Carbon particles, along with polyaromatic hydrocarbons (byproducts of burning fuel), are the main culprits. Both are released as a result of traffic pollution, and both, thanks to the way they just happen to bind with the skin, cause aging. In fact, the study, which was published in the Journal of Investigative Dermatology, found that the more airborne particles around, the more age spots appeared.
While we can rely on sunscreen to keep us shielded from sun-related skin damage, scientists are still working on ways to protect us from pollutants by blocking them from penetrating the skin. Until then, I'll be riding my bike more often.