This is a really tough problem. I have spent years studying and experimenting about this. Outdoor air contains tracers such as dust, pollen, etc. called aerosols. It is possible to use LIDAR to detect these at distances in the scale of kilometers away. But lasers of the power to use at even 1km. are very expensive and and the required light gathering power of the reciever requires telescopes of 8 inch or larger diameter and photon counting. I would like to use optics with a diameter of 3 inches maximum. I need a range of 1000 feet or more. A passive system using daylight would be preferable to an active system. I am aware that the polarization of light is changed by aerosols but my experiments in finding this effect (polaization filter and image processing) have not been successful. ANY other ideas about this topic are apreciated!