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Delhi has earned the unenviable distinction of becoming the most polluted city on Earth last year. The airborne particles and toxic chemicals that make up the smog and contributes to air pollution in NCR had reached the record level on Novermber 8, 2017 when some monitoring stations reported an Air Quality Index of 999, way above the upper limit of the worst category, Hazardous. (An extra-sensitive air quality instrument at the US embassy got a reading of 1,010). ‘Great Smog’ was deadly air pollution event in history of London between December 5-9, 1952,around 4,000 people died of illnesses linked to respiratory problems such as bronchitis and pneumonia, and the smog's effects causedanother 8,000 deaths over the next several months[1]. Great smog showed first time that it could be deadly. Term smog is derived from the words smoke and fog. There are several variants of smog but at least two distinct types of smog are recognized in literature: Classical (London-type) smog or sulfurous smog [1] and Photochemicalsmog [2] (Los Angeles-type). Sulfurous smog results from a high concentration of sulfur oxides in the air and is caused by the use of sulfur-bearing fossil fuels, particularly coal. London-type smog occurs in the regions where, emission of the sulfur-containing compounds is high (due to burning of coal to generate heat and energy) and air contains high liquid water contents or fog. Burning of coal produces sulfur dioxide, soot and other gases and particulates, which are commonly called smoke. It leads to the production of high concentrations of sulfuric acid in fog droplets. The classical smog is recognized by the human since a long. Here the primary pollutants are SO2, soot particles and secondary pollutants are H2SO4, sulfate aerosol. It predominantly occurs in early morning hours when temperature is low and relative humidity is high or foggy. Photochemical (Los Angeles-type) smog was first recognized during mid1940s. It predominantly occurs in noon-eveninghours and the required temperature is (> 75oF)and relative humidity is low and usually hot and dry environment. This type of smog occurs in the regions of high emissions of automobiles, large concentrations of reactive hydrocarbons (RH) (from automobile exhaust or from other natural or anthropogenic sources) and plenty of sunlight (high level of UV radiation).Photochemical smog typically develops in summer (when solar radiation is strongest) in stagnant conditions promoted by temperature inversions and weak winds. Photochemical smog is a ubiquitous urban problem and often blankets large populated regions such as the eastern United States and Western Europe and Asia for extended periods in summer.

Smog in Delhi

Delhi is theone of the most polluted [3] city in the world and according to one estimate, air pollution causes the death of about 10,500 people in Delhi every year[4]. During 2013–14, peak levels of fine particulate matter (PM) in Delhi increased by about 44%, primarily due to high vehicular and industrial emissions, construction work and crop burning in adjoining states. Delhi has the highest level of the airborne particulate matter PM_2.5 , (153 micrograms). Rising air pollution level has significantly increased lung-related ailments (especially asthma and lung cancer) among Delhi's children and women. The dense smog in Delhi during winter season results in major air and surface traffic disruptions every year.  Considering the serious impacts caused by smog, the effective management to meet out such conditions becomesinevitable. Therefore, to monitor and control of various air pollutants, Central Pollution Control Board (CPCB) had launched a nationwide program National Air Quality Monitoring Programme (NAMP)[5]. Moreover, under the NAMP, three major pollutants viz. sulphur dioxide (SO₂), nitrogen dioxide (NO₂), and particulate matter (PM₁₀) have been identified for regular monitoring at almost all the locations of Delhi. Other parameters like particulate matter (PM_2.5), carbon monoxide (CO), ammonia (NH₃), lead (Pb), ozone (O₃), benzene (C₆H₆), arsenic (As), and nickel (Ni) are also monitored at selected locations and are slowly being added to the monitoring networks under NAMP.Air quality monitoring in Delhi is carried out through a number of air quality monitoring stationssituated across the territory. The monitoring is undertaken by various organizations viz. CentralPollution Control Board (CPCB), Delhi Pollution Control Committee (DPCC), and System ofAir Quality and Weather Forecasting and Research (SAFAR) of Indian Institute of TropicalMeteorology (IITM), Pune.

Air quality for three major contents of smog (SO₂, NO₂, and PM) is determined to understand the trend of pollution in Delhi during recent years [5]. On the basis of annual average concentration of pollutants, air quality trend has been seen for the years 2009 – 2015 along with the comparison with existing national ambient air quality standards (NAAQS), 2009. Regarding the threepollutants viz. SO₂, NO₂, and PM₁₀, the concentration of NO₂ and PM₁₀ arefar exceeding the prescribed standard limits. The concentration of SO₂ is within the standardlimits. However, as far as NO₂ is concerned, continuous rise in concentration was observed in 2009 – 2015. Moreover, the problem of particulate matter (PM₁₀) is more critical. Since 2009, approximately 258–335% rise has been observed in PM₁₀ concentration compared to the standards. Further the concentration of volatile organic compounds (VOC) significantly increased during smog.

Chemistry of Photochemical Smog

This type of smog requires neither smoke nor fog, has its origin in the nitrogen oxides and hydrocarbon vapors emitted by automobiles and other sources[6-8].Photochemical smog forms primarily as a result of interactions among nitrogenoxides (NO_x = NO + NO₂), reactive hydrocarbons (RH), and sunlight. Primary pollutants NO_x and reactive organic vapors RH are emitted from automobiles and create secondary pollutantsi.e., ozone, peroxyacetylnitrate (PAN), smog etc.Ozone produced in stratosphere is known as good ozone and shield the planet from harmful ultraviolet radiations of Sun. Here, ground level or "bad ozone” is produced in troposphere, dangerous to human health when detected in our atmosphere and at ground level.

