Atmospheric field measurements and models of the stratospheric sulfate aerosol layer led to the suggestion that sulfuric acid (H2SO4) must photolyze at high altitudes. We propose that excitation of vibrational overtones of H2SO4 and its hydrate in the near-infrared and visible leads to photolysis, forming sulfur trioxide (SO3) and water. On the basis of absorption cross sections calculated with ab initio methods calibrated to experimental measurements, we estimated J values that are sufficient to explain stratospheric and mesospheric sulfur dioxide (SO2) concentrations and the observation of the sulfate layer.

We have measured and analyzed the infrared (IR), near-infrared (NIR) and vacuum ultraviolet (VUV) absorption spectra of vapor-phase sulfuric acid (H 2 SO 4 ). Transitions associated with the fundamental vibrations and the first and second OH stretching overtones of this molecule have been identified. Our measured vibrational spectrum extends and complements those in the literature and agrees well with ab initio calculated spectra. We have calculated the fundamental and overtone OH stretching intensities with the use of a simple anharmonic oscillator local-mode model with ab initio calculated dipole-moment functions. Theory and experiment have been used to investigate the OH stretching vibrational overtones of H 2 SO 4 and indicate that the OH stretching mode in H 2 SO 4 is an important aspect of the spectroscopy of this atmospheric chromophore. We have attempted to measure the VUV spectrum of vapor-phase sulfuric acid and, in the absence of observed bands, give upper bounds to the photoabsorption cross section. We conclude that H 2 SO 4 absorbs in the IR and NIR regions, with the OH stretching vibration playing the dominant role, and that the electronic excitation of H 2 SO 4 will only be significant at very high energies, well above those available from the sun in the earth's atmosphere.

We examine organic acids as precursors to aerosol formation. We do this by examining the hydration of formic acid with up to three water molecules. This approach starts at the molecular level, providing insight to the first steps of the processes that result in nucleation. Our methodology involves high-level molecular calculations. We present structures and energetics for these species. Using these methods, we predict a cooperative bonding effect that may be present in other aerosol precursors. This methodology also may provide some insight into the hygroscopic growth of these particles. We discuss some possible atmospheric implications of this work.

[1] We present the first microphysical calculations confirming that photolysis of sulfuric acid vapor by visible light is responsible for the formation of the springtime ''CN layer'' observed in the polar stratosphere. Our calculations show that the recently proposed photolysis mechanism is also sufficient to explain observations of vertically increasing SO 2 mixing ratios in the upper stratosphere. Such photolysis, however, does not sufficiently explain the limited observations above 40 km of vertically decreasing H 2 SO 4 and SO 3 vapor, suggesting an additional loss mechanism there. We rule out previous speculation regarding reaction of H 2 SO 4 with O( 1 D) or OH as inconsistent with observations of SO 2 . Rather, the loss is consistent with a permanent sink for sulfur, such as H 2 SO 4 neutralization by metals on meteoritic dust. In light of such removal, H 2 SO 4 photolysis to SO 2 gains added importance in preserving gaseous sulfur in the upper stratosphere. We also present new cross sections derived from ab initio calculations for H 2 SO 4 absorption at visible and Lyman-a wavelengths and evaluate their atmospheric implications. Our atmospheric model reveals that photolysis of H 2 SO 4 by Lyman-a radiation is responsible for up to one third of the SO 2 in the mesosphere and 10% of the particles in the polar stratospheric CN layer.