Kate is researching air pollution and finds some information on ozone. She knows that ozone is a good thing as part of the ozone layer, so why is it also an air pollutant?

Ozone gas contains the oxides of nitrogen and other reactive gases that makes it more harmful to the living beings and thus makes it as an air pollutant.

Explanation:

Ozone is the most reactive substance and it contains three oxygen atoms. Ozone gas may be consider as either bad or good substance depends upon the source it reacts. Ozone act as a protective layer that prevents the entery of harmful radiation from sun and thus act as a good substance. Ozone gas when reacts with oxides of nitrogen with air result in air pollution. This harmful gas affects the human beings and several living organism who enhales this ozone gas. It also cause several breathe problems like asthma, bronchitis and it affects the function of lungs. Throat irritation, Cough and heart pain are some of the minor issues caused by ozone gas.

The answer is that at ground level, Ozone contributes to smog, pretty sure.