mclina

Another thought is, rather than trying to capture air samples in flight, which would require a vacuum pump and some kind of activated charcoal filters, etc., you might want to design your experiment around light sensors or something instead. You may be able to use photocells to measure light intensity at various altitudes, which would give you a measure of the clarity of the air, rather than trying to get actual chemical contaminate readings.

You may also be able to so something similar from the ground, by photgraphing your plane at various altitudes, and comparing the color or image degradation as you go higher, indicating the amount of pollution in the air.

Of course, a balloon would probably work better than a plane for these types of things, but I know you are trying to work in the plane angle.

Good luck.