Smog is another secondary pollutant. This term was developed to describe a substance that is a hybrid of smoke and fog. The SOx aerosols are one source of smog formation. As discussed earlier, sulfuric acid droplets, or sulfuric acid absorbed on the surface of soot and fly ash particles, can attract moisture from the air to form what is often referred to as conventional or ‘classical’ smog. Such smog, whose principal components are NOx, SOx and particulates, was prevalent in the heyday of the coal-fueled Industrial Revolution, before the transportation revolution of the 20th century
