Air Composition and Pollution

Lessons

Senior 1 · Chemistry

Air Composition and Pollution

5 lessons

Lesson 5 of 5

Preventing Air Pollution: Rwanda's Actions

25 min

A Sunday without cars

On some Sunday mornings the main roads of Kigali close to cars. Thousands of people walk, run, cycle and do aerobics where buses and motos usually queue. Car-Free Day is a sports event, but it is also an experiment in air quality. Does keeping cars off the road for a few hours make the air measurably cleaner? And what else can a country, a school or a family do to cut the pollutants you met in Lesson 4?

Cyclists on a closed road in Kigali during Car-Free Day. With the cars and motos gone, the exhaust gases and soot of an ordinary morning are missing too. Photo: Fabos Ishimwe, CC BY-SA 4.0, via Wikimedia Commons

Preventing pollution at its source

Most air pollutants come from burning. There are three ways to cut them: burn less, burn more cleanly, or clean the gases before they leave the chimney or exhaust pipe.

Complete combustion. A fuel that burns with plenty of air forms carbon dioxide and water, not carbon monoxide and soot. Improved cookstoves let air reach the charcoal and waste less heat, so they need less fuel.
Cleaner fuels. Liquefied petroleum gas (LPG) and methane burn with a clean blue flame. For propane: $\mathrm{C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g)}$ . Electric cooking and electric vehicles release no smoke where they are used.
Catalytic converters in car exhausts turn carbon monoxide and nitrogen oxides into harmless gases (Figure 5.1).
Removing sulfur dioxide from factory chimneys by passing the gases through lime: $\mathrm{CaO(s) + SO_2(g) \rightarrow CaSO_3(s)}$ .
No open burning of rubbish, plastics or crop waste, which fills the air with smoke and poisonous gases.
Fewer journeys by car: walking, cycling, buses, and well-maintained vehicles.

Figure 5.1: A catalytic converter changes harmful exhaust gases into harmless ones.

Definition

Catalytic converter: a device in a vehicle's exhaust containing a catalyst (platinum and rhodium) that speeds up the conversion of carbon monoxide, nitrogen oxides and unburned fuel into carbon dioxide, nitrogen and water. Example: $\mathrm{2CO + 2NO \rightarrow 2CO_2 + N_2}$ .

What Rwanda is doing

Table 5.1: Rwandan actions against air pollution

Action	What it tackles	Source
Car-Free Day in Kigali. One Sunday a month from 2017, and the first and third Sundays of each month from 2018; main roads close to motor traffic from about 7 a.m.	exhaust gases and soot from traffic; health through exercise	Subramanian and others, Clean Air Journal, 2020
Electric motos. From January 2025 only electric motorcycles are registered for new moto-taxi use in Kigali; petrol motos already registered may continue. Kigali has about 30,000 motos, and petrol moto-taxis carry about a fifth of the city's trips.	exhaust from petrol engines	Infrastructure Minister Jimmy Gasore, reported by electrive.com, November 2024
Cooking gas (LPG). More than 80% of Rwandan households cooked with charcoal or firewood; the government set a target to cut that to 42% by 2024, with a national LPG Master Plan.	smoke and carbon monoxide from charcoal and firewood; cutting of trees	Čukić and others, Energies, 2021
Monitoring by REMA. Since September 2021 the public can see real-time air quality for 23 sites on aq.rema.gov.rw and a mobile app. From May 2025 vehicle emissions are tested separately, and motorbikes are included.	measures pollution so that action can be targeted and checked	Government of Rwanda (gov.rw), 2021; REMA, 2025
The Rwanda Climate Observatory on Mount Mugogo. A joint project of MIT and the Government of Rwanda, at 2,590 m, measuring carbon dioxide, carbon monoxide, methane, black carbon and ozone since May 2015.	tracks greenhouse gases and pollution over the whole region	DeWitt and others, Atmospheric Chemistry and Physics, 2019

Figure 5.2: Some of Rwanda's measures against air pollution.

Worked example

Problem. The Kigali study (Lesson 4) found that car-free Sundays appear to lower PM2.5 by 10 to 12 µg per cubic metre. Take an ordinary value of 52 µg per cubic metre and a fall of 11. (a) What is the percentage fall? (b) The authors wrote that this "needs further investigation". Why should we be careful before claiming that Car-Free Day cleans Kigali's air?

Solution.

(a) $\frac{11}{52} \times 100 \approx 21\%$ .
(b) Only a few Sundays were measured; the roads close for only about three hours; weather, rain and cooking fires change from day to day and also affect PM2.5; and the air on the other days of the month is unchanged. A fair conclusion needs many more car-free and ordinary Sundays compared under similar weather.

