Nitrogen Oxides (2)

What are the effects of nitrogen oxides?

High concentrations of NO2 can produce an abnormally high accumulation of fluid in lung tissue. For exposures ranging from several minutes to one hour, a level of 50 – 100 ppm NO2 causes inflammation of lung tissue for a period of 6 – 8 weeks, after which time the subject normally recovers. Exposure of the subject to 150 – 200 ppm of NO2 causes bronchititis fibrosa obliterans , a conditions fatal within 3 – 5 weeks after exposure. Death generally results within 2 – 10 days after exposure to 500 ppm or more of NO2.

NO2 also causes extensive damage to plants through its secondary products such as peroxy acyl nitrate formed in smog. Exposure of plants to several parts per million of NO2 in the laboratory causes leaf spotting and break down of plant tissue. It also causes fading of dyes and inks used in some textiles. Much of the damage to materials caused by NOx, such as stress – corrosion cracking of electrical apparatus, comes from secondary nitrates and nitric acid.

How to control nitrogen oxides' emission?

It is possible to lower nitrogen oxides by carrying out the combustion in two stages, the first of which is rich in fuel and the second of which is rich in air. In this way the fuel is burned completely, but the temperature is never as high as it would be for a stoichiometric mixture. This two-stage approach is being incorporated in power plants; it has been tried in cars but with less success.

The other method that is done to reduce emissions is to remove the pollutant from the exhaust gases. In automobiles, this is done by the use of a threeway catalytic converter.

In order to deal with both NO and unburned gases the converter has two chambers in succession. In the reduction chamber, NO is reduced to N2 by hydrogen, which is generated at the surface of a rhodium catalyst by the action of water on unburned fuel molecules.

HC +H2O→H2 + CO
2NO+ 2H2 →N2 + 2H2O

In the oxidation chamber, air added, and the CO and unburned hydrocarbons are oxidised to CO2 and H2O at the surface of platinum/palladium catalyst.

2CO + O2 →2CO2
HC + 2O2 → CO2 + 2H2O

Nitrogen Oxides (1)

How are nitrogen oxides formed?

In this post, I am going to discuss two of the nitrogen oxides that are important in the study of air pollution, namely, nitrogen monoxide (NO) and nitrogen dioxide (NO₂). The most abundant oxide is nitrous oxide. This is however chemically rather unreactive and is formed from the natural biological processes in the soil. Nitrogen monoxide first undergoes photochemical reaction. The formed atomic oxygen reacts with another molecule of N₂O to give NO. The formed nitric oxide reacts with ozone, thereby causing ozone depletion.

The following equations show how nitrogen and oxygen in the air combine to form nitrogen monoxide:
N₂O+O→2NO

The nitrogen monoxide then reacts with more oxygen to become nitrogen dioxide:
NO+O₂ → 2NO₂

Nitrogen monoxide is formed by the combustion of nitrogen-containing compounds (including fossil fuels). Thus all high temperature processes produce NO, which is then oxidised to NO2 in the ambient air. In the natural world, these reactions occurs in lightning and forest fires. What is nitric oxide used for in the natural world then? Actually, it is an important source of nitrogen for growing plants.

In contrast to nitric oxide, nitrogen dioxide is very reactive and significant species in the atmosphere. The principal reactions among NO,NO₂, and HNO3 are indicated below:

In conclusion, nitric oxide and nitrogen dioxide are important constituents of polluted air. These oxides collectively designated as NOx, enter the atmosphere mainly from combustion of fossil fuels in both stationary and mobile sources.

Sulfur Dioxide (2)

What produces sulfur dioxide?

The two natural sources of SO₂ are volcanic eruptions and sulfur containing geothermal sources like geysers and hot springs. Emission rates of sulfur dioxide from an active volcano range from 20 tonnes per day to 10 million tonnes per day according to the style of volcanic activity and type and volume of magma involved.

For example, the large explosive eruption of Mount Pinatubo on 15 June 1991 expelled 3-5 km3 of dacite magma and injected about 20 million metric tons of sulfur dioxide into the stratosphere. Together with the increased stratospheric chlorine levels from human-made chlorofluorocarbon (CFC) pollution, the sulfate aerosols dispersed from the volcanic eruption destroyed ozone and led to some of the lowest ozone levels ever observed in the atmosphere.

