An Atmosphere Processor is a Weyland-Yutani built multiple level installation and facility for terraforming colonys on planets like LV426. Atmosphere Processors don't feature in the film Prometheus but are included in the Timeline as installations that are built in a number of locations that are being terraformed.
This proprietary system creates a localized environment in the lowest atmospheric strata by mining the surrounding air and delivering electrostatic charge to correct the chemical composition of the planetary atmosphere. The system does this by converting toxic compounds into a breathable air ratio of approximately 1:3.1 oxygen-to-nitrogen ratio, depending on a colony’s oxygen needs. Once stable equilibrium is achieved, the AP continually monitors environmental gas compositions and corrects imbalances. By-products can often be isolated and condensed for use as fuel sources, industrial cleaning supplies or raw materials for manufacturing. Atmosphere Processors are used to initiate terraforming, the process can take anywhere from 5 to 20 years depending on conditions.