New particles form in the lowest layer of Earth's atmosphere through condensation of highly oxygenated compounds, a new study shows, and without sulfuric acid - previously considered essential to nucleation. Future atmospheric models should take these factors into account, the study's authors say, to better represent the process. Cloud condensation nuclei, or CCNs, are small particles on which water vapor from a cloud condenses. Many different types of atmospheric particulates can act as CCNs, including dust, soot, or salt from ocean spray, and the number and types of CCNs in the atmosphere at any one time affect properties of clouds, including their reflective abilities. Scientists are unsure about the fraction of CCNs attributable to newly formed atmospheric aerosol particles in the free troposphere. To date, studies of these particles and their role as CCNs have been limited. Further complicating matters, different models of atmospheric aerosol particle formation rates have provided varying views on what the process requires, with a questionable role for sulfuric acid. Here, to better characterize the process by which new atmospheric aerosol particles form, Federico Bianchi and colleagues used a suite of state-of-the-art mass spectrometers and particle counters at a high-altitude research station in Junfraujoch, Switzerland--a site where new particle formation occurs on 15 to 20% of days. The researchers collected measurements for one year. Among their observations, they found that new particle formation occurred nearly exclusively on days when the concentration of highly oxygenated organic compounds was high, but that these compounds could only contribute to nucleation for a short window (one to three days) after their vertical transport from the planetary boundary layer.