Environmental Chemistry
Chemistry where it matters most — the reactions in our air, water and soil that produce smog, acid rain and the ozone hole, and the green chemistry that can undo them
- What a pollutant is, and the layers of the atmosphere where pollution acts.
- The major tropospheric pollutants — oxides of S, N and C, and particulates.
- The difference between classical and photochemical smog.
- The chemistry of the greenhouse effect, acid rain and ozone depletion.
- How water quality is measured by BOD, and what drives eutrophication.
- The aims and methods of green chemistry.
Pollutants & the Atmosphere
Environmental chemistry studies the chemical species and reactions in our surroundings, and how human activity disturbs them. A pollutant is any substance present in a high enough concentration to harm living things or materials. Pollutants may be biodegradable (broken down by nature, like sewage) or non-biodegradable (persistent, like DDT and most plastics).
Tropospheric Pollution
Most everyday air pollution is tropospheric, caused by gases and tiny particulates from burning fossil fuels. The chief gaseous offenders are the oxides of sulphur, nitrogen and carbon.
| Pollutant | Source | Harm |
|---|---|---|
| \(\ce{SO2}\) | burning sulphur-rich coal/oil | respiratory damage, acid rain |
| \(\ce{NO},\ \ce{NO2}\) | high-temperature combustion | photochemical smog, acid rain |
| \(\ce{CO}\) | incomplete combustion | binds haemoglobin (toxic) |
| \(\ce{CO2}\) | complete combustion | greenhouse gas |
| Particulates | smoke, dust, fumes | lung disease, reduced visibility |
Smog: Two Kinds
"Smog" (smoke + fog) comes in two chemically opposite forms. Classical (London) smog is reducing, formed in cool, humid air from smoke and \(\ce{SO2}\). Photochemical (Los Angeles) smog is oxidising, formed in warm, sunny, dry air from nitrogen oxides and hydrocarbons.
| Feature | Classical smog | Photochemical smog |
|---|---|---|
| Chemistry | reducing (\(\ce{SO2}\), smoke) | oxidising (\(\ce{NO2}\), \(\ce{O3}\), PAN) |
| Conditions | cool, humid | warm, sunny, dry |
| Key products | \(\ce{H2SO4}\) aerosol | ozone, peroxyacetyl nitrate |
Sunlight splits \(\ce{NO2}\); the freed oxygen atom makes ground-level ozone. Ozone and unburnt hydrocarbons then form eye-stinging PAN (peroxyacetyl nitrate). The result is a brown, irritating haze.
The Greenhouse Effect & Global Warming
The greenhouse effect is natural and necessary — certain gases let sunlight in but trap the outgoing infrared, keeping Earth warm. The problem is its intensification: rising concentrations of these gases trap more heat, driving global warming.
Acid Rain
When \(\ce{SO2}\) and \(\ce{NO2}\) dissolve in atmospheric water and are oxidised, they form sulphuric and nitric acids. Rain with a pH below about \(5.6\) is acid rain.
Acid rain corrodes marble and limestone buildings ("marble cancer": \(\ce{CaCO3 + H2SO4 -> CaSO4 + H2O + CO2}\)), acidifies lakes and kills fish, and leaches nutrients from soil.
Stratospheric Ozone & Its Depletion
High in the stratosphere, ozone forms and breaks down in a natural cycle, absorbing harmful ultraviolet light as it does. Chlorofluorocarbons (CFCs) upset this balance: UV light frees chlorine atoms from them, and each chlorine atom destroys ozone catalytically — over and over.
The chlorine atom is regenerated, so it keeps going — a single \(\ce{Cl}\) can destroy thousands of \(\ce{O3}\) molecules. The thinning is worst over Antarctica (the "ozone hole"), and more UV-B at the surface raises skin cancer and cataract rates and harms phytoplankton.
Water Pollution & BOD
Water is polluted by pathogens, organic waste, industrial chemicals and fertiliser run-off. A key measure of organic pollution is the biochemical oxygen demand (BOD) — the oxygen that micro-organisms need to break down the organic matter in a water sample.
A high BOD means lots of organic waste, so bacteria consume the dissolved oxygen — suffocating fish. Eutrophication is the related danger: fertiliser nutrients (\(\ce{N},\ \ce{P}\)) trigger algal blooms that, on decaying, strip the water of oxygen.
Soil Pollution
Soil is polluted chiefly by pesticides, herbicides and industrial waste. The most persistent — like the insecticide DDT — are non-biodegradable, so they accumulate up the food chain in a process called biomagnification, reaching toxic levels in top predators.
Green Chemistry
Green chemistry is the design of products and processes that prevent pollution rather than clean it up — using safer reagents, less energy, and producing less waste. The guiding idea is to maximise atom economy (more of the reactants ending up in the product) and avoid hazardous substances altogether.
| Conventional | Greener alternative |
|---|---|
| Bleaching with \(\ce{Cl2}\) | bleaching with \(\ce{H2O2}\) |
| Dry-cleaning with tetrachloroethene | liquid \(\ce{CO2}\) with a detergent |
| Toxic solvent reactions | reactions in water or solvent-free |
Putting It to Work
Problem. A brown haze forms on a hot, sunny afternoon over a city full of traffic. Which smog is it, and is it oxidising or reducing?
Solution. Sun + vehicle \(\ce{NO_x}\) → ozone and PAN:
Problem. Write the reaction by which \(\ce{NO2}\) produces nitric acid in rain.
Solution. \(\ce{NO2}\) is oxidised and dissolves in water:
Problem. Explain why one chlorine atom from a CFC destroys many ozone molecules.
Solution. The chlorine is regenerated, so it acts catalytically:
Problem. Sample A has a BOD of 3 ppm and sample B has 20 ppm. Which is cleaner, and why?
Solution. Lower BOD means less organic waste demanding oxygen:
Problem. Why is carbon monoxide far more dangerous than carbon dioxide at low concentration?
Solution. CO binds haemoglobin much more strongly than \(\ce{O2}\):
Problem. Suggest a green-chemistry replacement for chlorine bleaching of paper, and state the benefit.
Solution. Hydrogen peroxide bleaches and decomposes to harmless products:
Chapter Summary
Biodegradable vs non-biodegradable; troposphere (smog, acid rain) vs stratosphere (ozone).
Oxides of S, N, C and particulates; CO binds haemoglobin and is silently toxic.
Classical (reducing, \(\ce{SO2}\)) vs photochemical (oxidising, \(\ce{O3}\), PAN).
Greenhouse warming, acid rain (\(\ce{H2SO4}/\ce{HNO3}\)), CFC-driven ozone depletion.
BOD measures organic pollution; eutrophication and biomagnification spread harm.
Prevent pollution at source — safer reagents, atom economy, benign solvents.
Problems
For each item, first decide whether it concerns air, water, soil or green chemistry — then apply the relevant idea. Difficulty rises down the list.
- Define a pollutant and distinguish biodegradable from non-biodegradable, with one example of each.
- Name three gaseous tropospheric pollutants and their main sources.
- Explain why carbon monoxide is toxic even at low concentration.
- Compare classical and photochemical smog in chemistry and conditions.
- Write the reactions that initiate photochemical smog from \(\ce{NO2}\).
- What is the greenhouse effect, and name three greenhouse gases.
- Write the reactions forming sulphuric and nitric acid in acid rain.
- Explain, with reactions, how CFCs deplete stratospheric ozone.
- Define BOD and state typical values for clean and polluted water.
- What is eutrophication, and how do fertilisers cause it?
- Explain biomagnification using DDT as an example.
- State two principles of green chemistry and give one practical example.