F. Gases and Atmospheric Chemistry · Gases and Atmospheric Chemistry

The Atmosphere: Composition, Layers and Pollutants

F3.1 F2.1About 65 minutes

Learning goals

I can identify the major, minor and trace components of the atmosphere and their approximate amounts.
I can describe the layers of the atmosphere.
I can use ppm and ppb, and convert them to percentages and partial pressures.
I can name the major air pollutants, their sources, and write equations for how they form.
I can tell primary pollutants from secondary pollutants.

Words to know

atmosphere
The layer of gases surrounding Earth, held by gravity.

parts per million (ppm)
For gases, moles (or volumes) of a substance per million moles (or volumes) of air; 1 ppm = 0.0001 %.

primary pollutant
A pollutant released directly into the air from a source.

secondary pollutant
A pollutant formed in the air by reactions of primary pollutants, such as ground-level ozone.

particulate matter (PM2.5)
Tiny solid or liquid particles 2.5 micrometres across or smaller, suspended in air.

volatile organic compound (VOC)
A carbon-based compound that evaporates easily, such as gasoline vapour or paint solvents.

acid deposition
Acids from SO₂ and NOx falling to Earth in rain, snow, fog or as dry particles.

The air you breathe

You take about 20 000 breaths a day. Each one is a mixture of gases, plus traces of things that shouldn't be there. To understand air quality (the next two lessons), you first need to know what clean air is made of and where pollutants come from. This is where everything you've learned about gases, partial pressures and moles comes together.

Remember from Grade 10

In the Grade 10 Climate Change unit you studied greenhouse gases and the greenhouse effect, the ozone layer and ozone depletion, and smog. In this lesson we look at the atmosphere through a chemist's eyes: exact composition, units like ppm, and the chemical equations for how pollutants form.

Composition of clean, dry air

The atmosphere is a mixture, not a compound. Its gases are well mixed in the lower atmosphere. Because percent by volume equals percent by moles for gases (Avogadro), these percentages are also mole fractions.

Major components: nitrogen, N₂ (78.08 %), and oxygen, O₂ (20.95 %).

Minor components: argon, Ar (0.93 %), and carbon dioxide, CO₂ (about 0.042 %, or a little over 420 ppm, and rising by about 2 to 3 ppm each year).

Trace components: neon, helium, methane (about 1.9 ppm), krypton, hydrogen, nitrous oxide, ozone and others.

Water vapour is not counted in dry air because it varies a lot: from almost 0 % in very cold Arctic air to about 4 % in hot, humid air.

For small amounts, chemists use parts per million (ppm) or parts per billion (ppb). For gases these are by volume (or by moles): 1 ppm = 1 mol in 1 000 000 mol of air = 0.0001 %.

Composition of clean, dry air by volume. Nitrogen and oxygen make up 99 %. Argon is next; carbon dioxide, about 420 ppm, is a tiny fraction but a very important one.

Layers of the atmosphere

The atmosphere is divided into layers by how temperature changes with height.

Troposphere (ground to about 8 km at the poles and 18 km at the equator; about 12 km over southern Canada): all our weather happens here, and it holds about 75 % of the atmosphere's mass. Temperature falls with height.
Stratosphere (up to about 50 km): contains the ozone layer. Temperature rises with height because ozone absorbs ultraviolet light.
Mesosphere (up to about 85 km): the coldest layer; meteors burn up here.
Thermosphere: extremely thin air; auroras glow here.

Layers of the atmosphere (left) and a typical temperature profile (right line). Temperature falls in the troposphere, rises in the stratosphere (ozone absorbs UV), falls again in the mesosphere and rises in the thermosphere. Boundary heights vary with latitude and season; the scale is approximate.

Air pollutants and where they come from

An air pollutant is a substance in the air at a concentration that can harm people, other living things or materials. A primary pollutant is released directly from a source. A secondary pollutant forms in the air from reactions of primary pollutants.

