Photochemical smog is a resultant of the reaction among

(a) NO2, O3 and peroxyacetyl nitrate in the presence of sunlinght
(b) CO, O2 and peroxyacetyl nitrate in the presence of sunlight
(c) CO, CO2and NO2at low temperature
(d) high concentration of NO2O3and CO in the evening



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To begin the chemical process of photochemical smog development the following conditions must occur.
1. sunlight
2. the production of oxides of nitrogen (NOx)
3. the production of volatile organic compounds
4. the temperature greater than 18 degrees celsius. 
reactions :
O3 + NO  ------------>   NO2 + O2
NO + RO2 ---------------->  NO2 + other products
NO2 + sunlight ----------->  NO + O
O + O2 ------------> O3
NO2 + R  --------------------> peroxyacetyl nitrate.
hence ,option (a) is correct.
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