What is Photochemical Smog?

Photochemical Smog is the contamination of the air, mainly in urban areas, by ozone caused by photochemical reactions and other compounds. The result is a reddish-brown atmosphere. Ozone is an oxidizing and toxic compound that can cause respiratory problems in humans. This type of smog was first described in Los Angeles in the 1940s, and usually occurs in cities with a lot of traffic (emission of nitric oxide, NO, and volatile organic compounds, VOCs), warm and sunny, and with little movement of air masses.

Photochemical Smog Formation

The main primary pollutants are nitric oxide (NO) and volatile organic compounds (especially those with C=C double bonds). Nitrogen oxide is formed from the high-temperature reaction of molecular nitrogen with molecular oxygen:

N_{2}\ + O_{2} \rightarrow 2\ NO

So vehicles are the main source of nitric oxide. In turn, this nitrogen monoxide oxidizes in the air in a short time to give nitrogen dioxide, NO2, which is what can give the atmosphere a yellow coloration. The gases NO and NO2 are generically referred to as NOx.

Volatile organic compounds (VOCs) include unburned hydrocarbons that can also be emitted by vehicles, as well as solvents or fuels that can evaporate easily. These can also come from arboreal areas, as hydrocarbons, mainly isoprene, pinene and limonene, are emitted naturally. Secondary pollutants, formed from the above, through a complex series of reactions, are ozone, HNO3, and other compounds.

Photochemical Smog Reduction

To reduce the formation of photochemical smog it is necessary to reduce the emission of NOx and VOCs. The amounts of volatile hydrocarbons in the atmosphere are quite large compared to those of NOx, so they are usually in excess. Thus, a reduction of the latter leads to a smaller than expected decrease in photochemical smog. In addition, naturally emitted hydrocarbons may be sufficient to continue to produce smog (although in urban areas these are not usually the most important). In any case, it is still important to reduce the levels of these volatile hydrocarbons in the atmosphere.

One of the largest sources of NOx is vehicles. Nitrogen oxide emissions are reduced by using three-way catalysts (two-way catalysts do not treat these gases) that reduce them to molecular nitrogen and oxygen. These catalysts, in the case of gasoline engines, are 80% to 90% effective, but only when they are hot. In addition, the catalytic converter wears out and becomes less effective over time. In the case of diesel engines, the effectiveness is lower.

Another major source of NOx is emissions from power plants. NOx can also be reduced by reduction processes, although there are other methods such as carrying out combustion in several stages or lowering the flame temperature.

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