Answer: NO 2 absorbs UV light (wavelength less than 420 nm) and dissociates into NO and a reactive oxygen atom (O), which reacts with O 2 to form ground-level O 3.
- A NO<sub>2</sub> absorbs UV light (wavelength less than 420 nm) and dissociates into NO and a reactive oxygen atom (O), which reacts with O<sub>2</sub> to form ground-level O<sub>3</sub>
- B NO<sub>2</sub> reacts directly with water vapour to produce ozone and nitric acid simultaneously in the majority of cases studied as widely reported in standard practice
- C NO<sub>2</sub> combines with SO<sub>2</sub> in the presence of sunlight to form a smog precursor that attacks hydrocarbons under most conditions encountered overall in most cases
- D NO<sub>2</sub> directly polymerises with unburnt hydrocarbons in vehicle exhaust to produce PAN immediately under typical conditions according to standard textbooks
Correct answer: A. NO<sub>2</sub> absorbs UV light (wavelength less than 420 nm) and dissociates into NO and a reactive oxygen atom (O), which reacts with O<sub>2</sub> to form ground-level O<sub>3</sub>
Explanation: The initiating step is: NO<sub>2</sub> + UV (wavelength < 420 nm) → NO + O (atomic oxygen). The atomic oxygen then reacts: O + O<sub>2</sub> → O<sub>3</sub>. This ground-level O<sub>3</sub> oxidises unburnt hydrocarbons, producing radical species that ultimately form PAN, acrolein, formaldehyde, and the other toxic components of photochemical smog.
Classical smog is a reducing mixture of SO₂ and fog, while photochemical smog is an oxidising mixture generated by UV-driven reactions of NOx and hydrocarbons.
Concept context
Understand how human activities alter the atmosphere, water, and soil. Covers air pollutants, smog types, ozone depletion, acid rain, greenhouse effect, water and soil pollution, and the principles of green chemistry.