- characteristics
- Some reactions that occur in the troposphere
- Primary and secondary air pollutants
- Ozone formation in the troposphere
- Causes of photochemical smog
- Effects of smog
- References
The photochemical smog is a dense fog formed due to chemical reactions of gases from combustion engines of automobiles. These reactions are mediated by sunlight and occur in the troposphere, a layer of the atmosphere that extends from 0 to 10 km above the ground.
The word smog comes from the contraction of two words in the English language: «fog», which means mist or mist, and «smoke», which means smoke. Its use began in the 1950s to designate a haze that covered the city of London.
Figure 1. Photochemical smog in Salt Lake City, USA. Source: Eltiempo10, from Wikimedia Commons
Smog manifests as a yellowish-brownish-grayish haze, originated by small drops of water dispersed in the atmosphere, which contain the chemical products of reactions that occur between air pollutants.
This haze is very common in large cities due to the high concentration of cars and the more intense vehicular traffic, but it has also spread to areas that were pristine, such as the Grand Canyon in the state of Arizona, USA.
Very often, smog has a characteristic, unpleasant odor, due to the presence of some typical gaseous chemical components. The intermediate products and the final compounds of the reactions that cause smog, seriously affect human health, animals, plants and some materials.
characteristics
Some reactions that occur in the troposphere
One of the distinctive features of the planet Earth's atmosphere is its oxidizing capacity, due to the large relative amount of diatomic molecular oxygen (O 2) it contains (approximately 21% of its composition).
Ultimately, virtually all gases emitted into the atmosphere are completely oxidized in the air, and the end products of these oxidations are deposited on the Earth's surface. These oxidation processes are of vital importance to clean and decontaminate the air.
The mechanisms of chemical reactions that occur between air pollutants are very complex. Below is a simplified exposition of them:
Primary and secondary air pollutants
The gases emitted by the combustion of fossil fuels in automobile engines mainly contain nitric oxide (NO), carbon monoxide (CO), carbon dioxide (CO 2) and volatile organic compounds (VOCs).
These compounds are called primary pollutants, since through chemical reactions mediated by light (photochemical reactions) they produce a series of products called secondary pollutants.
Basically, the most important secondary pollutants are nitrogen dioxide (NO 2) and ozone (O 3), which are the gases that most influence the formation of smog.
Ozone formation in the troposphere
Nitric oxide (NO) is produced in car engines through the reaction between oxygen and nitrogen in the air at high temperatures:
N 2 (g) + O 2 (g) → 2NO (g), where (g) means in the gaseous state.
Nitric oxide once released into the atmosphere is oxidized to nitrogen dioxide (NO 2):
2NO (g) + O 2 (g) → 2NO 2 (g)
NO 2 undergoes photochemical decomposition mediated by sunlight:
NO 2 (g) + hγ (light) → NO (g) + O (g)
Oxygen O in atomic form is an extremely reactive species that can initiate many reactions such as the formation of ozone (O 3):
O (g) + O 2 (g) → O 3 (g)
Ozone in the stratosphere (layer of the atmosphere between 10 km and 50 km above the earth's surface) works as a protective component of life on Earth, as it absorbs high-energy ultraviolet radiation that comes from the sun; but in the terrestrial troposphere, ozone has very damaging effects.
Figure 2. Smog in New York. Source: Wikipedia Commons
Causes of photochemical smog
Other pathways for the formation of ozone in the troposphere are complex reactions involving nitrogen oxides, hydrocarbons and oxygen.
One of the chemical compounds generated in these reactions is peroxyacetyl nitrate (PAN), which is a powerful tear agent that also causes difficulty in breathing.
Volatile organic compounds come not only from hydrocarbons that are not burned in internal combustion engines, but from various sources, such as evaporation of solvents and fuels, among others.
These VOCs also undergo complex photochemical reactions that are a source of ozone, nitric acid (HNO 3) and partially oxidized organic compounds.
VOCs + NO + O 2 + Sunlight → Complex mixture: HNO 3, O 3 and various organic compounds
All these organic compounds, oxidation products (alcohols and carboxylic acids), are also volatile and their vapors can condense into tiny liquid droplets that are distributed in the air in the form of aerosols, which scatter sunlight, reducing visibility. In this way a kind of veil or fog is produced in the troposphere.
Effects of smog
Soot or carbon particles produced by combustion, sulfuric anhydride (SO 2) and the secondary pollutant –sulfuric acid (H 2 SO 4) - are also involved in the production of smog.
Ozone in the troposphere reacts with the C = C double bonds in lung tissues, plant and animal tissues, causing severe damage. Additionally, ozone can damage materials such as automobile tires, causing cracking for the same reasons.
Photochemical smog is the cause of severe respiratory problems, coughing fits, nasal and throat irritation, shorter breathing, chest pain, rhinitis, eye irritation, lung dysfunction, decreased resistance to respiratory infectious diseases, premature aging of lung tissues, severe bronchitis, heart failure and death.
In cities such as New York, London, Mexico City, Atlanta, Detroit, Salt Lake City, Warsaw, Prague, Stuttgart, Beijing, Shanghai, Seoul, Bangkok, Bombay, Calcutta, Delhi, Jakarta, Cairo, Manila, Karachi, called In megacities, peak critical episodes of photochemical smog have been cause for alarm and special measures to restrict circulation.
Some researchers have reported that contamination caused by sulfur dioxide (SO 2) and sulfates cause a decrease in the resistance to contracting breast and colon cancer in populations that inhabit northern latitudes.
The suggested mechanism to explain these facts is that smog, by scattering incident sunlight on the troposphere, causes a decrease in available ultraviolet type B (UV-B) radiation, which is necessary for the biochemical synthesis of vitamin D Vitamin D works as a protective agent against both types of cancer.
In this way, we can see that an excess of high-energy ultraviolet radiation is very harmful to health, but also the deficiency of UV-B radiation has harmful effects.
References
- Ashraf, A., Butt, A., Khalid, I., Alam, RU, and Ahmad, SR (2018). Smog analysis and its effect on reported ocular surface diseases: A case study of 2016 smog event of Lahore. Atmospheric Environment. doi: 10.1016 / j.atmosenv.2018.10.029
- Bang, HQ, Nguyen, HD, Vu, K. et al. (2018). Photochemical Smog Modeling Using the Air Pollution Chemical Transport Model (TAPM-CTM) in Ho Chi Minh City, Vietnam Environmental Modeling & Assessment. 1: 1-16. doi.org/10.1007/s10666-018-9613-7
- Dickerson, RR, Kondragunta, S., Stenchikov, G., Civerolo, KL, Doddridge, B. G and Holben, BN (1997). The Impact of Aerosols on Solar Ultraviolet Radiation and Photochemical Smog. Science. 278 (5339): 827-830. doi: 10.1126 / science.278.5339.827
- Hallquist, M., Munthe, J., Tao, MH, Chak, W., Chan, K., Gao, J., et al (2016) Photochemical smog in China: scientific challenges and implications for air-quality policies. National Science Review. 3 (4): 401–403. Doi: 10.1093 / nsr / nww080
- Xue, L., Gu, R., Wang, T., Wang, X., Saunders, S., Blake, D., Louie, PKK, Luk, CWY, Simpson, I., Xu, Z., Wang, Z., Gao, Y., Lee, S., Mellouki, A., and Wang, W.: Oxidative capacity and radical chemistry in the polluted atmosphere of Hong Kong and Pearl River Delta region: analysis of a severe photochemical smog episode, Atmos. Chem. Phys., 16, 9891-9903, https://doi.org/10.5194/acp-16-9891-2016, 2016.