Which of the following is a secondary pollutant?

Ground-level ozone is a secondary pollutant. ‘Bad’ ozone formed in the lower atmosphere (troposphere) that harms plants and animals. Ozone, unlike the other criteria pollutants, is not emitted directly into the air by any one source. It is formed through chemical reactions of other molecules already in the air, specifically nitrogen oxides (NOx) and volatile organic compounds (VOCs).