Cars release approximately 333 million tons of carbon dioxide (CO2) into the atmosphere annually, which is 20% of the world's total, according to the Environmental Defense Fund. Motor vehicles also contribute 72% of the nitrogen oxides and 52% percent of reactive hydrocarbons.
This depends on where you are and when. In a city, most of the pollution is car or power-plant driven. On a dormant or active volcano, most of the "pollution" is natural. For places outside of cities and volcanos, it depends on what nearby cities there are and how far away they are, and what nearby volcanoes there are and how active they are at the moment.
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