12 November 1999 Ž . Chemical Physics Letters 313 1999 416–420 www.elsevier.nlrlocatercplett Sum frequency generation by water on supercooled H SO rHO 2 4 2 liquid solutions at stratospheric temperature Cheryl Schnitzer a , Steve Baldelli b , Mary Jane Shultz a, ) a Department of Chemistry, Tufts UniÕersity, Medford, MA 02155, USA b Department of Chemistry, UniÕersity of California, Berkeley, CA 94720, USA Received 7 June 1999; in final form 4 September 1999 Abstract Sum frequency generation spectra indicate that the surface water of liquid sulfuric acid solutions varies as a function of Ž . concentration, but not with temperature. At 0.1 x H SO where x smole fraction, 38 wt% , subsurface ionic species orient 2 4 Ž . surface water molecules. The surface of 0.2 x H SO 58 wt% solutions, however, features H SO rH O complexes both at 2 4 2 4 2 273 K and supercooled at 216 K. The results support a picture of stratospheric chemistry in which sulfuric acid aerosols are coated with hydrogen-bonded waterrsulfuric acid complexes. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction It is well known that gas-phase chemistry alone cannot explain the dramatic seasonal decrease in stratospheric ozone concentration. Instead, the rapid w x loss of ozone 1–3 is facilitated by stratospheric Ž . aerosols, i.e. polar stratospheric clouds PSCs and Ž . stratospheric sulfate aerosols SSAs . SSAs are im- portant players in atmospheric chemistry not only because they are the only particles present to partici- pate in heterogeneous chemistry at mid-latitudes but also because they serve as nucleation sites for PSCs wx 3 . Calculations indicate that SSAs range in concen- Ž tration between 40 and 80 wt% H SO 0.1–0.4 x, 2 4 . where x s mole fraction at temperatures of 205–240 w x K 4–6 . Since the freezing point of 0.2 x H SO is 2 4 wx 245 K 7 , it is either supercooled, i.e. still liquid, or ) Corresponding author. Fax: q1-617-627-3443; e-mail: mshultz1@emerald.tufts.edu frozen at stratospheric temperatures. The purpose of this Letter is to identify the molecules at the surface Ž of supercooled, liquid H SO P 4H O solutions 0.2 x 2 4 2 . H SO , 58 wt% to learn more about SSA chem- 2 4 istry. Our results indicate that water is tightly bound in sulfuric acid complexes on the surface of super- cooled 0.2 x H SO rH O solutions. 2 4 2 The surfaces of sulfuric acid solutions are exam- ined at the molecular level using sum frequency Ž . generation SFG . SFG is ideal for this application because of its surface sensitivity and molecular specificity. Under the electric dipole approximation, SFG is forbidden in the bulk, but allowed at the wx interface where the inversion symmetry is broken 8 . A vibrational spectrum of surface molecules is ob- tained by scanning an infrared laser beam that is overlapped in space and time with a 532 nm visible w x beam 9,10 . When the infrared light is in resonance with a molecular vibration, a peak appears in the spectrum. The intensity of the peak is enhanced both 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 01087-8