By Carolyn Gramling

    Scientists have sussed out a new source for the seeds of clouds. When the stratospheric layer of Earth’s atmosphere dips a toe into the underlying troposphere, the resulting chemical mixture becomes a fertile environment where tiny new particles, including the microscopic aerosols around which clouds begin to coalesce, can form.

    Data collected by four flights of NASA aircraft in different regions of the northern hemisphere in 2016 and 2017 confirmed that this mechanism, called stratospheric air intrusion, is not only a source of new particles but also may be the most productive place for these particles on Earth, Jiaoshi Zhang, an aerosol scientist at Washington University in St. Louis, and colleagues report in the July 12 Science.

    Previously, scientists have thought that most new particle formation happens in regions of the atmosphere where clouds float upward into the upper troposphere and dissipate. As the clouds there rain out, any existing particles are removed with the rainwater. What’s left behind in these “cloud outflow” regions is a blank slate, basically particle free, so that gaseous molecules have nothing existing to glom onto. Instead, they make new particles.

    But the airborne observations suggest that stratospheric air intrusions are even more productive when it comes to particle formation. Turbulence in the atmosphere caused by the jet stream, a fast-moving current of air, can cause fingers of stratospheric air to punch down and curl into the troposphere below.

    The two atmospheric layers have very different chemical makeups, and where those air masses mix, they generate very productive particle factories, says study coauthor Jian Wang, an aerosol scientist also at Washington University in St. Louis. The stratosphere is cold and ozone-rich, while the troposphere is warmer, moister and contains a variety of molecules such as sulfur dioxide. Catalyzed by sunlight and water, the chemical reaction of these air masses may generate a variety of tiny particles, including cloud-seeding sulfate.

    Exactly which and how many particles are being formed by these stratospheric air intrusions is a subject for future work, Wang says. “We don’t really understand the mechanisms in detail. We know from the data that … you need sunshine, high ozone and moisture” to produce highly reactive molecules known as OH radicals (SN: 6/4/09). Those molecules eagerly interact with other gases in the atmosphere. So there are probably many different chemical reactions going on in these regions, producing a variety of new molecules and particles.

Cloud seeding

Anatomy of aerosol formation