Have you ever thought about doing a water air filter? someone mentioned to me previously that larger data centers in dusty areas like in deserts use them to filter the air coming in, basically you use a chambered box so the air routes under a surface and you have basically irrigation misters spraying water across the entire opening and running back down into a water reserve. The misting water catches the dirt particles and keeps the air coming in pretty much as clean as the best air filter without the restriction of a media for the air to cross through..