Calcium carbonates are key materials to biomineralization, they are frequently used in industrial applications and also for carbon capture technologies. Finally, they serve as an important model system to test novel nucleation theories. Calcium carbonate crystalizes in a multi-step process, where amorphous calcium carbonate (ACC) is the most important precursor in the crystallization process. Existing synthesis protocols generate ACC of different stability and purity. To improve our mechanistic understanding of carbonate crystallization, reactivity and polymorph formation, the reproducible synthesis of clean and stable ACC is an important, and yet unresolved step. Here we use the fast reaction of CO2 with calcium hydroxide in airborne aerosols to reproducibly create pure and stable ACC, which may serve as a well-defined starting material for further chemical processing.