Researchers at Washington University in St. Louis, led by Jian Wang, professor and director of WashU's Center for Aerosol Science and Engineering, Lu Xu, assistant professor in the Department of Energy, environmental and chemical engineering at WashU's McKelvey School of Engineering, and scientists from NASA, NOAA, NCA, and European universities, have reportedly discovered something new in the sky: a mechanism that generates a significant amount of the particles in Earth's atmosphere. The study was published in Science.
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The atmosphere can be viewed as a massive chemistry set, a global churn of gaseous molecules and particles that continually bounce off and affect each other in complex ways. While the particles are extremely minute, frequently less than 1% of the thickness of human hair, they have a significant impact. Particles, for example, are the seeds of cloud droplets, and their quantity influences reflectivity, the number of clouds, rainfall, and climate.
The prevailing theory was that most particle creation happens in cloud outflow zones when clouds drift into the upper troposphere and eventually dissipate. During this process, clouds are wrung out, and rain removes most particles. As a result, the air in the outflow zones is pure and clean, leaving certain gaseous molecules with no option except to produce 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.
Jian Wang, Professor and Head, Center for Aerosol Science and Engineering, Washington University in St. Louis
While investigating this, Wang and colleagues discovered a completely different process at work when stratospheric and tropospheric air mix to create circumstances conducive to particle production.
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.
Jiaoshi Zhang, Study First Author and Research Scientist, Washington University in St. Louis
“We found this phenomenon is widespread around the globe and likely occurs more frequently than the particle formation in the cloud outflows,” Zhang added.
Future field observations and modeling studies will be required to validate and quantify the significance of this newly identified mechanism of particle formation. Humans add their own particles in the form of air pollution, but Wang believes what was uncovered in this study is a natural process that occurs all across the world, even in isolated and pristine areas.
There is also some indication that stratospheric air would drop into the troposphere more frequently in future climates, making this process even more essential, Wang stated. Including this previously unknown mechanism can enhance climate models, allowing for better simulation of climate change and prediction of future climate.
Wang concluded. “While we are puzzled by the observation initially, once we put everything together, afterwards it was not so surprising. It is well known that molecules forming new particles are generated through oxidation in the atmosphere. When the stratosphere and troposphere air mix, the OH concentration is very high, and it’s primed for particle formation.”
Journal Reference:
Zhang, J., et al. (2024) Stratospheric air intrusions promote global-scale new particle formation. Science. doi:10.1126/science.adn2961.