Atmospheric Chemistry

Sunlight (mostly visible light) passes through the atmosphere and warms Earth's surface. The warmed surface then re-radiates that energy back outward as infrared (heat) radiation. Certain gases — called greenhouse gases, most importantly ${\text{CO}}_2$, ${\text{CH}}_4$, and water vapour (${\text{H}}_2\text{O}$(g)) — absorb some of that outgoing infrared energy and re-radiate it in all directions, including back down toward the surface. This traps extra heat in the lower atmosphere, warming it.

High in the stratosphere, a layer of ozone (${\text{O}}_3$) gas absorbs most of the sun's harmful ultraviolet (UV) radiation before it reaches the surface, protecting living things from UV-related damage (like skin cancer and DNA damage). Chlorofluorocarbons (CFCs) — once common in refrigerants and aerosol propellants — drift up into the stratosphere, where UV light breaks them apart and releases chlorine atoms that catalytically destroy ozone molecules (a single chlorine atom can destroy thousands of ozone molecules before it's removed from the cycle). The cycle is two steps: $\text{Cl}+{\text{O}}_3\to \text{ClO}+{\text{O}}_2$, then $\text{ClO}+\text{O}\to \text{Cl}+{\text{O}}_2$. The chlorine atom comes out the other end unchanged and free to attack another ozone molecule, which is what makes it a catalyst. The 1987 Montreal Protocol, an international agreement signed in Montréal, phased out CFCs, and the ozone layer is now slowly recovering; it is often cited as the most successful environmental treaty. This is a distinct issue from the greenhouse effect, even though both involve atmospheric gases.

Burning fossil fuels (especially coal and vehicle fuel) releases sulfur oxides (${\text{SO}}_2$) and nitrogen oxides (${\text{NO}}_x$) into the atmosphere. These react with atmospheric water vapour to form sulfuric acid (${\text{H}}_2{\text{SO}}_4$) and nitric acid (${\text{HNO}}_3$), which fall back to the surface as "acid rain" — precipitation with an abnormally low pH. For sulfur the steps are ${\text{2SO}}_2+{\text{O}}_2\to {\text{2SO}}_3$ followed by ${\text{SO}}_3+{\text{H}}_2\text{O}\to {\text{H}}_2{\text{SO}}_4$ (sulfur dioxide can also combine directly with water to form sulfurous acid, ${\text{H}}_2{\text{SO}}_3$). Ordinary rain is already slightly acidic, around pH 5.6, because dissolved ${\text{CO}}_2$ forms carbonic acid; rain below that is called acid rain. In Ontario, lakes on the granite of the Canadian Shield were hit especially hard because the rock has little limestone to neutralize the acid, and smelter emissions near Sudbury were a major source until scrubbers and taller stacks were added. Acid rain damages forests, acidifies lakes (harming aquatic life), and corrodes buildings, statues, and infrastructure made from limestone or metal.

"Smog" (smoke + fog) forms from vehicle and industrial emissions reacting in sunlight to produce ground-level ozone and fine particulate matter. Unlike the protective ozone high in the stratosphere, ground-level ozone is a pollutant that irritates lungs and airways. Smog often gets trapped near the ground during a temperature inversion — a weather condition where a layer of warm air sits above cooler surface air, preventing the usual upward mixing that would otherwise disperse pollutants. To communicate air pollution risk to the public, agencies use an Air Quality Health Index (AQHI) (or similar AQI scales elsewhere) — a simple numerical scale that summarizes the health risk from a combination of pollutants (ground-level ozone, particulate matter, nitrogen dioxide) into one easy-to-understand number. On Canada's AQHI, 1–3 is low risk (little to no health risk for most people), 4–6 is moderate risk (people with heart or breathing conditions may want to reduce strenuous outdoor activity), 7–10 is high risk (sensitive groups should limit outdoor exertion and others may feel effects), and above 10 is very high risk (everyone should reduce or reschedule strenuous outdoor activity, and sensitive groups should avoid it).

• Improving energy efficiency (better building insulation, efficient appliances) reduces the fossil fuels burned for heating/electricity.
• Using public transit, cycling, or walking instead of driving reduces vehicle emissions of ${\text{CO}}_2$, ${\text{NO}}_x$, and particulate matter.
• Shifting electricity generation toward renewable or low-emission sources (hydro, wind, solar, nuclear) reduces ${\text{CO}}_2$ released per unit of energy used.
• Reducing, reusing, and recycling lowers the energy needed to manufacture new goods from raw materials.
• Reducing meat consumption can lower methane (${\text{CH}}_4$) emissions from livestock, since ${\text{CH}}_4$ is also a potent greenhouse gas.