Sum frequency generation by water on supercooled H2SO4/H2O liquid solutions at stratospheric temperature

Sum frequency generation spectra indicate that the surface water of liquid sulfuric acid solutions varies as a function of concentration, but not with temperature. At 0.1x H2SO4 (where x=mole fraction, 38 wt%), subsurface ionic species orient surface water molecules. The surface of 0.2x H2SO4 (58 wt%) solutions, however, features H2SO4/H2O complexes both at 273 K and supercooled at 216 K. The results support a picture of stratospheric chemistry in which sulfuric acid aerosols are coated with hydrogen-bonded water/sulfuric acid complexes.