What is the main reason the earth is warmer at the equator than at the north and south poles?

The poles receive more direct sunshine so it has to do with the angle that the light hits. The Earth is round and when light hits it the light wraps around the earth. At the poles the light is very stretched and at the equator it is very direct hence it is hotter at the poles.  

Air pollution: “combustion results in gases such as nitrogen oxides, nitrogen dioxide and particulates. the main causes of air pollution include transport, domestic combustion and industrial processes. pollutants from these sources can cause problems in the immediate vicinity and a distance away.”