Bioaerosols are typically introduced into the air via wind turbulence over a surface. Once airborne they typically remain in the planetary boundary layer (PBL), but in some cases reach the upper troposphere and stratosphere. Once in the atmosphere, they can be transported locally or globally: common wind patterns/strengths are responsible for local dispersal, while tropical storms and dust plumes can move bioaerosols between continents. Over ocean surfaces, bioaerosols are generated via sea spray and bubbles.