Two gases of suffocating odour are obtained when a green solid is heated, along with a residue. These gases are major air pollutants. When the vapours of the gases are collected and dissolved in water, the solution turns blue litmus to red. The colour of the residue becomes red. These compounds would be?

So, the two gasses of suffocating odors are respectively sulfur dioxide (SO₂) and sulfur trioxide ( SO₃). The green solid being ferrous sulfate (FeSO₄) which on heating releases the two mentioned gasses. On dissolution with water these oxides of sulfur form acids. That is why the solution turns blue litmus to red.

This can be shown in the following reaction:
$2\mathit{FeS}{O}_4 \to \mathit{ F}{e}_2{O}_3 +S{O}_2 \left(g\right)+\mathit{ S}{O}_3\left(g\right)$
$2S{O}_2 + 2H_2\mathit{O }\to 2H_{2}S{O}_3$
 

Hence, The compounds are FeSO₄,_, Fe₂O₃, SO₂, SO₃