Cosmic rays can incite up to a 10-fold increase in the nucleation rate of atmospheric aerosol particles from sulfuric acid vapor, according to a study published in Nature (DOI: 10.1038/nature10343). Aerosol particles play a role in both human health and climate, and a better understanding of the role of cosmic rays in aerosol formation may help elucidate solar effects on climate variability. Scientists conducted the cosmic ray experiments as part of the Cosmics Leaving Outdoor Droplets (CLOUD) project, in which cosmic rays are simulated using a proton accelerator at CERN, the European Organization for Nuclear Research. The researchers, led by CERN physicist Jasper Kirkby, studied the effects of cosmic rays on the nucleation of H2SO4 particles in a chamber filled with humidified N2, O2, and traces of H2SO4. Kirkby and colleagues found that cosmic rays led to a twofold to 10-fold increase in nucleation rate, depending on the temperature. They also found that ammonia is critical to aerosol particle formation, with particles nucleating fastest when they grow in a 1:1 NH3:H2SO4 ratio. But even with cosmic ray and ammonia enhancements, experimental nucleation rates did not match those observed in the field, so other compounds must also play a role, the researchers say.