Air Pollutants Emitted by Transport Systems

Author: Dr. Jean-Paul Rodrigue

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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 NO_x

• 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 +15^oC instead of -15^oC 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.