Pollutant	Main sources	How it forms
carbon monoxide, CO	vehicles, furnaces, wildfires	incomplete combustion: 2 C₈H₁₈ + 17 O₂ → 16 CO + 18 H₂O
nitrogen oxides, NOx (NO and NO₂)	vehicles, power plants, industry, lightning	N₂ and O₂ from air react at high temperatures: N₂ + O₂ → 2 NO; then 2 NO + O₂ → 2 NO₂
sulfur dioxide, SO₂	smelting of sulfide ores, burning coal and oil, volcanoes	sulfur is oxidized: S + O₂ → SO₂; e.g. 2 ZnS + 3 O₂ → 2 ZnO + 2 SO₂
fine particulate matter, PM2.5	vehicle exhaust, wood burning, wildfires, industry, road dust	primary particles, plus secondary particles formed from SO₂, NOx and VOCs
volatile organic compounds (VOCs)	gasoline vapour, solvents, paints, trees	evaporation; incomplete combustion
ground-level ozone, O₃ (secondary)	forms in air	NOx + VOCs + sunlight: NO₂ → NO + O, then O + O₂ → O₃

Major air pollutants. PM2.5 means particles 2.5 micrometres across or smaller, small enough to reach deep into the lungs.

Primary pollutants from vehicles, solvents and industry react in the air to form secondary pollutants: ground-level ozone (smog) and the acids in acid deposition.

Acid deposition

SO₂ and NO₂ react with oxygen and water in the air to form acids that fall as acid rain or snow, or settle as dry particles:

2 SO₂(g) + O₂(g) → 2 SO₃(g), then SO₃(g) + H₂O(l) → H₂SO₄(aq)
4 NO₂(g) + O₂(g) + 2 H₂O(l) → 4 HNO₃(aq)

In the 1970s and 1980s, acid rain from smelters and coal-burning power plants damaged lakes and forests on the Canadian Shield, where thin soils over granite have little ability to neutralize acid. (In the unit Solutions and Solubility you studied acids and pH in more detail.) The next two lessons look at the damage and at what Canada has done about it.

Worked example

Example 1: Working with ppm

The air contains about 420 ppm of CO₂. (a) Express this as a percentage. (b) Find the partial pressure of CO₂ at 101.3 kPa.

(a) 420 ppm = 420 ÷ 1 000 000 = 0.000 420 = 0.0420 %
(b) Mole fraction x = 0.000 420. P(CO₂) = x × P(total) = 0.000 420 × 101.3 kPa = 0.0425 kPa

(a) 0.0420 % (b) 0.0425 kPa (42.5 Pa)

Worked example

Example 2: CO₂ in a crowded classroom

In a crowded classroom with poor ventilation, CO₂ can reach 1000 ppm. If the room holds 200 m³ of air at SATP, what mass of CO₂ is in the air?

Total moles of air: 200 m³ = 200 000 L; n = 200 000 ÷ 24.8 = 8.06 × 10³ mol
Moles of CO₂: 1000 ppm = 1000 × 10⁻⁶ = 0.001 00; n(CO₂) = 0.001 00 × 8.06 × 10³ = 8.06 mol
Mass: 8.06 mol × 44.01 g/mol = 355 g

About 355 g of CO₂, mostly breathed out by the students. Opening a window brings in outdoor air at about 420 ppm.

Worked example

Example 3: Balancing a smelting reaction

Copper is often found as copper(I) sulfide, Cu₂S. Roasting it in air forms copper(I) oxide and sulfur dioxide. Write the balanced equation and say which product is an air pollutant.

