The troposphere (the bottom layer of the atmosphere extending upward from the surface for 6 to 10 kilometers) is well mixed due to turbulence caused by surface friction.
And convective activity caused by the uneven heating of the Earth's surface plays a significant role, too. We glider pilots call 'em thermals, and, as such, that's where I got my name. I've seen grass, shingles, leaves, spiders (and webs), and many other "heavier than air" objects well over 5 km aloft thanks to thermal activity. I even once saw an ultralight airplane picked up about 5' off the ground, turned 180 degrees and gently sat back down, much to the amazement of all who watched and the guy sitting in the pilot's seat.
In fact, I'm headed out right now to explore for some of these some times elusive critters....wish me luck!