A sample of air tums lime water milky and also turns acidified potassium dichromate green in aqueous solution has low pH. This is due to the presence of pollutants

SO₂ tums lime water milky.
SO₂ in aqueous solution gives H₂SO₃

${\text{H}}_2{\mathrm{SO}}_3\mathrm{\hspace{0.17em}}\stackrel{ }{\rightleftharpoons }\mathrm{\hspace{0.17em}}{\mathrm{H}}^{+}\mathrm{\hspace{0.17em}}+{\mathrm{HSO}}_3^{-}$

CO₂ also turns lime water milky.
CO₂ in aqueous solution gives H₂CO₃

${\mathrm{H}}_2{\mathrm{CO}}_3\stackrel{ }{\rightleftharpoons }{\mathrm{H}}^{+}+{\mathrm{HCO}}_3^{-}$

H₂SO₃ is stronger acid than H₂CO₃. Thus, ionization of H₂SO₃ is larger than H₂CO₃.
Thus, pH of H₂SO₃ is lower than that of H₂CO₃.
SO₂ also tums acidified K₂Cr₂O₇ solution green.

${\text{3SO}}_2\mathrm{\hspace{0.17em}}+{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}\mathrm{\hspace{0.17em}\hspace{0.17em}}+\mathrm{\hspace{0.17em}\hspace{0.17em}}2{\mathrm{H}}^{+}\mathrm{\hspace{0.17em}}\stackrel{ }{\to }\mathrm{\hspace{0.17em}\hspace{0.17em}}3{\mathrm{SO}}_4^{2-}\mathrm{\hspace{0.17em}}+\mathrm{\hspace{0.17em}\hspace{0.17em}}\underset{\mathrm{Green}}{2{\mathrm{Cr}}^{3+}}\mathrm{\hspace{0.17em}\hspace{0.17em}}+{\mathrm{H}}_2\mathrm{O}$

Thus, air sample has SO₂ as pollutants.