Chemistry · Ch 15 — Environmental Chemistry
Smog
Smog
Smog is a combination of smoke and fog that forms droplets remaining suspended in the air. Chemically, smog is a mixture of gases that forms a brownish-yellow haze over urban cities, mainly consisting of ground-level ozone, oxides of nitrogen, volatile organic compounds, SO₂, acidic aerosols and gases, and particulate matter. …
What this figure shows. A photograph of a thick, greyish fog blanketing a landscape, with the bare silhouettes of a few trees and the faint outline of a building barely visible through the haze -- illustrating the dense, low-visibility character …
Classical Smog or London Smog
Classical smog, also known as London smog, was first observed in London in December 1952. It consists of coal smoke and fog, and occurs in cool, humid climate; this atmospheric smog is found in many large cities. Its chemical composition is a mixture of SO₂, SO₃ and humidity, and it generally occurs in the morning, becoming worse as the sun rises -- this is mainly because of the induced oxidation of SO₂ to SO₃, which reacts with water to yield sulphuric-acid aerosol.
Chemically it is reducing in nature, because of the high concentration of SO₂, so it is also called reducing smog.
Effects of classical smog: (a) it is primarily responsible for acid rain; (b) it results in poor visibility and affects air and road transport; (c) it also causes bronchial irritation. …
Worked out. A case-study box: the Great Smog of London (or Great Smog of 1952) was a severe air-pollution event that struck London from Friday 5 December to Tuesday 9 December 1952, dispersing quickly once the weather changed. It caused major disruption by reducing visibility and even penetrating indoors. Government medical reports in the following weeks estimated that, up to 8 December, 4,000 people had died as a direct result of the smog and 100,000 more were made ill by its effe …
Photo Chemical Smog or Los Angeles Smog
Photochemical smog, also known as Los Angeles smog, was first observed in Los Angeles in 1950. It occurs in warm, dry and sunny climate, forming from the combination of smoke, dust and fog with air pollutants such as oxides of nitrogen and hydrocarbons in the presence of sunlight. It forms when the sun shines and becomes worse in the afternoon. Chemically it is oxidising in nature, because of the high concentration of the oxidising agents NO₂ and O₃, so it is also called oxidising smog.
Photochemical smog forms through this sequence of reactions:
NO and O₃ are strong oxidising agents, and can react with unburnt hydrocarbons in polluted air to form formaldehyde, acrolein and peroxy acetyl nitrate (PAN).
Effects of photochemical smog. The three main components of photochemical smog are nitrogen oxide, ozone, and oxidised hydrocarbons like formaldehyde (HCHO), acrolein (CH₂=CH-CHO) and peroxy acetyl nitrate (PAN). Photochemical smog causes irritation to eyes, skin and lungs, and increases the chances of asthma. High concentrations of ozone and NO can cause nose and throat irritation, chest pain and discomfort in breathing. PAN is toxic to plants, attacking younger leaves and causing bronzing and glazing of their surfaces. It also causes corrosion of metals, stones, building materials and painted surfaces. …