Skeleton: Cu₂S + O₂ → Cu₂O + SO₂
Balance S and Cu: one Cu₂S gives one Cu₂O and one SO₂.
Count O on the right: 1 + 2 = 3 O atoms, so 3/2 O₂. Double everything to clear the fraction.
2 Cu₂S + 3 O₂ → 2 Cu₂O + 2 SO₂ (check: Cu 4 = 4, S 2 = 2, O 6 = 6)

2 Cu₂S + 3 O₂ → 2 Cu₂O + 2 SO₂. The pollutant is SO₂, which causes acid deposition.

Hands-on activity

Catch the particles

You need: 4 white index cards, petroleum jelly, a hole punch or pencil, string or tape, a magnifying glass (or a phone camera zoomed in), a ruler.

Safety: a grown-up should help place any card near a road; stay on the sidewalk, away from traffic.

Draw a 2 cm × 2 cm square in the middle of each card. Smear a thin layer of petroleum jelly inside the square.
Label the cards and place them in four spots for 3 days: e.g. inside your bedroom, outside a back window, near a street or bus stop, and near a garden or park. Hang or tape them so they don't get wet.
Collect the cards. Use the magnifying glass to count particles in the square (if there are many, count one corner and multiply).
Make a table and a bar graph of your results.

What to look for: cards near traffic usually collect the most dark particles; indoor cards the fewest. You are only catching larger particles you can see: the PM2.5 that harms health is too small to see without a microscope. Write one sentence about the limits of this method, and one question you could test next (for example, does a windy day change the count?).

Just for fun

Argon's name comes from the Greek word for "lazy" or "idle", because it reacts with practically nothing. It makes up almost 1 % of every breath you take, just tagging along for the ride.

Your turn

Name the two major and two minor components of dry air, with their approximate percentages.

Why is water vapour left out of the composition of dry air?

Convert 1.9 ppm (methane) to a percentage.

What is the difference between a primary and a secondary pollutant? Give one example of each.

Write the balanced equations for the formation of NO and then NO₂ in a car engine and exhaust.

In which layer of the atmosphere is the ozone layer?

More practice: warm-up

What percentage of dry air is nitrogen?

What does ppm stand for?

In which layer does weather happen?

Name one natural source of SO₂.

What does PM2.5 mean?

More practice: on your own

What is the partial pressure of methane (1.9 ppm) at 101.3 kPa?

How many moles of oxygen are in 50.0 m³ of dry air at SATP?

Write balanced equations showing how SO₂ becomes sulfuric acid in the atmosphere.

Why do nitrogen oxides form in a car engine even though gasoline contains no nitrogen?

Balance: ZnS + O₂ → ZnO + SO₂

Challenge

Ground-level ozone is usually highest on hot, sunny summer afternoons and lower on cloudy days and at night. Explain using the reactions that form it.

Balance the equation for the incomplete combustion of octane that produces only carbon monoxide and water, and explain why this is dangerous indoors.

A city's air has an SO₂ concentration of 10 ppb. How many SO₂ molecules are in each litre of air at SATP?

Check your understanding

Quiz

The second most abundant gas in dry air is:

About 420 ppm is equal to:

Which is a secondary pollutant?

Nitrogen oxides from vehicles form mainly because:

The ozone layer is found in the:

Which equation correctly shows a step in forming acid rain?

For parents and teachers

Big idea (F3.1, F2.1): clean, dry air is about 78 % N₂, 21 % O₂, 0.93 % Ar and 0.04 % CO₂, with trace gases. Students should know the major sources of CO, NOx, SO₂, VOCs, PM2.5 and ground-level ozone, tell primary from secondary pollutants, and write the equations for how they form.

How to help: go for a walk and identify sources of air pollutants you can see (exhaust, wood smoke, a gas station). Ask your teen to name the pollutant and write its formation equation.

Common difficulties:

Thinking CO₂ is a large part of the air. It's about 0.04 %, but its effect on climate is large.
Mixing up "good" stratospheric ozone with "bad" ground-level ozone. Same molecule, different place.
ppm conversions: divide by 1 000 000 to get a fraction; multiply that by 100 for a percentage.

Safety: supervise placing particle cards near roads.