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

We present ground-based spectral measurements of downward direct and diffus e 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 approxim ately 0.001, with an implied upper limit for instantaneous clear-sky absorp tion 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) absorptio n of the instantaneous downward direct solar flux for an overhead Sun. Obse rvations of light scattered through a thick cloud reveal that when the atmo sphere is saturated with respect to water vapor, no observable dimer absorp tion occurs and there is no significant spectral shape change in water mono mer absorption at our spectral resolution.