Which factor drives photochemical processes that can influence ambient PM measurements?

• A. Solar radiation driving photochemical processes
• B. Humidity
• C. Atmospheric mixing
• D. Temperature

Answer: (A)

Sunlight energizes atmospheric chemistry. Solar radiation provides the energy needed for photolysis and the formation of reactive species like OH radicals and ozone, which drive the oxidation of gases such as VOCs and NOx. This photochemical activity leads to the production of secondary aerosols and changes the composition and mass of ambient PM that a monitor detects.

That daytime photochemistry logic is why ambient PM measurements can rise or shift in composition on sunny days—the sun’s energy accelerates the chemical reactions that create new particles and alter existing ones. Humidity, atmospheric mixing, and temperature influence aerosol growth, transport, and reaction rates in various ways, but they do not initiate photochemical processes themselves as directly as solar radiation does.