Pollutants Emitted by Transport Systems (Air, Water and Noise)

Home > Contents > Chapter 8 > Application 2

Author: Dr. Jean-Paul Rodrigue

1. Air Pollutants

One geographical dimension of air pollution is at the local and regional levels where its externalities are immediately felt. The higher the level of concentration of transport activities, the higher their environmental impacts are being felt by the local community. This is particularly the case for large transport terminals, such as ports, rail yards and airports. A salient example is Hong Kong, one of the world's most important container port. The container terminals are centrally located with with an acute concentration of large ships and tens of thousands of truck movements per day. Many air pollutants have been 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 vapors, 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.

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 climate change.

Sulfur Dioxide (SO₂)

Sulfur dioxide is a heavy, colorless gas with a strong odor. 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 climatic models.

Ozone

Ozone is a pale blue gas with a strong odor 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.

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 colorless 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 during 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.

2. Water Pollutants

Transportation contributes significantly to the pollution of the hydrosphere in various ways ranging from air pollution fallouts to the construction and maintenance of infrastructure such as roads, railways and ports. The first type of impacts are related to the transport modes.

Air Pollution Fallouts

Fallouts occur when a pollutant goes from an airborne state (gas, solid or liquid) towards a solute or colloidal state. Water is a very good solvent for several pollutants, notably acid depositions. Fallouts are accelerated and concentrated in an area by rainy conditions. As an important source of air pollution, transportation accounts on a similar scale for fallouts. In some areas transportation may account for up to 25% of nitrogen fallouts in water. It is estimated that acid rains may account for more than 75% of the growth of acidity of lakes.

Since fallouts are a continuous accumulation and occur over a longer period than most water pollution sources, they have a higher impact on still-water (lentic) environments than running-water (lotic). The most notable and destructive fallouts are sulfuric and nitric acids that may alter the pH of water if they are present in sufficient concentrations. Several northeastern United States and eastern Canadian lakes have seen their entire fish population destroyed as a result of increased acidity levels. It also includes damage to forests like reduced photosynthesis (sparse foliage) and acidified soils (limited nutrients). Nitrous oxides may affect the ecological balance of marine life by favoring algae blooms.

Other fallouts such as HC/VOC and lead are poisonous and may disrupt marine life if they accumulate in the aquatic food chain. Particulate fallouts, when in sufficient quantities, may increase the turbidity of water and thus reduce the photosynthesis capacity of aquatic plants. A long term accumulation of air pollution fallouts of various nature will contaminate and disrupt whole aquatic ecosystems.

Marine Vessels Discharges and Spills

After unloading their bulk loads like oil, coal, nitrates and mineral products, marine vessels require cleaning. Since this practice is restricted in several port and coastal areas, operators wait until they are in international waters to proceed. Oil products residuals carried by tankers are the major source for discharges. It is estimated that for every million tons of oil carried, one ton is spilled through washouts. Once a spill has occurred, it is extremely difficult to contain it. From 1989 to 1992, 105 accidental oil spills by tankers were accounted worldwide, totaling 991,000 tons of oil being spilled. Annually, an average of 1.1 million tons of oil comes from discharges and 400,000 tons are spilled. They depend on the nature of the residue discharged. Petroleum products are the most harmful and include environmental effects like the destruction/disruption of aquatic plant/animal life and of shore ecosystems. Since most marine life is in neritic (continental shelf) and epipelagic (less then 100 meters) zones, it is particularly vulnerable to marine vessels discharges.

De-Icing of Infrastructure and Runoffs

Salt (NaCl) has the characteristic of lowering the melting point of water and thus presents an useful compound for keeping safe road conditions in sub-zero climates. Other elements like sand and gravel are also added to provide adherence. Runoffs occur when substances accumulated by a surface (notably a road) are dissolved / carried by water and evacuated elsewhere. It is often the convergence of a surface to a point. De-icing of transportation infrastructure (roads, parking lots, airfields etc.) is almost the only artificial source of salt release in the environment. Salt mostly comes from mining (halite) or in fewer proportions from sea water evaporation. Other compounds like calcium and magnesium can be used, but they work more slowly and cost ten times as much. Lubricants (from car leakages - engine, brakes, and transmission), heavy metals (Zn, Cd, Cu, Ni, Cr and Fe from abrasion of tires and brake linings) and dry fallouts (HC/VOC, particulates) account for harmful sources of runoffs.

