Identify the major atmospheric pollutants, their sources, and the chemistry of photochemical smog formation.

What this dot point is asking

SACE expects you to classify pollutants as primary or secondary, write balanced equations for their formation, and explain the photochemical cycle that builds up ozone and smog.

Lead worked calculation

Primary pollutants and their sources

• Carbon monoxide ($\text{CO}$): from incomplete combustion of fuels when oxygen is limited: $2\text{C} + \text{O}_2 \rightarrow 2\text{CO}$. Toxic because it binds haemoglobin more strongly than oxygen.
• Nitrogen oxides ($\text{NO}_x$): $\text{N}_2$ and $\text{O}_2$ combine at the high temperatures inside engines: $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$, and $\text{NO}$ oxidises to $\text{NO}_2$ in air.
• Sulfur dioxide ($\text{SO}_2$): from burning sulfur-containing fuels: $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$. A cause of acid rain.
• Particulates: tiny solid/liquid particles (soot, ash) from incomplete combustion, harmful to lungs.
• Volatile organic compounds (VOCs): unburnt hydrocarbons from fuel and solvents, feeding smog.

The photochemical smog cycle

Photochemical smog needs three ingredients: $\text{NO}_x$, VOCs and sunlight. The key starter is the photodissociation of $\text{NO}_2$:

Ground-level ozone is the hallmark secondary pollutant. The cycle is self-sustaining because $\text{NO}$ is regenerated and re-oxidised, and reactive radicals from VOCs convert more $\text{NO}$ back to $\text{NO}_2$, allowing ozone to accumulate during sunny, still, traffic-heavy conditions. Other secondary pollutants such as peroxyacetyl nitrate (PAN) also form, irritating eyes and lungs.

Acid rain chemistry

Both $\text{SO}_2$ and $\text{NO}_x$ produce acids in the atmosphere. Sulfur dioxide is oxidised then hydrated: $2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$, then $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$. Nitrogen dioxide gives nitric acid: $3\text{NO}_2 + \text{H}_2\text{O} \rightarrow 2\text{HNO}_3 + \text{NO}$. These acids lower rainfall pH, damaging vegetation, leaching nutrients from soil, acidifying waterways and corroding limestone buildings.

Why it matters for monitoring

Air-quality networks track $\text{CO}$, $\text{NO}_x$, $\text{SO}_2$, ozone and particulates because each signals a different pollution source and health risk. Understanding which are primary and which are secondary tells regulators whether to control emissions at source or the conditions that drive secondary formation.