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 key idea is that these radicals keep the daytime photochemical system that connects VOCs, NOx, and ozone running. The hydroxyl radical is the main daytime oxidant, so it reacts with volatile organic compounds and sets off a chain that rapidly forms peroxy radicals (RO2). Those RO2 species can react with nitric oxide (NO) to produce nitrogen dioxide (NO2), and when NO2 is illuminated, it splits to NO and an oxygen atom that quickly joins O2 to form ozone (O3). This is how VOC oxidation translates into ozone production in the presence of NOx and sunlight.

Hydroperoxyl radical (HO2) also plays a crucial role by reacting with NO to form NO2 and another radical, such as OH, which helps replenish the hydroxyl pool. That reaction both converts NO to NO2 (feeding ozone formation via NO2 photolysis) and regenerates OH, sustaining the radical chain that drives ongoing VOC oxidation. Because the OH/HO2 cycle maintains a large pool of radicals, the atmosphere can keep producing ozone as long as NOx and VOCs are available and sunlight is present.

There are also termination steps where radicals combine to form stable products, which can temporarily trim the radical pool and slow ozone production. But the overall effect of OH and HO2 is to link VOC oxidation with NOx cycling in a way that promotes ozone formation under sunny conditions, while their cycling also sets the potential for ozone loss when radical concentrations or NOx/VOC levels shift.