The presence of ozone in troposphere:

Ozone ($$O_3$$) is present in both the stratosphere and the troposphere. In the stratosphere, it forms the protective ozone layer that shields the Earth from harmful ultraviolet (UV) radiation.

However, when ozone is present in the troposphere (the lower atmosphere), it acts as a pollutant. Tropospheric ozone is a secondary pollutant formed by the reaction of nitrogen oxides ($$NO_x$$) and volatile organic compounds (VOCs) in the presence of sunlight.

This tropospheric ozone is a key component of photochemical smog. The process involves: sunlight causes the photolysis of $$NO_2$$ to form $$NO$$ and atomic oxygen, which then combines with $$O_2$$ to produce $$O_3$$. This ozone, along with other oxidants like PAN (peroxyacetyl nitrate), constitutes photochemical smog.

Therefore, the presence of ozone in the troposphere generates photochemical smog.

The correct answer is Option 4.