You can think of our atmosphere as a big chemistry set, a global churn of gaseous molecules and particles that constantly bounce off and change each other in complicated ways. While the particles are very small, often less than 1% of the thickness of human hair, they have outsized impacts. For example, particles are the seeds of cloud droplets, and the abundance of the particles changes the reflectivity and the amount of clouds, rainfall and climate.

The conventional thinking was that most particle formation occurs in cloud outflow regions, where clouds float into the upper troposphere and eventually evaporate. In that process, clouds are getting wrung out and most particles are removed by rain. As a result, the air in the outflow regions is clear and clean, leaving some gaseous molecules with nowhere to go but form new particles.

However, using the data collected from NASA's global-scale aircraft measurements, we found that most of the new particles are not formed in the outflow regions as previously thought.

Stratosphere air often dips in troposphere due to meandering jet stream. As the ozone-rich stratospheric air and more moist tropospheric air mix, it leads to a high concentration of hydroxyl radical (OH), an important oxidant that helps produce the type of molecules that nucleate and form new particles.

We found this phenomenon is widespread around the globe and likely occurs more frequently than the particle formation in the cloud outflows.