Concept 01 / 05Environmental Chemistry
Smog and Particulate Pollutants
Classical (reducing) smog versus photochemical (oxidising) smog, the chemistry of photochemical smog with NO2, ozone, PAN and aldehydes, its effects and control, and particulate pollutants: viable and non-viable particles, sizes, sources and health effects.
Smog (smoke + fog) is the most familiar form of air pollution in cities. There are two very different kinds.
Classical (London) smog. Occurs in cool, humid climates. A mixture of smoke, fog and sulfur dioxide; chemically it is a reducing mixture, so it is also called reducing smog. The London smog of December 1952 killed thousands of people.
Photochemical (Los Angeles) smog. Occurs in warm, dry, sunny climates with heavy traffic. It results from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides from vehicle exhausts. It has a high concentration of oxidising agents, so it is called oxidising smog.
Formation (see figure):
Normally the last reaction removes ozone. But unburnt hydrocarbons react with O atoms and ozone to form radicals that convert NO to without consuming ozone, so ozone accumulates. Ozone and NO react further with hydrocarbons to give formaldehyde, acrolein () and peroxyacetyl nitrate (PAN, ). The daily pattern: NO peaks in the morning rush hour, a little later, and ozone and PAN in the afternoon.
Effects: ozone and PAN are powerful eye irritants; ozone and NO cause nose and throat irritation, headache, chest pain, dryness of throat and cough; smog cracks rubber and damages plants, metals and paint.
Control: catalytic converters in cars; planting trees such as Pinus, Juniperus, Quercus, Pyrus and Vitis, which metabolise nitrogen oxides.
Particulate pollutants. Minute solid particles or liquid droplets suspended in air.
Viable particulates: bacteria, fungi, moulds, algae, pollen (cause allergies and diseases).
Non-viable particulates: smoke (from burning organic matter: soot, oil smoke), dust (fine solid particles over from crushing and grinding: sand, cement, fly ash), mists (spray droplets, sulfuric acid mist, herbicide and insecticide sprays), fumes (condensed vapours during sublimation, distillation, metallurgical operations).
Effects depend on size: particles larger than are trapped in the nose and throat, while particles about (and fine PM2.5) penetrate deep into the lungs, causing bronchitis and other respiratory disease. Lead particulates from leaded petrol (tetraethyl lead) interfere with the development of red blood cells. Particulates also catalyse the oxidation of and scatter sunlight (haze, reduced visibility).
Control: electrostatic precipitators, cyclone separators, bag filters, wet scrubbers.
Key Takeaways:
- Classical smog: smoke + fog + ; cool and humid; reducing.
- Photochemical smog: + hydrocarbons + sunlight; warm and sunny; oxidising (, PAN, acrolein, HCHO).
- Daily sequence: NO, then , then .
- Fine particles (about ) reach deep into the lungs.
- Pitfall: ground-level ozone is a harmful pollutant, even though stratospheric ozone is protective.
Brain-Teaser 1: Classify each as characteristic of classical or photochemical smog: , PAN, cold humid air, ozone, reducing nature.
Solution: Classical: , cold humid air, reducing nature. Photochemical: PAN, ozone (oxidising).
Brain-Teaser 2: Why does ozone concentration in a polluted city peak in the afternoon rather than in the morning rush hour?
Solution: Morning traffic emits NO and hydrocarbons. Sunlight first converts NO to (via radicals from hydrocarbons), and then photolysis of produces O atoms that form ozone. This chain takes several hours and needs strong sunlight, so ozone builds up to a maximum in the early afternoon.
Brain-Teaser 3: Why are particles about in size more harmful to health than those of ?
Solution: Larger particles settle quickly or are trapped by hairs and mucus in the nose and throat. Particles of about stay suspended longer and follow the airflow deep into the alveoli of the lungs, where they lodge and cause inflammation; they can also carry adsorbed toxic compounds.
Practice this concept
Free · no account required · instant solutions