Water vapor dimers and atmospheric absorption of electromagnetic radiation

Using a simple model based on a quantum mechanical harmonic oscillator we estimate the average dimer absorption cross sections within six 3500 cm(-1) wide bands. The cross section is of the order of 6x10(-21) cm(2) in the first frequency band (1000 to 4500 cm(-1)) and it decreases by a factor of about 600 with increasing frequency from far infrared to visible portion of the spectrum. This relatively slow decrease is a result of anharmonic forces within a dimer, that weaken the selection rules of water monomer. Using numerical results of previous studies, we parameterize the concentration of dimers in the atmosphere and provide the formalism to calculate the absorption of solar radiation by water dimers as a function of total water vapor column amount and temperature. For water vapor column of 1 g/cm(2) and 80% relative humidity at 273 K, the dimers will absorb about 1.6% of the incident solar radiation. The absorbency increases with increasing temperature to about 4.6% at 298 K. Our estimates are accurate within a factor of about four. Most of the dimer absorption occurs at infrared wavelengths. Only about 5% of solar radiation absorbed by dimers is at wavelengths below 0.9 mu m.