Nitrite decomposition releases NO2 for PAN formation

Steps:

• PAN forms in photochemical smog via NO2 reaction with oxidized unburned hydrocarbons.
• Gas M must therefore be NO2.
• NO2 arises from thermal decomposition of metal nitrites.
• Ca(NO2)2 decomposes on heating to yield NO2 gas among products.

Why B is correct:

• Ca(NO2)2 → CaO + 2NO2 on heating, directly providing NO2 gas M as per decomposition reaction.

Why the others are wrong:

• A: Ca(NO3)2 is a nitrate; mild heating yields Ca(NO2)2 + O2, not direct NO2 gas.
• C: MgO is a stable solid oxide with no gaseous products on heating.
• D: Mg(NO2)2 decomposes to MgO + NO + O2, yielding NO gas, not NO2.

Final answer: B