Implications for water monomer and dimer solar absorption from observations at Boulder, Colorado

We present ground-based spectral measurements of downward direct and diffuse solar intensities between 624 and 686 nm. Using the rising Sun as a light source, it is shown that the water dimer does not absorb significantly in this wavelength region. Over Boulder, Colorado, on July 10, 1998, the upper limit to the dimer vertical optical depth at these wavelengths is approximately 0.001, with an implied upper limit for instantaneous clear-sky absorption of solar radiation of about 0.8 W/m(2) for an overhead Sun. The direct Sun measurements also show that in the observed wavelength region no other unknown structured absorber can account for more than 0.6 W/m(2) absorption of the instantaneous downward direct solar flux for an overhead Sun. Observations of light scattered through a thick cloud reveal that when the atmosphere is saturated with respect to water vapor, no observable dimer absorption occurs and there is no significant spectral shape change in water monomer absorption at our spectral resolution.