If technology that reduces hydrocarbon concentration had been utilized, which outcome would be most likely?

• A. The concentration of O3 would have been lower.
• B. The concentration of CO2 would have been lower.
• C. The concentration of PM would have increased.
• D. The concentration of NO2 would have increased.

Answer: (A)

Ozone in polluted urban air is formed through sunlight-driven reactions that require both NOx and volatile organic compounds (VOCs, such as hydrocarbons). VOCs help generate radicals that convert nitric oxide (NO) to nitrogen dioxide (NO2), and NO2 then photolyzes in sunlight to produce oxygen atoms that combine with O2 to form ozone (O3). If technology lowers hydrocarbon (VOC) levels, there are fewer radicals to sustain this chain of reactions, so ozone production slows and O3 concentrations decline under typical conditions where NOx is present. The other outcomes aren’t as directly tied to reducing VOCs: CO2 mainly comes from combustion processes and isn’t governed by VOC levels in the same way; particulate matter can include secondary organic aerosols formed from VOC oxidation, but reducing VOCs would more likely decrease PM rather than increase it; and reducing VOCs doesn’t inherently raise NO2 levels—the overall photochemical balance would tend to produce less ozone, not more NO2.