Industrial and highly populated cities are mostly seen with brown haze usually at the time of dawn and dusk. A mixture of <a href="https://botanylive.com/photochemical-smog/">pollutants formed</a> by the combination of nitrogen oxide, volatile organic compounds (VOCs), ozone, and peroxyacetyl nitrate (PAN) (CH3COOONO2) react with sunlight to create a brown haze called photochemical smog.