Gray smog is best described as which of the following?

• A. Industrial smog from coal burning.
• B. Photochemical smog formed by sunlight and NOx.
• C. Gray smog is primarily produced by coal burning, sulfur compounds, and particulates.
• D. Acid rain deposition.

Answer: (C)

Gray smog, often called sulfurous smog, arises mainly when coal is burned and releases sulfur compounds along with soot and other particulates. In cool, humid air, sulfur dioxide from coal can oxidize and combine with moisture to form sulfates, creating thick, grayish haze that reduces visibility and irritates the lungs. This makes gray smog a sulfur- and particulate-driven phenomenon tied closely to coal combustion.

Photochemical smog, by contrast, forms in sunny conditions from reactions involving nitrogen oxides and volatile organic compounds, producing a brownish haze rich in ozone—so it’s not the gray type described here. Acid rain deposition involves pollutants like sulfur and nitrogen compounds returning to the surface via precipitation, which is a related environmental issue but not the same as the smog layer formed in the air.

So describing gray smog as primarily produced by coal burning, sulfur compounds, and particulates best captures its origin and appearance.