At Kilauea Volcano, the recent effusive eruption of about 0.0005km3/day (500,000 m3) of basalt magma releases about 2,000 tonnes of sulfur dioxide into the lower troposphere. Downwind from the vent, acid rain and air pollution is a persistent health problem in Hawaii when the volcano is erupting. These large explosive eruptions would also inject a tremendous volume of sulfur aerosols into the stratosphere can lead to lower surface temperatures and promote depletion of the Earth's ozone layer.


There are also industrial production of SO2, including the following few.

Burning of fuels

Large amounts of coal and petroleum are burnt around the world in power stations to generate electricity and in industries to provide energy. Both types of fuels contain sulfur as an impurity, although coal contains a higher concentration of it. Diesel fuel, which is used in vehicles, also contains a little sulfur. It is estimated that 70% of worldwide atmospheric SO2 comes from power stations.

When these fuels are burnt, the sulfur undergoes oxidization to become sulfur dioxide. The chemical equation is as follows:

S (s) + O2 (g) --> SO2 (g)
*Sulfur is from fuel, while oxygen is from the air.

Oil refining:
Sulfur and hydrogen sulfide are constituents of crude oil and H2S is released as a gas during catalytic cracking. Since H2S is considerably more toxic than SO2 it is burned to produce SO2 before release to the ambient air.

Pulp and paper manufacture: 
The sulfite process for wood pulping uses hot H2SO3 and thus emits SO2 in air. The kraft pulping process produces H2S, which is then burned to produce SO2.

Sulfur Dioxide (1)

How would sulfur dioxide affect the world?

Sulfur dioxide is a colorless gas with a pungent odor that irritates skin and the tissues and mucous membranes of the eyes, nose, and throat. Sulfur dioxide chiefly affects upper respiratory tract and bronchi. Locally, sulfur dioxide gas can lead to acid rain and air pollution.

When sulfur is entrained in an aerosol, it is possible for sulfur oxides to reach far deeper into the lungs. The combination of particulate matter and sulfur oxides can then act synergistically, with the effects of both together being much more detrimental than either of them separately. Sulfur dioxide is one of the serious air pollutants which is responsible for smog formation, which has resulted in several incidents of loss of human lives. Atmospheric sulfur dioxide is harmful to plants and leaf tissue is killed with exposure to high levels of gas. A case study would further strengthen my point.

Case Study:
Linfen has been identified as one of its most polluted cities. Rapid development and unequivocal faith in industry has led to the development of hundreds of unregulated coal mines, steel factories and refineries which have polluted indiscriminately through the burning of fossil fuels, releasing soot, sulfur dioxide and excessive heat. The State Environmental Protection Administration (SEPA) has branded Linfen as having the worst air quality in the country. Levels of sulfur dioxide and other particulates are many times higher than limits set by the World Health Organization (WHO). Local clinics are seeing growing cases of bronchitis, pneumonia, and lung cancer. The children of Shanxi Province also have high rates of lead poisoning.

Sulfurous pollutants can discolour paint, corrode metals, and cause organic fibres to weaken. Airborne sulfates significantly reduce visibility and discolour the atmosphere. Prolonged exposure to sulfates causes serious damage to buildings made of marble, limestone and mortar, as the carbonates of these materials are replaced by sulfates, which are soluble in water.

CaCO3 +H2SO4→CaSO4 + CO2 +H2O

Furthermore, sulfur dioxide once released can convert to SO3, in a series of reaction which involve a free radical such as OH•. Sulfur trioxide react quickly with H2O to form sulfuric acid, which is the principal cause of acid rain.

SO₃ + H₂O → H₂SO4

Sulfuric acid molecules rapidly become particles by either condensing on existing particles in the air or by merging with water vapour to form H2O - H2SO4 droplets. Often significant fraction of particulate matter in the atmosphere consist of such sulfate aerosols. The formation is promoted by the presence of hydrocarbons and nitrogen oxides, which are key components of photochemical smog. Photochemical smog is more harmful than the separate pollutants. It burns the eyes of people and is dangerous to people with breathing or heart problems. It is poisonous to plants, and damages materials such as paint and rubber.

In relatively humid atmospheres, SO2 is oxidized by reactions occurring inside water aerosol droplets, which proceed faster in the presence of ammonia and catalysts such as manganese (II), iron (II), nickel (II), copper (II), etc.

Global Warming (2)

What is done to curb Global Warming?

