What are secondary pollutants and how do they form given example of a secondary pollutant?

Secondary pollutants are substances that are not directly emitted into the atmosphere but form through chemical reactions between primary pollutants and other atmospheric components. For example, ground-level ozone is a secondary pollutant that results from the reaction of sunlight with pollutants like volatile organic compounds (VOCs) and nitrogen oxides (NOx), which are emitted from vehicle exhaust and industrial processes. These reactions typically occur in the presence of sunlight, making ozone concentration higher during warmer months.