Carbon dioxide is not enough. To complete the chemical signature you add a cartridge that contains either octenol (a generic molecule that simulates plant chemicals) or Lurex (a proprietary mixture that simulates sweat chemicals). These chemicals act as strong attractants for different types of mosquitoes. Lurex-type chemicals work best on imported tiger mosquitoes, while octenol works best on mosquitoes native to the United States.
By mixing the chemical attractant with the carbon dioxide and moisture and then blowing it out into the surrounding air, the trap creates a plume of gas that mosquitoes find irresistible. They will fly upwind to follow the plume to its source.
When the mosquitoes get to the mosquito trap, they encounter a vacuum created by a fan, just like a vacuum cleaner. The fan sucks in air and the mosquitoes. The mosquitoes are trapped in a net bag, where they dehydrate and die.