The BP oil spill that sent 4 million barrels of crude into the Gulf of Mexico last year also created air pollution, and studying this pollution gave scientists clues into how these contaminants get into the atmosphere.

BP’s Deepwater Horizon rig exploded on April 20, 2010, killing 11 people and spewing oil from the underwater well that rose to the surface. It also created a plume of air pollution downwind of the spill, researchers reported in the journal Science.

The lightest chemicals in the oil evaporated within hours, as scientists expected them to do. What they didn’t expect was that heavier compounds — the ones with more carbon atoms per molecule — in the oil took longer to evaporate, spread out much more widely and contributed most to the formation of air pollution particles.