Could you explain the significance of your article to the non-specialist?

The liquid-air interface comprises many natural systems including atmospheric processes such as heterogeneous cloud formation and ozone depletion. In particular, sulfate aerosols are known as key players in stratospheric ozone depletion. Although some field observations and laboratory work corroborate this general picture, a fundamental understanding of the surface structure and the information of the ionic compositions at the surface are still lacking. This paper identified unequivocally the acid concentration of the first acid dissociation at the sulfuric acid solution surface, and we find the existence of the net orientation of the hydrate network at the H₂SO₄ aqueous surface. Incorporating our results into atmospheric models will lead to a fuller and more accurate picture of ozone depletion on/in the sulfate aerosols.

What has motivated you to conduct this work?

Although the first investigation of the aqueous air/H₂SO₄ interface by using SFG was conducted in 1997 by two different groups independently, their interpretations of the surface structure remain controversial. Moreover, the ionic composition at the interface is still uncertain from these earlier SFG work exploring the OH stretching region.

Where do you see this work developing in the future?

In the field of the SFG, plenty of room for further development is concerned with extension of the wavelength range. There is a wide range left from about 1400 to about 500 cm^-1 which is essentially unexplored but is of great interest since most of the chemical group frequencies appear in this fingerprint region. We successfully measured the SFG signals derived from the sulfate core signals around 1000 cm^-1. Our aim is to elucidate potential applicability of SFG to the characterization of the surface chemical species, and leaving plenty of room for studies in which SFG will be a key technique by which to study chemistry and physics at interfaces. On the other hand, our findings that the contrasting behavior of the SO and OH SFG signals will promote reconsideration of the hydrogen-bonding structure at H₂SO₄ aqueous the interface.

Are there any particular challenges facing future research in this area?

The result of our study was quite similar to the earlier second harmonic generation study of the air/H₂SO₄ interface (A. J. Fordyce, W. J. Bullock, R. D. Smith, and J. G. Frey, Phys. Chem. Chem. Phys., 2004, 6, 2415.). Our findings concerning the chemical nature of the species present at the air/H₂SO₄ interface are a strong complement to inferences obtained from some earlier SFG work exploring the OH stretching region. Based on these results, we are now performing the molecular dynamics simulation to provide more accurate picture about the interfacial structure of a H₂SO₄ aqueous solution. This experiments and the theoretical calculation will promise to determine the accurate surface structure of the sulfate aerosols.