Photochemical smog is a type of smog produced when ultraviolet light from the sun reacts with nitrogen oxides in the atmosphere. It is visible as a brown haze, and is most prominent during the morning and afternoon, especially in densely populated, warm cities.

Causes of Photo Chemical Smog

1) Stubble burning in areas of Punjab  and Haryana It is estimated that approximately 35 million tonnes of crop are set afire by these states. The wind carries all the pollutants and dust particles, which have got locked in the air. 

2) Rise in vehicular population: The Central Pollution Control Board (CPCB) and the National Environmental Engineering Research Institute (NEERI) have declared vehicular emission as a major contributor to Delhi’s increasing air pollution  which in turn increases photochemical smog

3)  As the winter season sets in, dust particles and pollutants in the air become unable to move. Due to stagnant winds, these pollutants get locked in the air and affect weather conditions, resulting in smog. 

4) Industrial pollution and garbage dumps are also increasing air pollution and building-up smog in the air. 

5)  Large scale construction in Delhi-NCR is another culprit that is increasing dust and pollution in the air.

Effects  of PhotoChemical Smog

1 Health Effects

Photochemical smog is capable of inflicting irreversible damage on the lungs and heart. Even short-term exposure to photochemical smog tends to have ill effects on both the young and the elderly. It causes painful irritation of the respiratory system, reduced lung function and difficulty breathing. High levels of smog also trigger asthma attacks because the smog causes increased sensitivity to allergens, which are triggers for asthma.

2 Environmental Effect :  The collection of chemicals found in photochemical smog causes problems for plants and animal life. Some plants such as tobacco, tomato and spinach are highly responsive to ozone, so photochemical smog can decimate these sensitive crops, trees and other vegetation. Ozone causes necrotic (dead) patterns on the upper surfaces of the leaves of trees. Ground-level ozone also can interfere with the growth and productivity of trees. The effects of smog on animals are also similar to its effect on humans; it decreases lung capacity and lung elasticity.

3 Transportation  : The  reduced  visibility  leads to accidents in many northern cities  during winter .

4 Acid rain :

Corrode architectural monuments  like Taj Mahal

Measures taken  to solve the problem

1. National Clean Air Programme (NCAP) was launched by MoEFCC in April, 2018 and this was preceded by Clean Air Programme in Delhi in February 2018 to sensitise the public in general and implementing agencies in particular.
2. Significant action has been taken in Delhi and NCR, including the formulation of Graded Response Action Plan (GRAP) by CPCB.
3. Other  Measures include ban on petcoke in Delhi and NCR; strict surveillance over coal-fired plants; strict action against construction activities; regular field surveillance by CPCB teams starting from September 2017; augmentation of air quality monitoring stations in Delhi and NCR; upgradation of Central Control Room in CPCB; integration of data on air quality from stations of Indian Meteorological Department (IMD); Delhi Pollution Control Committee (DPCC) and Central Pollution Control Board (CPCB); launching a Mobile App called SAMEER for grievance redressal;
4. Leap-frogging from BS-IV to BS-VI. 

The need  of hour is  to reduce effects of  photochemical smog , the  suggestion by NITI Aayog “Breathe Free India “  initiative also needs to be implemented.