Smog Is A Result Of Excessive Air Pollution.

The mix of smoke and fog is known as smog. It is a result of excessive air pollution. The term smog was first used in the year the 1900s. There are mainly two types of smog.

1. Photochemical smog
2. Sulfurous smog

Smog is mainly caused by nitrogen dioxides, sulfur oxide, smoke, coal combustion, vehicular emissions, forest and agricultural fire and other particulates combined with moisture in the air. What we see primarily is photochemical smog. 

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On the 26th of July 1943, photochemical smog was first observed in Los angles. It was like dark brown or grayish smoky mist and caused by the action of solar UV radiations on pollutants, volatile organic compounds and oxides of nitrogen which reduced the visibility. It caused panic as it had a chemical odor. It was highly suffocating and toxic for human beings, animals and plants. It formed in the afternoon of summer when there is bright sunlight. 

On the 5th of December 1952, sulfurous smog was first observed in London. It developed, due to the high concentration of sulfur in the air. A great air polluting substance that comes from sulfur-bearing fossil fuels like coal and volcanoes which also emit sulfur dioxide in the atmosphere. This affected mainly the eyes and lungs and caused 4000 respiratory deaths. It is formed in the mornings of winter when the temperature is low. Countries like China, Mexico and Poland experience a high level of photochemical smog. The government tried many methods to tackle it, from reducing the number of cars, to checking the health of vehicles, to considering the use of gas instead of wood etc. 

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The industrial smog was known to form when smoke from coal-burning, home stoves and factories combined with moisture in the air. But the smog behind the LA (1943) panic was different. It was yellowish with a chemical odor. Since the city didn't burn much coal, its cause would remain a mystery, until a chemist named Arie Hagen-Smit identified two culprits:

1. Volatile organic compounds, or VOCs
2. Nitrogen oxides.

VOCs are compounds that easily become vapors and may contain elements, such as carbon, oxygen, hydrogen, chlorine and sulfur. Some are naturally produced by plants and animals but others come from manmade sources, like solvents, paints, glues, and petroleum. Meanwhile, the incomplete combustion of gas in motor vehicles releases nitrogen oxide. That's what gives this type of smog its yellowish color. VOCs and nitrogen oxide react with sunlight to produce secondary pollutants called PANs and tropospheric or ground-level ozone. 

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PANs and tropospheric ozone cause eye irritation and damage lung tissue. Both are key ingredients in photochemical smog. Both industrial and photochemical smog combine manmade pollution with local weather and geography. High humidity made it a prime location for industrial smog. Photochemical smog is strongest in urban areas with calm winds and dry, warm, sunny weather. The ultraviolet radiation from sunlight provides the energy necessary to breakdown molecules that contribute to smog formation. Cities surrounded by mountains or lying in a basin are also especially vulnerable to smog. since there's nowhere for it to dissipate. That's also partially due to a phenomenon known as temperature inversion, where instead of warm air continuously rising upward, a pollution-filled layer of air remains trapped near the Earth's surface by a slightly warmer layer above. 

Smog isn't just an aesthetic eyesore. Both forms of smog irritate the eyes, nose, and throat. Exacerbate conditions like asthma and emphysema, and increase the risk of respiratory infections like bronchitis. Smog can be especially harmful to young children and older people and exposure in pregnant women has been linked to low birth weight and potential birth defects. Secondary pollutants found in photochemical smog can damage and weaken crops and decrease yield, making them more susceptible to insects. As the result the Clean Air Act of 1956 banned burning coal in certain areas of the city, leading to a massive reduction in smog. Similarly, regulations on vehicle emissions and gas content in the US reduced the volatile compounds in the air and smog levels along with them. 

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Smog remains a major problem around the world. Countries like China and Poland that depend on coal for energy, experience high levels of industrial smog. Photochemical smog and airborne particles from vehicle emissions affect many rapidly developing cities, from Mexico City and Santiago to Lahore, New Delhi and Tehran. Governments have tried many methods to tackle it, such as banning cars from driving for days at a time. As more than half of the world's population crowds into cities, considering a shift to mass transit and away from fossil fuels may allow us to breathe easier.

Classical smog is produced  by the combustion or burning at fossil  fuels like coal, petrol or gasoline  and caused by a mixture of smoke and sulfur dioxide present in air. Classical smog also known as London smog. It occurs in cool, humid climate. Major constituents are smoke, fog and sulfur dioxide. it can occur both during the day and at night. In December 1952, London was trapped in the deadly cloud of fog and pollution for five days. At that time, the city ran on cheap coal for everything, from generating power to heating homes which was the result of classical smog.

In the reaction of photochemical smog, at  first nitrogen and oxygen combine   in the combustion process to form nitric oxide. Then this nitric oxide, oxidized in the  atmosphere to produce nitrogen dioxide. Next sunlight breaks down nitrogen dioxide  into nitric oxide and an oxygen radical. The oxygen radical combines with  molecular oxygen to form ozone. Ozone is the main component of photochemical smog. Nitric oxide can remove ozone by reacting  with it to form nitrogen dioxide and oxygen. Nitric oxide and nitrogen dioxide can also  react with the hydrocarbons instead of ozone   to form other volatile compounds known  as PANs, It means a nitrate reacted hydrocarbon from out of exhaust interact  with ozone to from the hydrocarbon free radical  . This hydrocarbon free radical rapidly reacts with  oxygen to from another hydrocarbon free radical.  Then the free radical reacts with nitric  oxide to produce nitrogen dioxide and the   free radical hydrocarbon. This new free radical  next interacts with oxygen to yields is stable   aldehyde and hydro peroxide radical. Hydro peroxide  a de coal then reactive with another molecule of   nitric oxide to give nitrogen dioxide and  hydrogen radicals. The hydro peroxide a de   coal is extremely reactive and rapidly reacts  with a stable hydro carbon to yield water and   regenerate the hydro carbon free radical.  Thereby completing cycle. 

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Lahore is now one of the most polluted cities in the world and the reason for that is pretty much vehicular emissions and industrial emissions. An equivalent number of cars, poor public transportation system is not running on clean fuel, lot of power plants and industries surrounding Lahore. Smog is a perennial issue in Pakistan. The government is planning to launch some long term and short medium term buyers to control the smoke in the coming years, like it is being considered that electric based buses and rickshaws and bikes will be introduced by in the transport sector. And industry which is working in the within the urban areas will might be shifted or they will be switched over to the pollution control technologies for the control of the smoke. Prevention is the key, so must avoid public places, public gathering, markets and marriages. Because these are areas where maximum exposure to smoke occurs and that's how you can prevent

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