome in Germany .  They are spending 1 billion yen annually for repairs.  Acid rain icicles are famous in Japan .  The effects of gases which cause acid rain can be seen indoors and damage to irreplaceable sculptures, art works, and books are evident.  The damage done to swords was in the news in Japan

   On the other hand, the effects of acid rain to human health have also been a concern.  In Sweden where the acidification of the environment is significantly notable, half the nation uses groundwater for their water supply and acidified well water corrodes metal inside water pipes so that there are cases where the color of hair has changed to green when washed.  Also copper and zinc are thought to have contributed to the persistent diarrhea of unknown causes among infants.  In addition to these, the effect of acidic mist to people with asthma has become a problem..


3. Acid Rain chemistry

   Acid is a chemical component producing hydrogen ions (H+) and pH value indicates the concentration of hydrogen ions in precipitation.

   The pH value of clean water with well dissolved carbon dioxide in the atmosphere is 5.6 and precipitation below pH5.6 is considered polluted with acid from atmospheric pollutants.  Therefore, a pH value of 5.6 is often used as a guideline value for acid rain.  However, the pH value of precipitation can be affected by surrounding volcanoes and alkaline soil.  There are cases where pH5.0 is set as the guideline value considering natural environments.

Hydrogen ions (H+) are not the only component in precipitation.  Major ion components include ions such as sulfate ions (SO42-), nitrate ions (NO3-), chloride ions (Cl-), and cations such as sodium ions (Na+), potassium ions (K+), calcium ions (Ca2+), magnesium ions (Mg2+), and ammonium ions (NH4+).

    Acid concentration in precipitation chemistry is high where precipitation is low but on the other hand, the concentration is low despite the heavy precipitation.  Therefore, the amount of acid deposition (concentration × precipitation) might be used for evaluation.

4.Acid Rain in the world

    The term “acid rain” was first coined in the book “Air and rain” written by Angus Smith in the late 19th century to describe air pollution. 

    However, regional air pollution affecting human health was of more serious concern and acidification of precipitation was recognized as a byproduct of air pollution in those days.  Rather, precipitation was considered as a blessed rain that got rid of air pollutants.  Later, as a measure to solve the air pollution problem, they heightened chimneys to diffuse the air pollutants further away.  Therefore, the concentration of pollutants in the atmosphere around factories decreased but the amount of emissions increased so that more air pollutants in precipitation were observed in areas far from the emission sources.  As a result, in Europe and North America , which are called the leading countries of acid rain, much damage has occurred.

    Currently in Europe and North America , sulfur oxide emissions which cause acidification of precipitation have been decreasing but the emission of air pollutants in the world has not yet started to decrease.  On the contrary, it has been increasing since the industrial revolution, in particular in developing countries in Asia where energy consumption has sharply increased.  Thus, researchers around the world, who have been studying the acidification of precipitation, regard East Asia including Japan as the area that will have the most serious problems in the future.

    The acidification of precipitation in Japan was recognized for the first time because of health effects.  In the summer of 1973, people in the Kanto region started complaining about eye and skin irritation from acidic drizzle.  It was reported that over thirty thousand people complained about the effects the following year.  From this point, a number of studies involving acid rain have started in Japan .

5.The acidity in precipitation

   Figure 2” and “Figure 3” are distribution maps of the average pH value of actual acidification of soil and terrestrial water in Europe and North America .  Figure 4” and “Figure 5” are distribution maps of the average pH value in Japan .

Figure 2 Figure 3 Figure 4 Figure 5

“Figure 6” shows the concentration and deposition amount of SO42- and NO3- in the world.


In Europe and North America , acidification of rivers and lakes are observed in the area where the pH value is below 5.0 and the number of affected rivers and lakes increases in the area where the pH value is below 4.5. 

On the other hand, the pH value in most areas of Japan indicates below 5.0, and in some areas the average indication sometimes seems to be below 4.5.  As for the concentration and deposition of SO42- and NO3- in precipitation in the world, the median value of Japan is equal to the mean value of Europe and North America, and furthermore, they tend to be steady or have even started decreasing in Europe and North America .  However, they tend to increase in East Asia including Japan .  Thus, the pH value and the concentration and deposition of SO42- and NO3- in precipitation in Japan are almost the same as those in Europe and North America .  Therefore, damage may occur at any moment in areas vulnerable to acid rain.

   Figure 7” shows pH values in precipitation in Hokkaido .

    In Hokkaido , pH values are low in cities like the Sapporo metropolitan area and Asahikawa and pH values tend to be low along the Sea of Japan side of Hokkaido . The concentration of nss-SO42- tends to be higher and the nss-SO42- depositions tend to be large in the center and the Sea of Japan side of Hokkaido . The concentration of NO3- tends to be higher and the NO3- depositions tend to be larger in the center of Hokkaido .

   Figure 8” shows the results of a long-term survey in Sapporo .

From around 1990, the pH value has decreased and H+ depositions have increased.  That is due to the decrease of road dust (asphalt dust scratched by studded tires).  The concentrations and depositions of nss-SO42- and NO3-, which are acidic components, tend to be either decreasing or remaining the same and the decline of alkaline components lead to the acidification of precipitation.  Acidification is more significant in winter and this trend can be seen anywhere in Hokkaido although there are slight differences in the degree of acidification and time.  Just like in Hokkaido , the acidification of snow has been progressing in the Tohoku region where studded tires were previously used.

☆Long term trend of pH concentrations and depositions

East of Tomakomai
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   The acidification of snow, in another words “acid snow” is a very serious problem in Hokkaido and the Tohoku region where they have a prolonged snow season.  Acid snow accumulates pollutants until snow melting season starts in early spring.  At the beginning of snow–melting season, melted snow with condensed low pH value flows into rivers and lakes and affects both the aquatic and terrestrial ecosystems greatly.

Figure 9” shows distribution of pH values of snowcover and its long-term trends in Hokkaido .

   The Sea of Japan side of Hokkaido where most snow falls tends to have lower pH values.  As was previously mentioned, asphalt dust in the atmosphere decreased so that pH values in 1992, 1996, 2000, and 2004 were lower than that of 1988.  Therefore effects from snow–melted water have become a concern in areas along the Sea of Japan .

Figure 9: pH values of snowcover in Hokkaido
Distribution maps of contents of components

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Distribution maps of concentrations of components

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6.The future measures against the acid rain

   Currently in the world, besides two monitoring networks like EMEP in Europe and NAPAP in North America, the Acid Deposition Monitoring Network in East Asia (EANET) started monitoring since fiscal year 2000 in East Asia where the acidification of the environment is expected to be on the rise.

EANET monitoring stations were set up on Rishiri Island and Cape Ochiishi in Nemuro , Hokkaido.  The monitoring network system (other than EANET stations, composed of 27 stations in Japan including the one in Sapporo ) was established throughout Japan by the Ministry of Environment and a nationwide survey has been carried out by JELA (Japan Environmental Laboratory Association: prefectural governments and environment laboratories in government–designated cities).  Thus national and regional governments cooperate with each other and monitor acid rain. 

There are many questions that remain unanswered, such as; “Do air pollutants emitted from one country affect some areas of another country and to what extent?” and “Which forest is dying or in which river or lakes has water acidification been progressing?”  These will be answered by utilizing research and monitoring reports.  Furthermore by making those reports public not only in one’s own country but also to the world.  This will promote work on policies against acid rain and it will become possible to control the emission of air pollutants which cause acid rain.

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