The major source of unburnt hydrocarbons in the atmosphere is/are:

Explanation

There are many sources of unburned hydrocarbons. Unburned hydrocarbons (UHC) are typically an output of incomplete combustion due to unfavorable engine design, low fuel quality, or failure in the control system. Sometimes unburned hydrocarbons are released into the air during incomplete combustion. Burning fossil fuels, including gasoline in automobile engines, releases some hydrocarbons into the air. In a typical urban environment, the atmospheric concentration of hydrocarbons is around 3 ppm (parts per million).