Dry ice is solid carbon dioxide, with a condensation temperature of minus 78.5 degrees Celsius and a boiling point temperature of minus 57 degrees Celsius. Once it falls like snowflakes, faced with the unshielded "Atlantis," it encounters not only extreme cold but also high concentrations of carbon dioxide.
The oxygen proportion in this planet's atmosphere is already low; not only humans need oxygen, but other plants and animals do as well. The "Wall of Sighs" did not carry enough greenery; even with "Atlantis's" agricultural crops, there is still a reliance on mechanical and chemical oxygen production as essential supplements, barely maintaining supply and demand balance in "Atlantis's" underground spaces.
