Clouds play an important role in Earth’s climate by reflecting sunlight. But the intricate details of how clouds form remain uncertain. A new study reveals that when acetaldehyde molecules (CH3CHO) in the atmosphere absorb onto inorganic aerosol particles, the aerosols more readily take on water to form cloud droplets. Incorporating this knowledge into climate models would help make them more accurate, according to Columbia University’s V. Faye McNeill and Georgia Tech’s Athanasios Nenes, who led the study (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1204838110). The scientists used a laboratory cloud chamber to expose ammonium sulfate aerosols to different small aldehydes. These species form in the air through oxidation of natural and man-made compounds. The researchers then watched for water vapor to coalesce around the aldehyde-modified aerosols to form cloud droplets. They found that acetaldehyde increases the aerosols’ ability to attract and hold water—their hygroscopicity—by 20%. At that rate, the number of cloud droplets in the atmosphere would increase by 10 to 20%, the researchers note.