Violent cometary or asteroidal impact events comparable to that which terminated the Cretaceous era must produce large amounts of nitrogen oxides, sufficient to privide NO_x mole fractions much larger than 100 ppm over areas of millions of square kilometers. Rapid production of NO₂, and subsequent rainout of HNO₃ and other strong acids can acidify surface waters, selectively destroying CaCO₃ shells. This scenario holds the promise of explaining both the scope of Cretaceous biological extinctions and their extraordinary selectivity.