I live directly in line with a the main runway at an International airport and about 8 Km from it. We see a yellow substance on our cars and laundry. Could this be from pollution of the aircraft?

Most definitely. I used to live in the runway path at Ft. Lauderdale International Airport in Florida and my cars and home were coated with substances all the time - usually jet fuel residue.

Possibly, but they've cleaned up particulate matter from jet engines quite a bit over the past years. Also 8 km is a good way out and one would not think that any jet emissions would be so concentrated at that distance (i.e. the planes would be at such an altitude) as to leave noticeable residue. There are other possible sources, or a combination of sources - including cars, trains , industrial activities, and even plants (e.g. certain trees at certain times of the year), etc. that could be contributing to the residue. Look around your local environment.

Jet fuel is perfectly healthy for you and the environment... :)
Actually, it is only kerosene - the same as the exhaust from a space heater, but of course a much larger concentration of it. I'm sure its not that great for you, but neither is living near a highway and all the pollutants that will cause.