Air Pollutants Emitted by Transport Systems
Author: Dr. Jean-Paul Rodrigue
1. Local and Regional Impacts
One geographical dimension of air pollution is at the local and regional
levels where its externalities are immediately felt. Many pollutants
identified as being closely related to transportation:
Carbon Monoxide CO
- Carbon monoxide is a colorless, odorless gas, the result of
the incomplete combustion of hydrocarbons.
- Transportation accounts from 70 to 90% of total carbon monoxide
emissions. It is thus the air pollutant the most strongly associated
with transportation. Carbon monoxide is often present near major
traffic intensive arterials, notably in urban areas. Carbon monoxide
is a poisonous gas.
- When inhaled, it combines with hemoglobin to form carboxyhemoglobin,
preventing absorption of oxygen and resulting in asphyxiation. 0.5%
of carbon monoxide in air may prove fatal in less than half an hour
by transforming over 50% of the hemoglobin in carboxyhemoglobin.
Lower concentrations of carbon monoxide (3 ppm) may cause poisoning
symptoms and affect people with heart, lung and circulatory system
weaknesses. It also effects the respiration of plants by inhibiting
photosynthesis.
- Since carbon monoxide is not chemically very stable, direct
global effects are strongly limited (probably non existent). Indirectly,
carbon monoxide contributes to the formation of greenhouse gazes
as a catalyst.
Nitrogen Oxides NOx
- Nitrogen oxide (NO or NO2) is a brown, odorless gas.
A by-product of combustion when energy is used to oxide nitrogen
instead of an hydrocarbon.
- Transportation accounts from 45 to 50% of total emissions
of nitrogen oxides. Other sources are chemicals (notably nitrates)
industrial production and combustion of fossil fuels in thermal
power plants.
- Nitrogen oxides are not very harmful to humans (particularly
NO), but when released from an internal combustion engine, high
concentrations are often toxic. It irritates and infects the respiratory
system and the eyes. Some decreases in the ability to resist bacterial
infection were also observed when the subject is exposed to significant
concentrations of nitrogen dioxide. Nitrogen oxides are known to
prevent the growth of crops and thus reduce agricultural yields.
- Nitrogen oxides are known to be associated with several global
effects and have increased at a rate of 0.2% annually over the last
decades. They are a catalyst for ozone, a component of acid rain
and a component of smog. Depositions of nitrogen oxides influence
the nitrate cycle, particularly in water where it influences algae
blooms.
Hydrocarbons and Volatile Organic Compounds - (HC/VOC)
- Hydrocarbons (HC) are a group of chemical compound composed
of carbon and hydrogen. When in a gaseous form, HC are called Volatile
Organic Compounds (VOC). Several HC and VOC are heavy gazes or volatile
compounds with a strong odor. They are mostly the result of the
incomplete combustion of gasoline or by-products of the petrochemical
industry. They include methane (CH4), gasoline (C8H18)
and diesel vapours, benzene (C6H6), formaldehyde
(CH2O), butadiene (C4H6) and acetaldehyde
(CH3CHO).
- Transportation accounts from 40 to 50% of total emissions
of HC/VOC. They can be emitted by incomplete combustion (70%),
during refueling (10%) or by evaporation from storage units (20%),
particularly gas tanks. For instance, a car parked overnight during
summer emits approximately 4 grams of HC/VOC. Other important sources
are petrochemical (plastics and solvent) industries.
- All HC/VOC are carcinogen (cases of leukemia linked with benzene)
to some extent, fatal at high concentrations, harmful to crops and
accumulates within the food chain (poisoning). However, heavy hydrocarbons
(like benzene) are far more carcinogen than light hydrocarbons (like
methane).
- All HC/VOC have several global effects. They are components
of smog, catalysts for ozone and components of acid rain.
Particulates
- Particulates include various solids in suspension in the atmosphere
such as smoke, soot, and dust and results of the incomplete combustion
of fossil fuels, notably coal. They may also carry traces of other
toxic substances like HC/VOC.
- Transportation accounts for around 25% of total emissions
of particulates. Diesel engines are the main emitters. Other
important sources are thermal power plants using coal.
- Particulates are carcinogen. They are also harmful to lungs
tissue and worsen respiratory and cardiovascular problems, notably
if their size is smaller than 5 microns. Particulates depositions
may alter the aesthetic of structures.
- The accumulation of particulates in the atmosphere and deposition
on leafs may reduce photosynthesis and plant growth.
Smog
- Mixture of solid and liquid fog and smoke particles formed through
the accumulation of carbon monoxide, ozone, HC/VOC, nitrogen oxides,
sulfur oxide, water, particulates, and other chemical pollutants.
Photochemical smog are those with a higher concentration
of ozone and HC/VOC.
