SMOG

Fog made darker and heavier by chemical fumes and smoke. A fog-like layer, often brown in color, that hangs over many areas under certain atmospheric conditions. Smog, a contraction of the words smoke and fog, is compounded from smoke, moisture and numerous chemicals produced by combustion (from power plants, automotive engines, etc.) and from natural and industrial processes. Formation of smog is aided by the presence of sunlight which reacts with many chemicals and combustion products such as hydrocarbons and oxides of nitrogen to form other more noxious compounds .