Edited sentence: A photochemical smog is the chemical reaction of sunlight, nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere, which leaves airborne particles (called particulate matter) and ground-level ozone.All of these chemicals are usually highly reactive and oxidizing.Because it travels with the wind, it can affect sparsely populated areas as well.In the 1950s a new type of smog, known as photochemical smog, was first described.It is present in all modern cities, but it is more common in cities with sunny, warm, dry climates and a large number of motor vehicles.Nitrogen oxides are released by nitrogen and oxygen in the air reacting together under high temperature such as in the exhaust of fossil fuel-burning engines in cars, trucks, coal power plants, and industrial manufacturing factories.Photochemical smog is therefore considered to be a problem of modern industrialization.This forms when sunlight hits various pollutants in the air and forms a mix of inimical chemicals that can be very dangerous.This noxious mixture of air pollutants can include the following:.VOCs are released from man-made sources such as gasoline (petrol), paints, solvents, pesticides, and biogenic sources, such as pine and citrus tree emissions.aldehydes (R'O).nitrogen oxides, such as nitrogen dioxide.peroxyacyl nitrates (PAN).tropospheric ozone.volatile organic compounds (VOCs)