Which of the following is a secondary pollutant involved in photochemical smog?

Tropospheric ozone (O3) is a secondary pollutant that plays a significant role in the formation of photochemical smog. It is categorized as a secondary pollutant because it is not directly emitted into the atmosphere but forms in the presence of sunlight through complex reactions involving primary pollutants such as volatile organic compounds (VOCs) and nitrogen oxides (NOx). These primary pollutants are often released from vehicle emissions, industrial activities, and other combustion processes.

During sunny conditions, the interaction between sunlight and these primary pollutants leads to chemical reactions that produce ozone and other photochemical oxidants, contributing to smog formation, especially in urban areas. This is why tropospheric ozone is a key component of photochemical smog, affecting air quality and human health.

In contrast, carbon monoxide and sulfur dioxide are considered primary pollutants as they are directly emitted from sources like vehicles and industrial processes, while particulate matter can be both primary and secondary depending on its formation processes but does not specifically characterize photochemical smog as distinctly as tropospheric ozone does.