"Both Aedes aegypti and Anopheles gambiae mosquitoes — which transmitdengue and malaria respectively — were much less attracted to the scented beads after being exposed to the chemical. These results showed that the receptor responsible for detecting carbon dioxide also detects skin odorants."

https://www.nih.gov/news-events/nih-research-matters/how-mos...

And thermal features:

http://www.caltech.edu/news/mosquitoes-use-smell-see-their-h...

In the cold, dry air of winter your breath vents upward, but getting near sunny 90 degree weather, during summer time, it might descend and pool at the feet of gatherings of people at barbecues.

Carbon dioxide in ambient, matched temperature mixtures tends to be dense, and sinks toward the floor in many conditions (similarly true for carbon monoxide), but local conditions obviously vary, and odor travels differently than entire air masses. If mosquitos are already out, they probably travel along the odor gradient, toward the most concentrated area of scent.

The problem is people don't see "air", but there are ways to visualize it (e.g. Schlieren photography). Or just watch smoke.