No one else has included this but - when the plants are active, they will be emitting terpenes into the air, a natural source of volatile hydrocarbons that produce haze.

The amount of isoprene emitted by plants is so significant that it filters sunlight, forming the beautiful blue haze that intrigued Leonardo da Vinci and is seen today in scenic locations such as the Blue Ridge Mountains of the United States and the Blue Mountains of Australia.