Atmospheric aerosols, particles once thought to need human-generated sulfur dioxide to form, can form from other naturally occurring compounds alone, according to a series of new reports (Nature 2016, DOI: 10.1038/nature18271 and 10.1038/nature17953; Science 2016, DOI: 10.1126/science.aad5456).
The discovery of this additional avenue for aerosol formation has significant implications for modeling the effects of these particles on climate. The studies also imply that the preindustrial atmosphere was cloudier than once thought.
Formation and growth of aerosols are some of the most poorly understood aspects of atmospheric science. Yet these particles’ importance is hard to overstate. Aerosols can seed cloud formation, and they also have detrimental effects on human respiratory and cardiovascular health. Because they reflect and scatter sunlight, aerosols are also likely responsible for an as-yet-unknown amount of global cooling that climate scientists think has somewhat counteracted the effects of global warming.
Previously, scientists had thought that sulfuric acid, which is largely produced in the atmosphere by the oxidation of sulfur dioxide, was essential for aerosol formation. Sulfur dioxide spews into the atmosphere from oil and gas burning and, to a lesser extent, from volcanoes and marine plankton.
But now, experiments that are part of the Cosmics Leaving Outdoor Droplets (CLOUD) project run by CERN, the European Organization for Nuclear Research, have expanded researchers’ knowledge about aerosol formation and growth mechanisms. The facility uses a proton accelerator to simulate the high-energy cosmic rays that shower Earth.
A team led by Urs Baltensperger of Paul Scherrer Institute and Jasper Kirkby, a particle physicist at CERN and a founder of CLOUD, studied the molecule α-pinene. When oxidized, this compound, which is emitted by pine trees, forms various types of highly oxygenated molecules (HOMs).
Research led by Kirkby showed that, under simulated pristine, sulfur-free atmospheric conditions, the ozonolysis of α-pinene leads to the condensation of nanometer-scale aerosol particles. The team also found that cosmic rays enhance this aerosol production by up to two orders of magnitude. Baltensperger led an additional study of the growth of these particles.
Another team led by Federico Bianchi, also at Paul Scherrer Institute, and Baltensperger studied atmospheric samples at a research station in the Swiss Alps. Their observations, made with mass spectrometers and particle counters, back up the CLOUD experiments: The researchers found that similar aerosols were produced in an unpolluted mountain atmosphere when the atmosphere contained high levels of HOMs.
The new studies show that these organic compounds can enable the nucleation of new particles, says Paul O. Wennberg, an atmospheric chemist at Caltech. This is important for understanding Earth’s atmospheric processes. Wennberg emphasizes: “Remember that at the core of every cloud or raindrop is one of these particles.”