As you have seen, the main cause of global warming is actually due to the exponential increase in concentration of CO2 in the atmosphere. So, in the natural state, what is the major sink for CO2? The major important sink for CO2 is ocean. This could be seen from the following diagram:

Because sea water is alkaline and CO2 is acidic, the oceans are vast reservoir of CO2 as reaction would occur. However only the surface layer of ocean, the top 75 meters, is in equilibrium with the atmosphere and its capacity to absorb CO2 is limited. Exchange of surface layer with deep oceans takes hundreds of years. The location of the remaining carbon dioxide has been a subject of considerable debate, but it is now widely accepted that vegetation absorbs much of the CO2.

But even with this natural sink sources, the CO2 level is continuously increasing resulting in global warming. Global warming can shift the climate zones and the existing forests may not be able to adapt, especially if the shift is rapid and again the result may be loss of biomass. Global warming may also lead to increased evaporation of water thereby reducing water available for agricultural, municipal, and industrial use.

Just a note, global leaders have also taken a step forward. International leaders gathered for the first time to address the emission of greenhouse gases during the  negotiation of the Kyoto Protocol in 1997, which only came into force only in 2005 due to lack of support by all countries. Kyoto Protocol recommends reduction in emission of greenhouse gases by 5.2% between 2008 and 2012. However, effectiveness of the Kyoto Protocol depends on full co-operation from all countries – thus, due to the importance of fossil fuels in the development of many countries, and that many were concerned that the profits of their industries may be affected by strict emissions control that might eventually lead to job losses, the number of countries agreeing to the terms of the Kyoto Protocol was relatively small. As recently as 2005, many developing countries like China  still refused the terms of the Kyoto Protocol, whereas Singapore only acceded to the agreement, joining the list of 141 countries committed to environmental protection in 2006.


Scientists have also worked hard to reduce carbon dioxide emissions due to the fear of the impending impacts of global warming on the Earth. Here is a video regarding some efforts on the reduction of carbon dioxide:

Global warming (1)

Effects of global warming

The increment in carbon dioxide concentration in the atmosphere is one of the indubitably serious environmental concern. The atmosphere consists of methane, chlorofluorohydrocarbons, nitrous oxide, water vapour and carbon dioxide, which leads to global warming. Statistics from Indian Institute of Technology Madras shows that carbon dioxide contributes to 50% of the world's temperature rise, while the gas succeeding it, methane, or CH4, only contributes to 19% of the world's rise in temperature.

Chlorofluorocarbons, nitrous oxide, CO2 and methane has been significantly increasing each year, however, the largest effect of global warming is still due to CO2. Anthropogenic production of CO2 from burning fossil fuels exceeds that of the other greenhouse gases. Even though the concentration of CO2 in the atmosphere in the natural state due to the constant respiration of the biosphere is much greater, this flux is in balance with photosynthesis. CO2 exist in the atmosphere naturally to trap heat so that the warmth of the Earth is maintained for animal's survival. CO2, hence, is also known as a type of greenhouse gas.


Greenhouse gases trap solar energy and keep Earth warm. Greenhouse effect is this natural phenomenon of warming Earth’s surface and lower atmosphere whereby gases in the atmosphere trap heat from the Sun's rays to keep Earth warm.  Some of the rays that pass through the atmosphere are reflected back into outer space when they reach Earth.  However, gases in the atmosphere trap some of the reflected heat from these rays just like glass panes of a greenhouse. As such, Earth is kept warm enough at about 15 degrees Celsius for life to exist. On the other hand, without Greenhouse Effect, heat from the Sun would be lost to outer space and Earth would be approximately 33 degrees Celsius lower, which is too cold to support life. Greenhouse effect in its natural state is ideal because greenhouse gases occur naturally, trapping solar energy and keeping Earth desirably warm to sustain life.

However, an excess of it would mean an increasing Earth temperature, also known as the Enhanced Greenhouse Effect or Intensified Greenhouse Effect. It is a process whereby excessive heat from the Sun is being trapped by atmospheric gases due to an increase in the amount of greenhouse gases as a result of human activities. This excessive trapping of solar energy leads to an increase in average temperature of the Earth's surface known as Global Warming. It is estimated that Earth's temperature has increased by about 0.3 degrees Celsius in the last 40 years.If left unchecked, Earth's average temperatures will rise a further 1.5 to 4.5 degrees Celsius in the next 100 years.

Since the Industrial Revolution in the 1700’s, human activities, such as the burning of oil, coal and gas, and deforestation, have increased CO2 concentrations in the atmosphere. In 2005, global atmospheric concentrations of CO2 were 35% higher than they were before the Industrial Revolution.Carbon dioxide (CO2) concentrations in the atmosphere increased from approximately 280 parts per million (ppm) in pre-industrial times to 382 ppm in 2006 according to the National Oceanic and Atmospheric Administration's (NOAA) Earth Systems Research Laboratory, a 36 percent increase. Almost all of the increase is due to human activities. The current rate of increase in CO2 concentrations is about 1.9 ppmv/year. Present CO2 concentrations are higher than any time in at least the last 650,000 years.

