When Captain America and Banner first board SHIELD's Helicarrier, agent Romanoff (Black Widow) warns them to get inside because it would become 'hard to breath' while on deck. Later during the flight when Fury was talking with Loki about 'ants and boots' as Loki was locked in the carrier Hulk cage, Fury disclosed that the carrier was flying at 30,000 feet.
Knowing that between 8,000 and 12,000 feet, hypoxia causes the first signs of diminished performance and at 15,000 feet the hypoxic effect becomes increasingly apparent. At 20,000 feet, even the most hardy, durable and experienced pilots can scarcely be able to see, much less read the instruments or demonstrate complex motor skills. His or her hearing, perception, judgment, comprehension, and general mental and physical faculties are practically useless. The pilot would be on the verge of complete collapse and unconsciousness. An inexperienced person would be far more adversely affected by the hypoxic effects of high altitude.
How then can Hawkeye perform such feats of visual and mental acuity as well as complex fine motor skills after strenuous physical exertion at 30k feet? Granted, his team is shown to have been wearing small oxygen generators but Barton was mask-less. Does he have some heretofore unknown or extra powers beyond archery?