Gaseous Pollutants Origins and Effects for the ESAT

The Origins of Carbon Oxides

Carbon containing fuels, such as coal, oil, and natural gas, consist primarily of hydrocarbons. When these fuels are burned in a plentiful supply of oxygen, complete combustion occurs. The carbon atoms are fully oxidised to form carbon dioxide, $CO_2$. The general equation for the complete combustion of a hydrocarbon can be represented by the reaction of methane: $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$.

However, if the oxygen supply is limited, incomplete combustion takes place. In this scenario, carbon is not fully oxidised, resulting in the formation of carbon monoxide, $CO$, and sometimes carbon particulates (soot). The formation of carbon monoxide can be shown as: $2C(s) + O_2(g) \rightarrow 2CO(g)$.

Environmental and Health Effects of Carbon Oxides

Carbon dioxide is a significant greenhouse gas. It absorbs infrared radiation that is reflected from the surface of the Earth, trapping heat within the atmosphere. While the greenhouse effect is a natural process that keeps the planet habitable, the excessive release of $CO_2$ from burning fossil fuels is a primary driver of enhanced global warming and climate change.

Carbon monoxide is a highly toxic, colourless, and odourless gas, making it extremely dangerous as it cannot be easily detected by human senses. When inhaled, $CO$ binds irreversibly to the haemoglobin in red blood cells. Because haemoglobin has a much higher affinity for carbon monoxide than for oxygen, the blood becomes unable to transport sufficient oxygen to the body tissues and organs. This can lead to fainting, brain damage, and death.

Sulfur Dioxide and Acid Rain

Sulfur dioxide, $SO_2$, is produced when fossil fuels containing sulfur impurities are burned. Most coal and some fractions of crude oil contain small amounts of sulfur. During combustion, this sulfur reacts with oxygen: $S(s) + O_2(g) \rightarrow SO_2(g)$.

Once in the atmosphere, sulfur dioxide can react further with oxygen and water vapour to form sulfuric acid, $H_2SO_4$. This is a major contributor to acid rain. Acid rain has several devastating effects: it acidifies lakes and rivers, killing fish and other aquatic life: it damages the waxy cuticle on the leaves of trees, making it difficult for them to absorb nutrients: and it reacts with metals and carbonate rocks, such as limestone and marble, causing buildings and statues to corrode and deteriorate. The reaction with limestone can be represented as: $CaCO_3(s) + H_2SO_4(aq) \rightarrow CaSO_4(aq) + H_2O(l) + CO_2(g)$.

Nitrogen Oxides and Their Impacts

Nitrogen oxides, collectively referred to as $NO_x$, primarily include nitrogen monoxide, $NO$, and nitrogen dioxide, $NO_2$. Unlike carbon and sulfur oxides, nitrogen oxides do not usually come from impurities in the fuel itself. Instead, they form inside high temperature environments, such as internal combustion engines in cars.

At these high temperatures, the nitrogen and oxygen from the air, which are normally unreactive toward each other, gain enough energy to react: $N_2(g) + O_2(g) \rightarrow 2NO(g)$. This nitrogen monoxide then reacts with more oxygen in the air to form nitrogen dioxide: $2NO(g) + O_2(g) \rightarrow 2NO_2(g)$.

Nitrogen oxides contribute to the formation of acid rain by reacting with water to produce nitric acid, $HNO_3$. Additionally, $NO_x$ gases are respiratory irritants that can aggravate conditions like asthma and contribute to the formation of photochemical smog, which reduces visibility and poses further health risks to the lungs.