Acid
Rain Facts
What causes acid rain?
Acid
deposition is a general term that includes more than simply acid rain. Acid
deposition primarily results from the transformation of sulphur dioxide (SO2)
and nitrogen oxides into dry or moist secondary pollutants such as sulphuric
acid (H2SO4), ammonium nitrate (NH4NO3)
and nitric acid (HNO3). The transformation of SO2 and NOx
to acidic particles and vapours occurs as these pollutants are transported in
the atmosphere over distances of hundreds to thousands of kilometers. Acidic
particles and vapours are deposited via two processes - wet and dry deposition.
Wet deposition is acid rain, the process by which acids with a pH normally
below 5.6 are removed from the atmosphere in rain, snow, sleet or hail. Dry
deposition takes place when particles such as fly ash, sulphates, nitrates, and
gases (such as SO2 and NOx), are deposited on, or absorbed onto,
surfaces. The gases can then be converted into acids when they contact water.
What does acid mean?
An
acid is a substance with a sour taste that is characterized chemically by the
ability to react with a base to form a salt. Acids turn blue litmus paper (also
called pH paper) red. Strong acids can burn your skin.
A
pH scale is used to measure the amount of acid in a liquid-like water. Because
acids release hydrogen ions, the acid content of a solution is based on the
concentration of hydrogen ions and is expressed as "pH." This scale
is used to measure the acidity of rain samples.
- 0 = maximum acidity
- 7 = neutral point in the middle of the scale
- 14 = maximum alkalinity (the opposite of acidity)
The
smaller
the number on the pH scale, the more acidic the substance is. Rain measuring
between 0 and 5 on the pH scale is acidic and therefore called "acid
rain." Small
number changes on the pH scale actually mean large changes in acidity.
For
example, a change in just one
unit from pH 6.0 to pH 5.0 would indicate a tenfold increase in acidity. Clean rain
usually has a pH of 5.6. It is slightly acidic because of carbon dioxide which
is naturally present in the atmosphere. Vinegar, by comparison, is very acidic
and has a pH of 3.
Where is acid rain a problem?
Acid
rain is a problem in eastern Canada
because many of the water and soil systems in this region lack natural
alkalinity - such as a lime base - and therefore cannot neutralize acid
naturally. Provinces that are part of the Canadian Precambrian Shield, like Ontario, Quebec, New Brunswick and Nova
Scotia, are hardest hit because their water and soil
systems cannot fight the damaging consequences of acid rain. In fact, more than
half of Canada
consists of susceptible hard rock (i.e., granite) areas that do not have the
capacity to effectively neutralize acid rain. If the water and soil systems
were more alkaline - as in parts of western Canada
and southeastern Ontario
- they could neutralize or "buffer" against acid rain
naturally.
In
western Canada,
there is insufficient information at this time to know whether acid rain is
affecting these ecosystems. Historically, lower levels of industrialization -
relative to eastern Canada - combined with natural factors such as eastwardly
moving weather patterns and resistant soils (i.e., soils better able to
neutralize acidity), have preserved much of western Canada from the ravages of
acid rain.
However,
not all areas in western Canada
are naturally protected. Lakes and soils resting on granite bedrock, for
instance, cannot neutralize precipitation. These are the conditions found in
areas of the Canadian Shield in northeastern Alberta,
northern Saskatchewan and Manitoba,
parts of western British Columbia, Nunavut and the Northwest
Territories . Lakes in these areas are as defenseless
to acid rain as those in northern Ontario.
If sulphur dioxide and nitrogen oxide emissions continue to increase in western
Canada, the same sort of
harmful impacts that have happened in eastern Canada could occur.
Where do sulphur dioxide emissions
come from?
Sulphur
dioxide (SO2) is generally a byproduct of industrial processes and
burning of fossil fuels. Ore
smelting, coal-fired power generators and natural gas processing are the main
contributors. In 2000, for instance, U.S. SO2 emissions were
measured at 14.8 million tonnes - more than six times greater than Canada's 2.4
million tonnes. But the sources of SO2 emissions from the two
countries are different. In Canada,
68% of emissions come from industrial sources and 27% comes from electric
utilities (2000). In the U.S.,
67% of emissions are from electric utilities (2002).
Canada
cannot win the fight against acid rain on its own. Only reducing acidic
emissions in both Canada and
the U.S.
will stop acid rain. More than half of the acid deposition in eastern Canada originates from emissions in the United States.
Areas such as southeastern Ontario (Longwoods)
and Sutton, Quebec
receive about three-quarters of their acid deposition from the United States.
