Think of our atmosphere as a giant chemistry set, full of gas molecules and tiny particles that constantly interact and change in complex ways.
These particles are incredibly small, often less than 1% the thickness of a human hair, yet they play significant roles in our environment.
For instance, they act as seeds for cloud droplets, influencing cloud formation, reflectivity, rainfall, and climate.
Researchers at Washington University in St. Louis have now uncovered a new mechanism in the atmosphere that produces a large number of these tiny particles.
The study, led by Professor Jian Wang and his team, was recently published in the journal Science.
The team included Lu Xu from WashU’s Department of Energy, Environmental and Chemical Engineering and scientists from NASA, NOAA, NCA, and several European universities.
Traditionally, scientists believed that most particle formation occurred in cloud outflow regions. These are areas where clouds move into the upper troposphere and eventually evaporate. During this process, rain removes most particles, leaving clear air where new particles can form from leftover gas molecules.
However, the new study challenges this conventional thinking. Using data from NASA’s global aircraft measurements, the researchers found that most new particles are not formed in these outflow regions.
Instead, Wang and his colleagues discovered a different process. When air from the stratosphere (the second major layer of Earth’s atmosphere) mixes with air from the troposphere (the lowest layer of the atmosphere), conditions become ideal for particle formation.
This mixing happens because the jet stream can cause stratospheric air to dip into the troposphere.
When the ozone-rich stratospheric air mixes with the more humid tropospheric air, it leads to a high concentration of hydroxyl radicals (OH).
These radicals are powerful oxidants that help produce the types of molecules that form new particles. Jiaoshi Zhang, the first author of the study, explained that this mixing creates perfect conditions for particle formation and occurs more frequently than previously thought.
The research team found that this phenomenon is widespread across the globe, even in remote and pristine areas. Although humans contribute to air particles through pollution, this newly discovered process is a natural one that happens worldwide.
There is also evidence that in the future, stratospheric air will dip into the troposphere more often due to climate changes. This means that this particle formation process could become even more important. Understanding and including this process in climate models can improve our predictions of climate change and future weather patterns.
Initially puzzled by their observations, the researchers eventually realized that the high concentration of OH radicals when stratospheric and tropospheric air mix was not surprising.
It’s well-known that these molecules are generated through oxidation in the atmosphere, and this mixing creates a perfect environment for new particle formation.
This discovery sheds new light on how particles are formed in our atmosphere, enhancing our understanding of atmospheric chemistry and its impact on climate.
