scrub them

tree pollen? pressure washer and a little Simple Green cleaner.

mix TSP in water then use a scrub brush on the chairs rinse with hose

I believe the powdery stuff is oxidation. You can A. get some Brite lume acid and it takes oxidation off of anything, or B. replace the chairs and try not to let them sit for long periods of time in the direct sunlight. Take into mind, if you use brite lume acid, it does work great but if you get it on your skin, it will soak directly to the bone and begin eating your bones away. It will also cause blindness if you get it in your eyes. Always wear protective equipment when using this type of acid.