In 1995, the estimated transboundary flow of sulphur dioxide from the United States to Canada was between 3.5 and 4.2
millions of tonnes per year.
Have SO2 emission levels
changed at all?
Initiated
in 1985, the Eastern Canada Acid Rain program committed Canada to cap SO2 emissions in the
seven provinces from Manitoba
eastward at 2.3 million tonnes by 1994, a 40% reduction from 1980 levels. By
1994, all seven provinces had achieved or exceeded their targets. In 1998, the
provinces, territories and the federal government signed The Canada-Wide Acid
Rain Strategy for Post-2000, committing them to further actions to deal with
acid rain. Progress under both the Eastern Canada Acid Rain Program and under
the Post-2000 Strategy, including data on emissions, is reported in the
respective annual reports of these two programs. Between 1980 and 2001,
emissions of SO2 declined by approximately 50% to 2.38 million
tones. In eastern Canada
, emissions of SO2 declined by approximately 63% between 1980 and
2001.
Where do NOX emissions
come from?
The main source of NOx emissions is the combustion of fuels
in motor vehicles, residential and commercial furnaces, industrial and
electrical-utility boilers and engines, and other equipment. In 2000, Canada's
largest contributor of NOx was the transportation sector, which accounted for
approximately 60% of all emissions. Overall, NOx emissions amounted to 2.5
million tonnes in 2000. By comparison, U.S. NOx emissions for 2000 amounted to
21 million tonnes - 8 times more than Canada 's emissions.
The
influence of transboundary flows of air pollutants from the United States into Canada is significant. Overall
about 24% of the regional-scale ozone episodes that are experienced in the United States also affect Ontario. An analysis of ozone concentrations
at four sites in extreme southwestern Ontario
taking wind factors into account provides an estimate that 50 to 60% of the
ozone at these locations is of U.S.
origin (Multi-stakeholder NOx/VOC Science Program 1997b).
Have NOX emission levels
changed at all?
In
Canada
, total NOx emissions have been relatively constant since 1985. As of 2000,
stationary sources of NOx emissions have been reduced by more than 100,000
tonnes below the forecasted level at power plants, major combustion sources and
metal smelting operations. In 2000, as part of the Ozone Annex to the Canada-US
Air Quality Agreement, Canada
committed to an annual cap on NO2 emissions from fossil-fuel power
plants of 39,000 tonnes in central and southern Ontario
and 5,000 tonnes in southern Quebec.
It also committed to new stringent emission reduction standards for vehicles
and fuels and measures to reduce NOx emissions from industrial boilers. These
commitments are estimated to reduce annual NOx emissions from the Canadian
transboundary region (defined as central and southern Ontario
and southern Quebec)
by approximately 39% from 1990 by 2010.
What is the difference between a
target load and a critical load?
The
critical load
is a measure of how much pollution an ecosystem can tolerate; in other words,
the threshold above which the pollutant load harms the environment. Different
regions have different critical loads. Ecosystems that can tolerate acidic
pollution have high critical loads, while sensitive ecosystems have low
critical loads.
Critical
loads vary across Canada.
They depend on the ability of each particular ecosystem to neutralize acids.
Scientists have defined the critical load for aquatic ecosystems as the amount
of wet sulphate deposition that protects 95% of lakes from acidifying to a pH
level of less than 6. (A pH of 7 is neutral; less than 7 is acidic; and greater
than 7 is basic.) At a pH below 6, fish and other aquatic species begin to
decline.
A
target load
is the amount of pollution that is deemed achievable and politically acceptable
when other factors (such as ethics, scientific uncertainties, and social and
economic effects) are balanced with environmental considerations. Under the
Eastern Canada Acid Rain Program, Canada
committed to cap SO2 emissions in the seven provinces from Manitoba eastward at 2.3
million tonnes by 1994. The program's objective was to reduce wet sulphate
deposition to a target load of no more than 20 kilograms per hectare per year
(kg/ha/yr), which our scientists defined as the acceptable deposition rate to
protect moderately sensitive aquatic ecosystems from acidification.
Under
the Canada-Wide Acid Rain Strategy for Post-2000, signed in 1998, governments
in Canada
have adopted the primary long-term goal of meeting critical loads for acid
deposition across the country. Recently, maps that combine critical load values
for aquatic and forest ecosystems have been developed. These maps indicate the
amount of acidity (reported as acid equivalents per hectare per year (eq/ha/yr))
that the most sensitive part of the ecosystem in a particular region can
receive without being damaged.
.
No comments:
Post a Comment