Photochemical smog exists in big cities with large quantities of exhausts from automobile, factories and more. These exhaust gases produce substances such as ozone (O3), aldehyde and peroxyacyl nitrates (PAN) which are harmful for humans, animals and plants in various ways. In this tutorial, we will discuss followings about photochemical smog.
Smog is a mixture of dust particles and small drops of fog. Smog is the major pollutant in large cities. Smog is categorized to two sections as followings.
In this tutorial, we are specifically going to learn about photochemical smog which is considered as a significant environmental pollution factor.
Photochemical smog is formed by the combination of smoke, dust and fog with an air pollutant in the atmosphere as a result of a photochemical smog.
Warm, dry and sunny climate is required to form a photochemical smog. Also it needs an atmosphere which is loaded with large quantities of automobile exhausts and stagnated air masses.
Automobile exhausts contain carbon dioxide as the main gas component after the combustion. Due to high temperature inside the engine, oxides of nitrogen (mainly NO and NO2) may be produced. Nitric oxide is a major gaseous pollutant.
If fuel contains sulfur, sulfur dioxide gas also can be formed as another gaseous pollutant. Like that, due to incomplete combustion, carbon monoxide and unburnt hydrocarbons (in fuel) are also given as the products.
Nitrogen dioxide and hydrocarbons are the primary precursors of photochemical smog.
If a city is rounded from mountains, exhaust gases are unable to leave the surrounded area. We called this air "stagnated air". Stagnated air helps to increase the photochemical smog.
Photochemical smog was observed in cities which have warm, sunny and dry climates such as Los Angeles and Denver in USA. So this was earlier called Los Angelis Smog.
Due to existing characteristic brown haze, the cities such as Los Angeles are sometimes called as brown air cities
There should be high concentration of photochemical oxidant.
Although it should have following environmental conditions.
NO2 and O3 are strong oxidizing agents and they can react with unburnt hydrocarbons.
Those reaction will start a chain reaction and produce formaldehyde, acrolein, organic peroxides, organic hydroperoxides, peroxyacyl nitrates(PAN) as below.
Acrolein and Peroxyacetylnitrate are very noxious substances.
Peroxyacetylnitrate is formed from aldehydes. This reaction is started by hydroxyl radicals. OH. is formed by reaction between atomic oxygen and water.
PAN is formed due to presence OH..
There are so many researches were conducted and are conducting in the world to reduce this photochemical smog. In this section, we are going to discuss some actions we can take to control or reduce the effect of photochemical smog.
Catalytic converters are installed to reduce emission of nitrogen oxide and hydrocarbons. These catalytic converters will convert nitrogen oxide to nitrogen. Hydrocarbons will be converted to carbon dioxide and water.
Some plants, pinus, Juniparus, Quercus, Pyrus and Vitis can metabolism nitrogen dioxide (NO2).
Some compounds which are capable of trapping free radicals in the photochemical smog are applied. These added compounds generate free radicals which combine with free radicals precursors of photochemical smog.
There are common constituents in acid rains and photochemical smog. Nitrogen dioxide and sulfur dioxide gases are the reasons for acidic rains. Also nitrogen dioxide initiate the photochemical smog. Difference is, for acid rains, hydrocarbons are not required.
Photochemical cases are found everywhere in the world and we will list out some cases here for your reference.
Kandy, a world heritage city is known as a very scenic location around the world. However, there are concerns regarding the air pollution in this beautiful city.
Kandy city is covered with a range of mountains and lot of vehicles are entered to the city for official and tourism purposes. Unfortunately, roads and streets are not developed to the current requirement and huge traffics are occurred during the morning, school and evening times. Emissions caused due to the automobiles are stagnated in the City because emissions are poorly transferred to the outer areas due to the covering of mountains. Due to the accumulation of these emissions, Photochemical smog is occurred when other conditions are met. There were several experiments done by universities to observe air pollution in Kandy, they have confirmed that air quality is not upto the standard values in long traffics.