Understanding Air Pollutants from Combustion

Air pollution comes from many sources, including natural events and human activities. Combustion, which is the process of burning, is a major source of many common air pollutants. Let's look at the pollutants listed and determine which one is generally not produced by combustion.

Analyzing the Options

• Carbon dioxide ($\text{CO}_2$): This is a primary product of the complete combustion of any material containing carbon, such as fossil fuels (coal, oil, natural gas) and biomass. The chemical reaction involves carbon reacting with oxygen: $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$. It is a well-known greenhouse gas.
• Nitrogen dioxide ($\text{NO}_2$): Combustion, especially at high temperatures in engines and power plants, causes nitrogen and oxygen in the air to react, forming nitrogen oxides ($\text{NO}_x$), initially mainly nitric oxide ($\text{NO}$). $\text{NO}$ then reacts with oxygen in the atmosphere to form nitrogen dioxide ($\text{NO}_2$). So, $\text{NO}_2$ is indirectly but significantly linked to combustion processes.
• Ammonia ($\text{NH}_3$): Ammonia is a colorless gas with a pungent smell. Its main sources are not direct combustion. It is primarily produced from the decomposition of organic matter, agricultural activities (like animal waste and fertilizers), and various industrial processes (like the manufacturing of fertilizers). While nitrogen compounds can be present in fuels, ammonia itself is not a typical direct product of fuel combustion under atmospheric conditions; it might be converted or participate in other reactions during burning.
• Sulphur dioxide ($\text{SO}_2$): This pollutant is produced when fuels containing sulfur, such as coal and heavy fuel oil, are burned. The sulfur in the fuel reacts with oxygen: $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$. $\text{SO}_2$ is a major contributor to acid rain.

Conclusion

Based on the typical sources of these air pollutants, Carbon dioxide, Nitrogen dioxide (formed from $\text{NO}_x$ produced during combustion), and Sulphur dioxide are all commonly obtained directly or indirectly from combustion processes. Ammonia, however, is primarily produced through biological decomposition and industrial synthesis, not typically as a direct product of fuel combustion.

Therefore, the air pollutant not obtained by combustion is Ammonia.