2Cp01S

Atmospheric applications ofvibrational local modes

Henrik G. Kjaergaard, Department of Chemistry,
University of Otago, Dunedin, New Zealand

We have calculated fundamental and overtone OH-stretching vibrational band
positions and intensities of molecules and mono-hydrated complexes ofatmospheric interest. The spectral intensities were determined with aharmonically coupled anharmonic oscillator local mode model and ab initio
calculated dipole moment functions. The basic principles of the local mode
model will be outlines and the effect of various ab initio methods on these typesof calculations discussed. Atmospheric implications of our results arehighlighted.
The water dimer is a weak van der Waals complex consisting of a hydrogen
doner and a hydrogen acceptor unit.

Our calculations predict that the frequencies of the hydrogen-bonded OHstretching transitions are red-shifted compared to those of the monomers andthat the shifts increase with increasing overtone. For the fundamental region, wesee a significant increase in intensity for the hydrogen-bonded transitions
compared to the monomer transitions. For the first overtone, the hydrogenbonded transitions are weaker than the monomer transitions, and for the higher
overtones the intensities are comparable. Our calculated water dimer spectrum
compares well with recent matrix isolation spectra in the fundamental and firstovertone region. The OHb bond of the hydrogen donor unit is the bond mostly
affected by the complexation. We show that transitions to higher overtones in
the OHb-stretching vibration are well seperated from water monomer transitions.

Resent atmospheric observation of one of these OHb-stretching transitions has
provided the first atmospheric detection of the water dimer. Our calculated
spectra of water dimer and hydrated complexes of oxygen, nitrogen, and argon
combined with estimates of their atmospheric abundance have been used toaccess the impact these complexes have on absorption of solar radiation.
Sulfuric acid is one of the major sulfur species in our upper atmosphere. The
concentration and distribution of sulfur compounds in the stratosphere have beenof increasing interest due to the important role they play in the formation ofatmospheric aerosols. Atmospheric field measurements and models of thestratospheric sulfate aerosol layer led to the suggestion that sulfuric acid must
photolyze at high altitudes.
High level ab initio calculations found that the lowest lying electronic transitionin sulfuric acid occurs at wavelengths of about 144 nm. However, very few
photons with this much energy are available in our atmosphere and thus
photolysis via electronic transitions is not very likely.
We propose that excitation of vibrational overtones of sulfuric acid and its
hydrate leads to photolysis forming sulfur trioxide and water in the near infraredand visible.

The calculated spectra of fundamental and overtone OH-stretching transitions insulfuric acid are in very good agreement with observed OH-stretching transitionin vapor phase sulfuric acid spectra. Based on our calculated spectra, themeasured stratospheric and mesospheric sulfur dioxide concentrations and theobservation of the sulfate layer can be explained.
These findings will allow scientists to improve their models of the Earth’s
atmosphere and will no doubt change our understanding of the atmosphere.
It is likely to affect climate modelling and our planet’s energy budget throughradiative and chemical effects.