Many air pollution monitors require a long optical path for making accurate measurements. This usually dictates large fixed instruments that sometimes resort to folding optics like Herriott cells.
NASA has come up with a clever way to make a physically small but optically long path using a cylindrical cavity that could be precsion machined in an aluminum billet. A varity of pollution monitoring tools could be inexpensive and potentially accurate. This kind of precision maching isn't cheap, but Apple has the manufacturing chops to make these cavities at scale. Not that they'd do it, but it would make a wonderful addition to their phones and pads - imagine if every edu iPad came with one. The density of pollution measurements would could improve by several orders of magnitude.