Sulfate salts in the atmosphere play a critical role in nucleating and enlarging aerosol particles, which can seed precipitation, reflect or absorb solar radiation, and harm health. But the mechanisms behind the formation of salts such as ammonium sulfate have yet to be fully detailed. Computational work now indicates that ammonium bisulfate can form from the pollutants ammonia and sulfur trioxide, mediated by water (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.5b13048). NH3 is emitted by agricultural operations and SO3 by fossil-fuel burning. A team at the University of Nebraska, Lincoln, led by Joseph S. Francisco and Xiao Cheng Zeng found that water molecules from a water trimer or droplet form a looplike network connecting NH3 and SO3 molecules. The network facilitates water-mediated proton transfer in which one water breaks apart, giving OH– to SO3 to form HSO4– and an H+ to another water. The water that receives the H+ transfers one of its original H+ ions to NH3 to form NH4+. The reaction has a low barrier and consequently may play a significant role in the formation of aerosol particles.