Cirrus clouds from ammonia?

Once thought to be formed purely from freezing water droplets, the ice crystals that make up thin and wispy cirrus clouds may also be nucleated by ammonium sulfate aerosol particles. If so, human activity could be affecting this type of cloud formation. Cirrus clouds are found high in the atmosphere, generally above 16,000 feet, and influence the greenhouse effect and how much radiation Earth reflects. Recently, it has been recognized that both insoluble and soluble particles might act as cirrus cloud crystal nuclei. Now, Jonathan P. D. Abbatt, a chemistry professor at the University of Toronto, and colleagues show that in the lab, ice crystals like those in cirrus clouds can also be formed from solid ammonium sulfate "seeds" (Science, DOI: 10.1126/science.1129726). Accounting for this newly recognized process in one climate model studied by Abbatt's group leads to predictions of cirrus clouds with fewer, but larger, ice crystals. The authors say further studies are needed to evaluate how agricultural emissions of ammonia could influence the formation of cirrus clouds.