First, RHs are chemically transformed to radicals, denoted by R^∙. The organic radical R^∙, can be composed of many atoms and have a complex molecular structure.

RH + OH = R^∙+ H₂O...............................(1)

R^∙+ O₂ + M = RO₂^∙+ M........................... (2)

Then these organic radicals react with nitric oxide (NO) to form nitrogen dioxide (NO₂).

RO₂^∙+ NO = RO^∙+ NO₂............................(3)

The photochemical reaction breaks NO₂ back to NO and O at wavelength < 0.42 µm

NO₂ + hn= NO + O...................................(4)

 Atomic oxygen produces ozone O₃, which is of primary concern in photochemicalsmog.

O + O₂ + M = O₃ + M..................................(5)

Thus, the overall process of smog formation can be summarized as

RH + HO + NO + hn=O₃ + NO₂ + HC..........(6)

Hundreds of different compounds may be produced by the reactions of RH.One of the most important is peroxyacetyl nitrate (PAN). Thus, secondaryhydrocarbons are important components of photochemical smog.Hence in photochemical smog,emission of NO, CO, RH occurs in initial stage then due to presence of sunlightO₃, NO₂, PAN (and other hydrocarbons), haze (aerosols) are formed in the final stage.

Before 1970s it was commonly believed that tropospheric ozone was basically transporteddownward from the stratosphere and eventually destroyed at the earth's surface [9-10]. Ozone production in heavily polluted summer-smog was recognized by many researchers [11-12]. In the 1970s it was discovered that ozone is produced and destroyed in the troposphere during the oxidation of methane and carbon monoxide (CO) catalyzed by free radicals and nitrogen oxides, NO_x=NO+NO₂[6-8,13].In Radical chemistry, formerly ozone production was considered to be important in the stratosphere but later it was found to be strongly involved in troposphericchemistry as well, and ozone was identified as the main precursor of the important hydroxyl radical(OH) [14]. OH is the dominant sink for methane, carbon monoxide andmany other organic substances in the atmosphere and is therefore often called the detergent ofthe atmosphere".

Thus, the role of ozone in the atmosphere is manifold. It is an effective greenhouse gas, andlargely controls the oxidizing power of the atmosphere. Additionally, ozone is a toxic gas whichcan have detrimental effects on human health and crops. The controllingfactor in determining ozone production or loss in large parts of the troposphere is the abundanceof NO_x, which is largely emitted by human activity.

Visibility and smog

Aerosols or SPM present in smog also have important irradiative effects in the atmosphere. Particles are efficientscatter of light due to their size which is of the order of wave length of visible light. SPMmodify the direction of radiation beams without absorbing radiation. Scattering of sunlight by SPM is the principalmechanism limiting visibility in the atmosphere, as it prevents us from distinguishing an object from the background. Air molecules are inefficient scatterers because their sizes are orders of magnitudesmaller than the wavelengths of visible radiation (0.4 to 0.7 micrometers). Further in humid environment, SPM absorb water, whichcauses them to swell and increases their cross-sectional area for scattering, creating haze. Without aerosol pollution our visual range would typically be about 200 miles, but haze can reduce visibility significantly.

1. Black, J., Intussusceptions and the great smog of London, December 1952. Archives of Disease in Childhood, 88(12): p. 1040-1042(2003). 2. Husar, R.B. and W.H. White, Color of Los-Angeles Smog. Atmospheric Environment, 10(3): p. 199 (1976). 3. “Delhi is most polluted city in world, Beijing much better: WHO study". Hindustan Times. Retrieved 8 May 2014 4. "Delhi's Air Has Become a Lethal Hazard and Nobody Seems to Know What to Do About It". Time magazine. Retrieved 10 February 2014 5. Air pollution in Delhi. An analysis Published by ENVIS Centre on Control of Pollution (Water, Air, & Noise) 2016 6. Crutzen, P. J., A discussion of the chemistry of some minor constituents in the stratosphere and troposphere. Pure and Applied Geophysics, 106-108 , 1385-1399, 1973. 7. Crutzen, P. J., Photochemical reactions initiated by and influencing ozone in unpolluted tropospheric air. Tellus, 16, 47-56, 1974. 8. Crutzen, P. J., The role of NO and NO2 in the chemistry of the stratosphere and troposphere. Ann. Rev. Earth Planet Sci., 7 , 443-472, 1979. 9. Regener, V. H., Vertical ux of atmospheric ozone. J. Geophys. Res., 62 , 221-228, 1957. 10. Junge, C. E., Global ozone budget and exchange between stratosphere and troposphere. Tellus,14 , 363-377, 1962. 11. Haagen-Smit, A. J., Chemistry and physiology of Los Angeles smog. Ind. Eng. Chem., 44 ,1342-1346, 1952. 12. Leighton, P. A., Photochemistry of air pollution. Academic Press, 1961. 13. Chameides, W. L., and J. C. G. Walker., A photochemical theory of tropospheric ozone. J. Geophys. Res., 78 , 8751-8760, 1973. 14. Levy, H., Photochemistry of the lower tropopshere. Planet.Space. Sci., 20 , 919-935, 1972. 15. Times of India, 16 Nov 2017. 16. What has Delhi done over the decades to combat air pollution?, The Hindu, November 09, 2022.