- Smog is strongly linked with transportation and industrial
activities, notably in urban areas. Smog is particularly dense
during a thermal inversion (static regional air masses that enable
the accumulation of pollutants).
- The effects of smog are the conjunction of those of its major
components (see the effects of carbon monoxide, sulfur dioxide,
nitrogen oxide, HC/VOC, particulates and ozone). Based upon historical
observations (like London in the 50s), the number of deaths among
susceptible persons (respiratory and cardiovascular problems) grows
sharply during thermal inversions.
- Several large cities (like Los Angeles, Tokyo and Mexico) have
serious smog problems to the point that emissions reduction policies
are established. Smog impairs visibility considerably and causes
different annoyances (odors, irritations, etc.). Because of its
components, smog is highly associated with acid rains and greenhouse
effects.
Lead Pb
- Lead is a toxic metal mainly used as an anti-knock agent
in gasoline (Lead tetraethyl - Pb(C2H5)4)
and in batteries (lead dioxide as an anode and lead as a cathode).
- Until recently, lead tetraethyl was a main source of atmospheric
lead emissions in developing countries. This contribution has dropped
in absolute numbers but still accounts for 30 to 40% of total emissions.
Batteries are now an important source of lead for transportation,
but a very limited amount of this lead is carried through the atmosphere
(see water pollution).
- Extremely poisonous metal. Lead has effects on the metabolism
and accumulates in living tissues. May causes anemia, and mental
retardation for young children. For instance, an extremely high
occurrence of mental retardation in some parts of Mexico city was
directly linked with lead poisoning. Small doses may cause behavioral
changes.
- Lead is fixed by plants and animals and re-contaminate the food
chain. It has a high potential to accumulate in the environment.
Lead can also be transported in the atmosphere over wide distances.
Odors
- Odors are the subjective perception of the sense of smell. They
exists different "shapes" of odors perceived as pleasant, neutral,
or unpleasant. A long run exposition to specific odors will attenuate
their perception.
- Diesel and gasoline engines are the major sources of odors
accounted by transportation. Odors are particularly prevalent during
smog conditions. Odors are at worst an annoyance, but they are linked
with the presence of harmful air pollutants like sulfur dioxide,
ozone and HC/VOC. People tend to stay or move away from areas having
a significant prevalence of odors.
2. Global Impacts
Although the pollutants below can have local and regional impacts,
their scope is more global.
Carbon Dioxide CO2
- Carbon dioxide is a colorless, odorless gas that composes 0.04%
of the atmosphere. Whenever there is combustion (oxidation) of fossil
fuels, there is an emission of carbon dioxide. Important temperature
regulator for the atmosphere, keeping it a +15oC instead
of -15oC if carbon dioxide was absent.
- Transportation accounts for around 30% of total carbon dioxide
emissions in developed countries (15% worldwide).
- About 66% of carbon dioxide emissions from transportation come
from the combustion of gasoline, 16% from diesel fuel and
15% from jet fuel. Carbon dioxide emissions by transportation have
the following modal breakdown: cars (43%), light trucks (20%), heavy
trucks (14%), airplanes (14%), rail and marine (7%) and non-oil
based (2%). Other significant natural sources are volcanic eruptions
and the metabolic respiration of living organisms (including decomposition).
- Carbon dioxide is a harmless gas and an essential element of
photosynthesis. Although limited concentrations of carbon dioxide
have no effects on human beings, high concentrations (5000 ppm)
may be harmful by causing breathing disorders. Growing quantities
of carbon dioxide in the atmosphere are assumed to be linked with
the greenhouse effect. Concentrations
of CO2 have rose on the average of 0.4% per year over
the last decades. Before the industrial revolution (early 19th century),
the concentration of carbon dioxide was estimated to be around 275
ppm, while by 2002 it has reached 372 ppm. It is advocated that
a level of 400 ppm is the limit after which climate modifications
become important and difficult to predict. Levels of about 600 ppm
are anticipated by the middle of the 21st century.
Sulfur Dioxide SO2
- Sulfur dioxide is a heavy, colorless gas with a strong odour.
It is the result of the combustion of fossil fuels like coal (particularly
bituminous coal) and hydrocarbons.
- Transportation accounts for around 5% of total sulfur dioxide
emissions. Although transportation is a minor source of SO2,
related activities like steel and petrochemical industries are important
emitters. One of the most important artificial source are thermal
power plants using low quality coal. Volcanic eruptions are an important
natural source of sulfur dioxide.
- Sulfur dioxide causes and worsens respiratory and cardiovascular
problems. In sufficient concentration, it irritates the eyes and
causes discomfort (odor). Sulfur is an essential nutrient for plants
but sulfur dioxide is regarded as an inhibitor of physiological
activity. Most affected plants are those having a high physiological
activity like crops and commercial timber forests.