Causes of Global Warming

The expansion of agricultural activities is one of the human causes of global warming. Carbon dioxide is absorbed by plants during photosynthesis process. If natural environment is left untouched, optimum level of carbon dioxide remains. However, increase in agricultural activities makes it necessary to create more space through deforestation. Deforestation by burning releases carbon dioxide in the atmosphere. This is worsened by the presence of fewer trees to use up and remove excess carbon dioxide from the atmosphere. This problem is especially prevalent in the Indonesia area, where many environmental officials are actually involved in illegal logging and hurting its own forest.

Another gas that cause global warming, as mentioned, is methane. Methane is produced in wet rice cultivation because flooded fields are ideal for methane-releasing bacteria to thrive, such as Rice Paddies in Japan. In livestock farms when sheep and cattle digest and belch, like cattle manure in México, and where organic waste matter and decayed vegetation decomposes, methane is too, released into the atmosphere.

Furthermore, as the human population increases, more food is needed. More nitrogen-based fertilisers are used to increase crop yields. Nitrous oxide is emitted, increasing the amount of greenhouse gases. For example, cropland soils in China are turning acid from the overuse of nitrogen fertilisers, decreasing productivity, polluting the environment, and contributing huge amounts of greenhouse gas emissions. 

In addition, there is the industrial development and expansion of manufacturing industries. With industrial development, more fossil fuels are burnt in power stations and factories to cater to increasing demands for energy to run machinery, resulting in more carbon dioxide emitted. Since Singapore started her development toward a first-world country, CO₂ emissions has increased about 83% from 1990 to 2007. Next up, the use of solvents in industries and making of foam packaging release CFCs. Also, rising affluence results to increase in private car ownership. As society gets richer, private cars become affordable and people tend to drive instead of taking public transport. More nitrous oxide is released from vehicle exhausts.

Chlorofluorocarbons (CFCs) are also effective in trapping heat, with one molecule of CFC having the ability to trap as much heat as the ability of 10 000 molecules of carbon dioxide. CFCs are released through the use of aerosol sprays which act as propellant that releases CFCs, and careless and inconsiderate disposal of old refrigerators and air conditioners, where the CFCs are used as coolants.

Last but not least, the lifestyle changes and indifferent attitude of people are of paramount importance in today's warming world. 

Carbon Monoxide (3)

How to stop carbon monoxide emissions?

You have understood that the major contribution to CO pollution is from transportation sources and gasoline fed internal combustions. Therefore, in order to curb the emission of CO pollution, control measures have been concentrated on the automobiles.

Carbon monoxide emissions may be lowered by using a relatively low air-fuel mixture, that is one in which the weight ratio of air to fuel is relatively high. With air fuel ratios ratios exceeding approximately 16 : 1, an internal combustion engine emits virtually no carbon monoxide.

Automobiles in the present day install catalytic exhaust reactors to reduce carbon monoxide emissions. It works by pumping excess air into the exhaust gas, which is then passed through a catalytic converter in the exhaust systems, resulting in oxidation of CO to CO2.

However, there are still problems retaining to this issue. Catalytic reactors basically lack of sufficiently durable (50,000 driven miles) catalytic material. The catalysts now in use are subjected to poisoning by the adsorption of materials on their surfaces. One of the most efficient catalytic poisons is lead and this is one reason for the development of lead free gasoline.

Carbon Monoxide (2)

How is carbon monoxide produced?

Now, knowing the deadly effects of carbon monoxide, aren't you interested in the source of this gas?

Industrial processes:
Carbon monoxide is formed during the incomplete combustion of compounds containing carbon.

2C +O₂ →2CO

It is also formed in huge quantities during the reaction between carbon-containing materials at high temperatures, such as, in blast furnaces.

CO2 + C→2CO

Carbon monoxide is also produced during the dissociation of CO2 at high temperature, although, being rather rare.

CO emission from vehicle exhaust:
One of the major sources of CO in the ambient air is vehicle exhaust. Internal combustion engines do not burn completely to CO2 and water; some unburnt fuel will always be exhausted, with CO as a component. It tends to accumulate in areas of concentrated vehicle traffic.
For example, the number of motor vehicles on Beijing's roads more than quadrupled from 1 million in 1997 to 4.76 million at the end of 2010, and researches has shown that more than three quarters of total air pollution comes from vehicle carbon monoxide and hydrocarbon emissions in large cities including Beijing, Shanghai and Guangzhou.

