In stratosphere, ozone is useful but in troposphere it is harmful to us- explain.

Explanation

The layer of ozone gas present in stratosphere which is extending from a height of 15 km to 75 km from the sea-level of the earth's surface is known as ozone layer or ozonosphere. The harmful ultraviolet rays (UV rays) coming from the sun is mostly absorbed in the ozone layer, because these rays are utilised in the production and dissociation of ozone gas.
But, ozone gas present in the troposphere acts as a greenhouse gas. Ozone gas contributes nearly 7-8% to the creation of greenhouse effect. Due to greenhouse effects, the surface temperature of the earth will rise and this eventually will melt the polar caps accumulated in polar regions which will cause colossal devastation by tidal waves, cyclone, super cyclone. Thus ozone, in stratosphere is useful but in troposphere is harmful. Absence of this ozone layer would allow the ultraviolet rays coming from the sun to reach the earth's surface entirely. This would have heated the earth's surface and the adjacent air to such an extent that the existence of the living world in land and water would have been jeopardised.