Air Composition and Pollution
Senior 1 · Chemistry
Air Composition and Pollution
Lesson 4 of 5
25 min
At dusk, a haze settles over Kigali's valleys as thousands of charcoal stoves are lit for the evening meal and traffic crawls home. The smoke stings your eyes. Far more dangerous is a gas in that smoke which has no colour and no smell at all, and which can kill a family sleeping in a closed room with a charcoal stove. What are the pollutants in the air, where do they come from, and what do they do?
Definition
Air pollution: the presence in the air of gases or particles in amounts that harm people, animals, plants or buildings. A pollutant is any such substance. Example: carbon monoxide from a charcoal stove in a closed kitchen.
Carbon monoxide forms when carbon fuels burn with too little oxygen (incomplete combustion), for example charcoal smouldering in a stove or petrol in a badly tuned engine:
Sulfur dioxide forms when fuels that contain sulfur, such as diesel, heavy fuel oil and coal, are burned; volcanoes also release it:
Nitrogen oxides (written ) form inside hot engines, where the temperature is high enough for the nitrogen and oxygen of the air to react; nitrogen monoxide then reacts further with oxygen:
Particulates are tiny solid or liquid particles: soot (black carbon) from charcoal, firewood and diesel, smoke from burning rubbish and crop waste, and dust from unpaved roads. The most harmful, called PM2.5, are less than 2.5 micrometres across, thin enough to pass deep into the lungs.
Table 4.1: Major air pollutants, their sources, effects and prevention
| Pollutant | Main sources | Effects | Prevention |
|---|---|---|---|
| Carbon monoxide, | charcoal and firewood burning with little air; vehicle and generator exhaust | poisonous: stops the blood carrying oxygen; headache, drowsiness, death | burn fuel with plenty of air; never use charcoal or generators indoors; maintain engines |
| Sulfur dioxide, | diesel, heavy fuel oil and coal in engines, generators and factories; volcanoes | irritates the lungs and eyes; causes acid rain | low-sulfur fuels; remove from chimney gases with lime |
| Nitrogen oxides, | hot engines of cars, buses and motorbikes; power stations | irritate the lungs; cause acid rain and smog | catalytic converters; well-maintained or electric vehicles |
| Particulates (PM2.5, soot) | smoke from charcoal and firewood cooking; burning rubbish; diesel exhaust; road dust | heart and lung disease, asthma; blacken buildings | cleaner stoves and fuels; no open burning; paved roads |
| Smoke from charcoal cooking | the traditional charcoal stove (imbabura) and firewood fires, often indoors | a mixture of CO and particulates breathed daily by cooks and children | improved stoves, LPG or electricity; cook outside or with good ventilation |
| Vehicle exhaust | old or poorly maintained cars, buses and petrol motorbikes | a mixture of CO, , unburned fuel and soot | emissions testing, catalytic converters, electric motos, public transport |
Warning
Charcoal indoors kills. Never bring a burning or glowing charcoal stove into a closed room to warm it or to finish cooking, and never sleep in a room with one or with a running generator. Carbon monoxide has no colour and no smell. Headache, dizziness, sleepiness and vomiting are warning signs: open doors and windows, move everyone into fresh air at once, and get medical help.
Rain is always slightly acidic, about pH 5.6, because it dissolves a little carbon dioxide:
Sulfur dioxide and nitrogen oxides make it much more acidic. In the clouds they react with water and oxygen:
The wind can carry these gases hundreds of kilometres, so the acid often falls far from where it was made (Figure 4.1).
Figure 4.1: How acid rain forms and what it damages.
Definition
Acid rain: rain with a pH below about 5.6, made acidic by sulfuric, sulfurous and nitric acids that form when sulfur dioxide and nitrogen oxides dissolve in the water of clouds. Example: rain falling downwind of power stations that burn high-sulfur coal.
Effects of acid rain. It damages leaves and washes nutrients out of soil, so trees and crops grow poorly; it lowers the pH of lakes until fish die; and it attacks limestone and marble buildings and iron roofs:
Worked example
Problem. Balance the equation for the conversion of sulfur dioxide into sulfur trioxide in the air: .
Solution.
Check: S: 2 = 2; O: 6 = 6. Balanced.
Scientists from Carnegie Mellon University and the University of Rwanda measured the air at sites across Kigali from March 2017 to July 2018 (Subramanian and others, Clean Air Journal, 2020). They found that PM2.5 rises sharply in the morning rush hour, from traffic, and again late in the evening, from traffic and cooking fires, and that it is about twice as high in the dry seasons as in the wet seasons. Figure 4.2 compares their average with the World Health Organization's levels.
Figure 4.2: Average PM2.5 in Kigali compared with WHO levels.
Worked example
Problem. Use Figure 4.2. (a) How many times the WHO 2021 guideline was Kigali's average PM2.5? (b) By what percentage did it exceed WHO interim target 1? (c) Suggest why PM2.5 is about twice as high in the dry season.
Solution.
Answer: about 10 times the guideline and about 49% above interim target 1. Kigali's air is cleaner than many large cities', but it still needs to improve.
Tip
Which pollutant causes acid rain? Look for the S and the N: sulfur dioxide and nitrogen oxides. Carbon monoxide poisons, but it does not make rain acidic.
FuseSchool: Effect of air pollutants on health (4 min). For each pollutant in the video, find its row in Table 4.1.
Practice
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
