I don't understand very well your question, do you need to eliminate impurities from NOx gas? A zero air generator is an instrument that produces laboratory-grade purified air for supplying to FID or other detectors. This system is composed by some filters and a catalytic converter and it is used to remove hydrocarbons from an existing oil-free compressed air supply.
Like Irene said - A zero air generator simply purifies ambient air - requires a source of high pressure (compressed) air to operate. Compressed air is usually generated with an air compressor, similar to those used in a workshop to operate pneumatic tools like a nail gun. When you fill your tires in your car you are using compressed air - zero air is just taking that compressed air and getting rid of the impurities (hydrocarbons and particulates).