Hi there, Contamination usually means that there is too much of a certain substance in something. It doesn't necessarily have to be harmful but it can be. Pollution is when there is so much of a substance in something that it begins to cause harm. Usually, when talking about the atmosphere in particular, we use the word pollution. In your example I would definitely use 'pollute' because the exhaust fumes cause real damage to the atmosphere. The words are often used interchangeably in general English, but when something is causing real harm it is definitely better to use 'pollute'. By the way I would probably change the tenses in the sentence to present continuous. We use present continuous to describe processes which are happening now. I would say this: More and more vehicles such as cars and motorbikeS are BEING used. This IS LEADING to an increase in exhaust emissions which POLLUTE / ARE POLLUTING the atmosphere.

Usually, they are synonymous with only a subtle difference. Contaminate means to add in outside matter (implied: unwanted). Pollution is the similar, but adding in outside matter makes things harmful. Full answer, see- http://the-difference-between.com/pollute/contaminate