 What
causes acid rain?
Acid rain is rain, snow or fog that is polluted by acid in the atmosphere
and damages the environment. Two common air pollutants acidify rain: sulphur
dioxide (SO2) and nitrogen oxide (NOX). When these
substances are released into the atmosphere, they can be carried over long
distances by prevailing winds and return to earth as acidic rain, snow,
fog or dust. When the environment cannot neutralize the acid being deposited,
damage occurs.
What does acid mean?
Acid is a substance with a sour taste that will react with a base to form
a salt. Acids turn blue litmus paper (also called pH paper) red. Strong
acids can burn your skin.
What is pH?
A pH scale is used to measure the amount of acid in a liquid—like water.
The acid content 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 cannot
neutralize the effects of acid rain. If the water and soil systems were
more alkaline—as in Western Canada—they could neutralize or "buffer" against
acid rain naturally.
Atlantic region
In Western Canada, acid rain is not a problem now but could always become
one if we are not careful. 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. If we monitor the environment carefully and apply strict pollution
controls when necessary, we should be able to prevent acid rain from becoming
an environmental concern in Western Canada.
However, not all areas in Western Canada are naturally protected. Lakes
and soils resting on granite bedrock, for instance, cannot neutralize precipitation.
These districts include areas of the Canadian Shield in northeastern Alberta,
northern Saskatchewan and Manitoba, and parts of western British Columbia.
Lakes in these areas are as defenseless to acid rain as those in northern
Ontario. They must be shielded from exposure to acid rain—if not environmental
damage could be swift and serious.
Visit The NatChem Website for Deposition Maps
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 1995, for instance,
U.S. SO2 emissions were measured at 16.8 million tonnes—a
full six times greater than Canada’s—2.7 million total tonnes. But the
sources of SO2 emissions from the two countries are quite different.
While 61% of Canada’s emissions come directly from industrial sources,
66% of the U.S.’ emissions are from the electrical utilities.
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 Muskoka-Haliburton and Quebec City
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 to 4.2 millions of tonnes per year.
SO2 Emissions from Canada and the United States in 1995
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, emissions
in eastern Canada totalled just 1.84 million tonnes—20 % below the 2.3-million
tonne cap and a 52% reduction from 1980 levels.
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 1995, Canada’s
largest contributor of NOX was the transportation sector, which
accounted for approximately 60% of all emissions. Overall, NOX
emissions amounted to 2.25 million tonnes in 1995. By comparison, U.S. NOX
emissions for 1995 amounted to 21.7 million tonnes—10 times more than Canada’s.
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 occur simultaneously
in 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).
NOx Emissions from Canada and the United States in 1995
Have NOX emission levels changed at all?
So far, Canadian NOX emissions have increased only slightly
from 2.25 million tonnes in 1995 to 2.33 million tonnes in 1998. This was
attributed largely to increased economic activity and industrial fuel use.
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.
The critical load varies across the Canada. It depends on the ability
of a particular ecosystem to neutralize acids. The critical load for aquatic
ecosystems is defined 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 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 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.
The maximum amount of acid deposition that a region can
receive without damage to its ecosystems is known as its critical load.
It depends essentially on the acid-rain neutralizing capacity of the water,
rocks, and soils and, as this map of eastern Canada shows, can vary considerably
from one area to another.
Will acid rain remain a problem after the Canadian and U.S. control programs
are fully implemented?
Yes. Areas of southern and central Ontario, southern and central Quebec,
New Brunswick and Nova Scotia will continue to receive mean annual sulphate
deposition amounts that exceed their critical loads. The critical load
will be exceeded by up to 10 kg/ha/yr of wet sulphate in parts of central
and southern Quebec. As a result, about 95,000 lakes will remain damaged
by acid rain. Lakes in these areas have not responded to reductions in
sulphate deposition as well as, or as rapidly as, those in less sensitive
regions. In fact, some sensitive lakes continue to acidify.
Even in 2010, with full implementation of the Canadian and U.S. programs,
almost 800,000 km2 in southeastern Canada—an area the size of France and
the United Kingdom combined—will receive harmful levels of acid rain; that
is, levels well above critical load limits for aquatic systems.
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