Dessens' Droplets in the Near and Middle Infrared Spectrum of the Sun

Abstract

Measurements of the monochromatic atmospheric extinction of solar radiation (in the 0.65?13.5 µm wavelength interval) in the Po Valley and neighboring mountainous areas show the occurrence during the morning (after the night inversion) of an evolution both in concentration and size distribution of airborne particulate matter. The time pattern of a (?), the monochromatic particle optical thickness, interpreted on the basis of a linear combination of Deirmendjian haze models, indicates that atmospheric particles turn into water droplets during the morning, probably near the top of the convective boundary layer where the relative humidity increases. The results are discussed in relation to other experimental and theoretical work and in terms of local meteorological evolution. They are consistent with the growth in the local atmosphere of condensation nuclei active below water saturation. The attenuation of solar radiation occurring in the 8?13 µm continuum is also discussed in terms of observed time patterns of a (?) in this spectral region. Growing water droplets appear to contribute to a lesser extent to the extinction occurring in this wavelength interval. In a polluted atmosphere, however, the contribution of Dessens droplets may be significant in extinguishing radiation in the 8?13 µm window also.