What is the role of radical species OH and HO2 in the atmospheric chemistry of ozone formation and destruction?

• A. OH initiates VOC oxidation producing RO2; HO2 participates in NOx cycles to convert NO to NO2, influencing ozone formation and loss; they regenerate OH in the radical cycle.
• B. OH and HO2 do not affect ozone.
• C. OH consumes ozone directly; HO2 breaks down PM.
• D. OH and HO2 convert CO2 to CO.

Answer: (A)

The main idea is that daytime ozone chemistry is driven by radical chains started by the hydroxyl radical and linked by the hydroperoxy radical. OH is the atmosphere’s primary oxidant and begins the oxidation of volatile organic compounds, producing RO2 radicals. Those RO2 radicals propagate the oxidation and feed the peroxy radical pool, which interacts with nitrogen oxides and other species to advance or limit ozone production.

HO2 enters the scene in the NOx cycle. It reacts with NO to form OH and NO2, and NO2 can then photolyze to regenerate NO and atomic oxygen, with O combining with O2 to form ozone. Through this NO-to-NO2 cycling, HO2 helps control how efficiently NOx contributes to ozone formation, while also tying into radical regeneration that keeps the chain going. The radicals collectively help determine the balance between ozone formation and loss in the daytime atmosphere, with OH and HO2 being continually regenerated in these interconnected cycles.

In short, OH starts VOC oxidation to build the radical pool; HO2 participates in NOx cycling to steer ozone production and loss; and the two radicals regenerate each other, sustaining the photochemical processes that control atmospheric ozone.