Answer: about 21% lower while the roads are closed; a promising sign, not yet proof.

Worked example

Problem. At Mount Mugogo, black carbon (soot) is highest in Rwanda's two dry seasons, December to February and June to August, which are also the main seasons for burning grassland and forest in the region. The scientists estimated that local sources account for up to 35% of the black carbon measured over a year. (a) What fraction, at least, comes from outside the local area? (b) What does this mean for Rwanda's policies?

Solution.

(a) At least $100\% - 35\% = 65\%$ comes from further away, carried by the wind from regional burning.
(b) Rwanda can reduce its own share with cleaner cooking, electric vehicles and no open burning, but much of its soot is blown in, so cleaner air also needs cooperation with neighbouring countries.

Answer: at least 65%; national action matters, but air pollution crosses borders.

Worked example

Problem. Balance the main reaction in a catalytic converter: $\mathrm{CO + NO \rightarrow CO_2 + N_2}$ .

Solution.

Nitrogen: 1 on the left, 2 on the right, so write $\mathrm{2NO}$ .
Now the left has 2 O from NO. Write $\mathrm{2CO}$ and $\mathrm{2CO_2}$ to balance carbon.
Oxygen: left $2 + 2 = 4$ ; right $2 \times 2 = 4$ .

$\mathrm{2CO(g) + 2NO(g) \rightarrow 2CO_2(g) + N_2(g)}$

Answer: balanced, with two poisonous gases turned into two harmless ones.

What you can do

Cook with LPG, electricity or an improved stove if you can; always cook where fresh air can reach the fire.
Never burn rubbish or plastic; take it to be collected (Unit 11).
Walk or cycle for short journeys, and join Car-Free Day.
Report vehicles that pour out black smoke, and keep family vehicles serviced.
Plant and care for trees during umuganda: they trap dust and take in carbon dioxide.
Check the air quality in your district on aq.rema.gov.rw before hard exercise outdoors.

Warning

Common mistake: thinking that electric motos or cookers cause no pollution at all. They release nothing where they are used, but the electricity has to be generated somewhere. They are cleanest when the electricity comes from hydropower, solar power or methane burned efficiently in a power station.

Tip

"Less, cleaner, cleaned." Burn less fuel, burn cleaner fuel, and clean the gases that remain.

City of Kigali: highlights of Car-Free Day, 13 September 2026 (1 min). Notice which kinds of transport are missing from the roads.

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Practice

Name three ways of reducing pollution from burning fuels.
Write the balanced equation for the reaction of carbon monoxide with nitrogen monoxide in a catalytic converter.
How does lime remove sulfur dioxide from chimney gases? Write the equation.
Why does an LPG cooker produce less carbon monoxide and smoke than a charcoal stove?
Give two actions taken in Rwanda to reduce air pollution from vehicles.
What is the Rwanda Climate Observatory, and where is it?
A district's PM2.5 falls from 45 to 36 µg per cubic metre after a ban on burning rubbish. Calculate the percentage fall.
A classmate says: "Electric motos will solve Kigali's air pollution." Evaluate this claim.

Show answersHide answers

Any three: burn less fuel; burn it completely with enough air; use cleaner fuels such as LPG or electricity; fit catalytic converters; remove sulfur dioxide with lime; stop open burning.
$\mathrm{2CO(g) + 2NO(g) \rightarrow 2CO_2(g) + N_2(g)}$
Calcium oxide reacts with sulfur dioxide to form solid calcium sulfite: $\mathrm{CaO(s) + SO_2(g) \rightarrow CaSO_3(s)}$ .
The gas mixes well with air and burns completely to carbon dioxide and water, whereas smouldering charcoal burns with too little air, making carbon monoxide and soot.
Any two: Car-Free Days; registering only electric motorcycles for new moto-taxi use in Kigali from January 2025; emissions testing that includes motorbikes from May 2025.
An air-monitoring station run by the Government of Rwanda with MIT on the summit of Mount Mugogo (2,590 m), measuring greenhouse gases and pollutants since 2015.
$\frac{45 - 36}{45} \times 100 = 20\%$ .
Only partly true. Electric motos remove the exhaust of petrol motos, which carry about a fifth of Kigali's trips, but cars, buses, charcoal cooking, rubbish burning, road dust and smoke blown in from the region would remain. Many actions are needed together.

1A Sunday without cars

2Preventing pollution at its source

3What Rwanda is doing

4What you can do

A Sunday without cars

Preventing pollution at its source

What Rwanda is doing

What you can do