Since road infrastructure (parking lots, roads, drainage systems) occupy a significant land surface in developed countries, it is the major source of runoffs. For instance, while highways occupy 5-8% of the urban catchment area, it contributes for as much as 50% of the total suspended solids, 16% of the total HC and 75% of the total metal inputs to a receiving stream. High concentrations of salt, notably chlorine ions, in fresh water environments disrupt life cycles and may be fatal to some organisms like larvae. Runoffs from infrastructure will alter the turbidity and the oxygen level of water (warm water holds less oxygen), and contaminate the food chain. It may increase the eutrophication process of several lakes, particularly in recreational areas where dirt roads are dense. De-icing salt has the tendency to accumulate in snow and soils beside roadways. During early springtime, nearly all the salt accumulated will be released in the hydrographic system where it will contaminate ground water and interfere with the growth of plants and the reproduction cycle of aquatic life, particularly vulnerable at this time of year. Infrastructure runoffs collected by the sewage system of urban areas often converge at evacuation points and contaminate whole hydrographic systems at high concentrations. It is worth noting that most cities have 30 to 70% of their surface occupied by roads and parking space. They thus represent important sources of runoffs.

Construction and Maintenance of Infrastructure

Several transportation infrastructures have important territorial handholds. When a transportation infrastructure is built over a hydrological environment like a river, wetland or a coastal area, disruption occurs. The maintenance of transportation infrastructure, particularly harbor and waterways (dredging), have also a significant impact. Each mode needs a specific set of infrastructure that interfere with hydric systems. Road infrastructure accounts for most of the territorial handhold of transportation with structures like bridges and parking facilities. Railways have also an important handhold over continental hydric systems. Maritime transportation, by its intrinsic link with hydric systems have several disruptive infrastructure like piers, canals, harbors and terminals. Airports have similar effects when constructed over wetland. Dredging accounts alone for 80% of the waste released in aquatic environments.

The most widespread effect of transportation infrastructure on hydric systems is the removal of natural habitats along shorelines. The aquatic / land interface to which several animal and vegetal species depend is considerably reduced. Further, a modification of the aquatic environment occurs, particularly during dredging in port harbors and along waterways. This notably influences the turbidity of water and destroys habitats. Roads and rails, when running through wetland, reduce the water regeneration / purification capacity by splitting available areas and disrupting water flows. Large ports occupy extensive areas along the shorelines of waterways and coasts. The construction and maintenance of those infrastructure have thus extensive impacts over aquatic environments. The construction of canals changes whole hydrographic systems by altering water flows (quantity and speed) at regional and often at continental levels.

3. Noise

Road accounts for approximately 70% of total noise emissions by transportation. It must be noted that different road transportation modes have different scales of noise emissions. Main sources of noise come from the engine and the friction of the wheels over the road surface. Further, travel speed and the intensity of traffic are directly linked with its intensity of noise. For instance, one truck moving at 90 km/hr makes as much noise as 28 cars moving at the same speed. Ambient noise is a frequent result of road transportation in urban areas. The addition of all the noise generated by cars, trucks and buses creates a permanent ambient noise (ranging from 45 to 65 db) that impairs the quality of life in urban areas and thus the property values of residences. Nearby road arterials, ambient noise is replaced by direct noise and vibrations. The acoustics created by the surrounding environment (hills, buildings, trees, open space, etc.) alleviate or worsen local conditions. Noise level grows arithmetically with speed. For instance a car traveling at 20 km/hr emits 55 db of rolling noise, at 40 km/hr 65 db, at 80 km/hr 75 db and at 100 km/hr 80 db. Available evidence underlines that around 45% of the population in developed countries live in high levels of noise intensity (over 55 db) generated by road transportation. Along major highway arterials in inter-urban areas, noise emissions are likely to alter the living environment of wildlife species.

Rail accounts for 10% of total noise emissions by transportation. Noise comes from the engine (mostly diesel), the friction of wheels over the rails, and whistle blowing. Furthermore, when trains are moving at high speed, areoacoustic noise becomes more important than other sources. Depending of the train aerodynamics, noise emissions are from 50 to 80 times the logarithm of train speed and become significant at speeds higher than 200 km/hr. When rail / truck transshipment is involved, the convergence of trucks towards railyards provides an additional source of noise related to rail transportation activities. Around 3% of the population may be exposed to high noise levels from rail transportation in OECD countries. The level of exposure is obviously related to the importance and location of rail transportation infrastructure. The most important noise impacts of rail operations are in urban areas where the majority transshipment functions are performed. Furthermore, rail terminals are often located in the central and high density areas of cities.

Air transportation accounts for 20% of total noise emissions by transportation. As air transportation took a growing importance in inter-city transportation and that jet engines were predominantly used, noise emissions have increased significantly to the point of becoming a major concern near airports. Noise comes from the jet engine, the aerodynamic friction and ground craft operations. Even if the turbofan is the least noisy jet propulsion technology available, aircrafts are an acute source of noise in several urban areas. Noise from aircraft operation is known to have direct impact on property values around airports. This effect is distributed along major approach and takeoff lanes. The establishment of heavily used flight paths between major cities creates noise corridors where ambient noise is almost prevalent. This is particularly noted when those corridors are over densely populated areas.