Dogs smell in stereo. 

As air enters into the dogs nose, dogs can smell several hundred million times more (with their Olfactory Receptor Cells) than what humans can smell.  As air enters into their nose and then exits through slits on the side of its nose, creating an airflow swirling new odour molecules to help odour concentration to build up over multiple sniffs.

See this amazing video explaining this complex behaviour of how dogs smell.

By: Alexandra Horowitz