Natural processes:
Volcanic action, natural gas emission, electrical discharge during storms, seed germination, marsh-gas production etc, are the natural processes that contribute to a small measure for the presence of CO in the atmosphere. Forest fires contribute to 7.2% of CO emissions and agricultural burning contribute 8.3% of emissions. The atmospheric back ground concentration of CO is 0.1 ppm.



Major sink process:
Now, are you wondering since there are so many sources of carbon monoxide gas, wouldn't people be dying? What on earth controls the amount of carbon monoxide gas?

In soil, the major CO sink is by soil microorganisms. The major sink process in the atmosphere is however is the conversion to CO2 by reaction with hydroxyl radical. This process is however rather slow and the reduction in CO level away from the source area is almost entirely a function of atmospheric
dilution processes. The residence time of CO in the atmosphere is of the order of 4 months
and it is removed from the atmosphere by reaction with hydroxyl radical, HO•:

CO+HOi→CO2 +H..................................................................(3)

The reaction of atomic hydrogen with atmospheric oxygen produces hydroperoxyl radical.

Carbon Monoxide (1)

What is carbon monoxide?

Carbon monoxide is a colourless, odourless tasteless gas, that is by far the most abundant of the criteria pollutants. This deadly gas can kill, and let us first look at a case study.

Case Study:

A male patient, Mr S., presented to the ED having been driven there by his wife after she found him "barely breathing in the garage with his car running." She stated she was at work and came home to find him in his car "very groggy." Mr S. was in a state of respiratory arrest and was intubated shortly after being admitted to the ED and placed on 100% oxygen. His pupils were dilated and very sluggish. He was very lethargic and disoriented. His blood pressure was 166/102 mm Hg, pulse is 102 beats per minute, temperature is 96.4°F, and oxygen saturation is 92% on room air. Breath sounds were diminished, and his respiratory rate was 6 breaths per minute before being intubated. The ED physician obtained a venous blood sample, which revealed the patient's COHb level to be 49%, blood sugar of 185 mg/dL, and total cholesterol level of 260 mg/dL; cardiac enzymes and troponin T were within reference range. His electrocardiogram showed sinus tachycardia with no ST-T wave changes. An arterial blood gas was ordered, and plans were under way to transport Mr S. to a facility where hyperbaric oxygenation (HBO₂) could be provided

As seen, carbon monoxide threatens human health. This is because carbon monoxide has a strong affinity for haemoglobin, Hb, which can be represented by the following equilbrium equation.

Hb(aq) + 4CO(g)  --> Hb(CO)4(aq)

Thus, a person breathing air that contains carbon monoxide converts his/her haemoglobin to Hb(CO)₄, thus reducing the blood’s normal oxygen-carrying capacity. After entering the blood stream through the lungs, carbon monoxide reacts with haemoglobin (Hb) to convert oxyhaemoglobin (O2Hb) to carboxyhaemoglobin (COHb). However, at levels of CO that occur in urban air, there are apparently no detrimental effects on materials or plants, but those levels can adversely affect human
health.

Carbon monoxide, in fact, has a much greater affinity for haemoglobin than does oxygen, so that even small amounts of CO can seriously reduce the amount of oxygen conveyed throughout the body. With this blood stream carrying less oxygen, brain function is affected and heart rate increases in an attempt to
offset the oxygen deficit.

Introduction to Air Pollutants

Pure air is colourless, odourless, and safe to inhale. They give us a healthy life, where our family and friends can live safely in. Yet, there are often pernicious substances in the atmosphere that are harmful to the living things and environment, resulting to air pollution. Although there are substances like smoke and soot, of which are pollutants that could be seen or smelt, generally, pollutants are colourless. In this blog, the focus would be placed on chemicals which pollute the air --- these chemicals are also known as air pollutants. Air pollutants can be arbitrarily classified according to chemical composition as (1) inorganic air pollutants and (ii) organic air pollutants.

The following inorganic air pollutants are going to be discussed:

• Oxides of carbon (eg., CO and CO₂)
• Sulphur Dioxide (i.e., SO₂)
• Nitrogen Oxides (i.e., NOx)
• Others (including ozone, methane and unburnt hydrocarbons, of which are going to be mentioned under the several environmental problems)

The following environmental problems are to be introduced:

• Ozone hole
• Global warming
• Acid rain