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GATE 2026 ES – Question 28

Environmental Chemistry · Atmospheric Chemistry · 1 mark · Multiple select

Which reactions are related to tropospheric smog formation? Select all that apply.

  1. $\mathrm{CFCl_3}+h\nu\to\mathrm{CFCl_2}+\mathrm{Cl}^{\bullet}$
  2. $\mathrm{CH_3O_2+NO+O_2\to HCHO+HO_2+NO_2}$
  3. $\mathrm{NO_2+O_2}+h\nu\xrightarrow{M}\mathrm{O_3+NO}$
  4. $\mathrm{CH_4+8O_2\to CO+H_2O}+2\mathrm{OH}^{\bullet}+4\mathrm{O_3}$

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Correct answer: (B) $\mathrm{CH_3O_2+NO+O_2\to HCHO+HO_2+NO_2}$; (C) $\mathrm{NO_2+O_2}+h\nu\xrightarrow{M}\mathrm{O_3+NO}$; (D) $\mathrm{CH_4+8O_2\to CO+H_2O}+2\mathrm{OH}^{\bullet}+4\mathrm{O_3}$

Explanation

Photochemical smog forms through sunlight-driven NOx and organic-radical chemistry. B converts NO to NO₂ via a peroxy radical; C represents NO₂ photolysis followed by ozone formation with a third body M. D is a net methane-oxidation/ozone-production chain, with sunlight and NOx cycling implicit. A represents CFC photolysis associated with stratospheric chlorine chemistry. Thus **B, C, D**.

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