" In one form or another, calcium carbonate turns up everywhere—in coral reefs, in the pores of basalt, in the ooze at the bottom of the ocean. It’s the main component of limestone, which is one of the world’s most common sedimentary rocks. “There are vast amounts of limestone dust blowing around in the troposphere, where we live,” Keutsch observed. “So that makes it attractive. “It has near-ideal optical properties,” he went on. “It dissolves in acid. So I can say with certainty that it will not have the same ozone-depleting impact that sulfuric acid has.” Mathematical modeling has confirmed the mineral’s advantages, Keutsch told me. But until someone actually throws calcium carbonate into the stratosphere, it’s hard to know how much to trust the models. “There’s no other way around it,” he said. — "

― Elizabeth Kolbert , Under a White Sky: The Nature of the Future