- A major component favoring the genesis of acid rain. Sulfur
dioxide has a counter effect on greenhouse
gases by blocking radiation. This effect is significant enough
to be included in global warming climatic models.
Ozone
- Ozone is a pale blue gas with a strong odour and a powerful
oxidant. It is the most common photochemical oxidant. Ozone is created
naturally in the high atmosphere when an oxygen molecule is broken
apart by ultraviolet radiation and combines with another oxygen
molecule.
- Ozone is also the result of the action of light over a mixture
of HC/VOC and nitrogen oxides in the lower atmosphere. It is thus
directly linked with transport emissions, notably in urban
areas.
- Ozone is poisonous, hampers breathing and irritates the eyes
and the respiratory system at concentrations higher than 0.15 ppm.
The normal/natural concentration is around 0.01 ppm at ground levels.
It degrades structures (metal and concrete) through oxidation. It
damages crops and vegetation and leads to losses of leafs. Depending
on the crops and the concentration involved, ozone may reduce yields
from 1 to 20%. Ozone impairs visibility.
- Ozone is essential in the upper atmosphere, as it absorbs light
in the ultraviolet band. A drop of 5% in the concentration of ozone
may lead to an increase of 10% of skin cancer and eye cataracts.
Ozone is also a greenhouse gas.
Acid Rain and Acid Depositions (Sulfuric and Nitric Acid - H2SO4,
HNO3)
- Sulfuric acid is a corrosive, oily colorless liquid, which forms
when sulfur oxides and water vapors are mixed. Nitric acid is a
corrosive and colorless liquid and forms when nitrogen oxides and
water vapor are mixed. The level of formation of acid (sulfuric
and nitric) is influenced by the level of exposition to sun light.
It may also exists in dry form, which is called acid deposition.
When dissolved in water, sulfuric and nitric acids lower the pH
(higher concentrations of hydrogen ions). The standard pH of fresh
water ranges between 6.5 and 7.5.
- Since transportation accounts for 5% of sulfur dioxide
emissions, 45% of nitrogen oxides emissions and for 40% of HC/HOV
emissions, sources may range from 10 to 30% of acid rains,
depending on regions. This figure is of 25% in Western Europe.
- Sufficient concentrations of sulfuric of nitric acids are known
to damage artificial structures, thus historical monuments are particularly
vulnerable. When inhaled as a mist, may cause respiratory organs
irritation.
- Change the chemical composition of soils by breaking down complex
organic matter in simpler elements. At a small scale, this is beneficial,
but at a large scale, it reduces the available biomass. By altering
the pH of fresh water, acid rains gradually destroy life in lake
and rivers.
- Sulfuric and nitric acids are carried over large distances through
weather systems. It later falls down either as rain or fog. Acid
rain and acid depositions are known to alter the ecological balance
of continental ecosystems, notably in industrialized areas.
Chlorofluorocarbons (CFCs)
- CFCs are colourless and poisonless gases (or liquids). They
are very stable, non-flammable and non-toxic components and they
have been widely used as dispersing agents (aerosols) or as refrigerants
(notably Freon, R-12).
- For transportation, motor vehicle air-conditioning systems
are the main source and account for about 20% of all CFCs emissions.
In fact, during its life cycle, an air-conditioning system will
release 100% of its CFCs in the atmosphere. With recent legislations,
CFCs emissions have considerably subsided in developed countries
but not in developing countries.
- Because of its chemical properties (stable and non-toxic), CFCs
have no noticed effects on living organisms.
- Current concentrations of CFCs in the atmosphere reach about
0.35 ppm (all types of CFCs) but the most widely used type, R12,
has 20,000 times more infrared absorbency than carbon dioxide. Thus
one ton of Freon will have the same
greenhouse effect than 2,000 tons of carbon dioxide. CFCs reduce
the concentration of stratospheric ozone, which absorbs harmful
ultraviolet rays. CFCs may stay in the atmosphere from 70 to 200
years, due to their extremely stable properties. They are a long
term component of the atmosphere. CFCs emitted in the 1990s are
likely to damage the ozone layer for 200 years.
- Indirect effects of CFCs (increase in ultraviolet rays exposition)
include growths in the incidence of skin cancer, eye cataracts,
damage to crops and plants, deficiencies of the immune system and
increase of ozone at ground levels (through photochemical smog).
Even though transportation contributes significantly to the emission
of air pollutants new technologies (catalytic converters) and policies
have reduced emissions significantly, notably
in the United States.
Media

Contribution of Man-Made Gases to the Greenhouse Effect

Emissions of Major Air Pollutants in the United States, 1970-1991
(in million metric tons)