Gray smog is primarily produced by

• A. Industrial smog from coal burning.
• B. Photochemical smog formed by NOx and VOCs.
• C. Coal burning, sulfur compounds, and particulates.
• D. Acid rain from sulfuric deposition.

Answer: (C)

Gray smog, also called industrial smog, forms mainly from burning coal that releases a lot of soot (particulates) and sulfur compounds into the air. In cold, humid conditions, sulfur dioxide from coal combustion reacts to form sulfate aerosols, and these mingle with the soot particles to create a thick, grayish haze. The combination of smoky particulates and sulfur-based aerosols gives that characteristic gray color and the heavy, opaque appearance of the pollution.

Photochemical smog, by contrast, comes from sunlight-driven reactions of nitrogen oxides and volatile organic compounds, producing ozone and other oxidants that create a hazy, often brownish look. Acid rain is the deposition of sulfur and nitrogen compounds from the atmosphere to the surface, not the visible smog itself, though it stems from the same emissions. So the description involving coal burning, sulfur compounds, and particulates best explains gray smog.