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Ottavio Vittori

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Ottavio A. Vittori and Vittorio Prodi

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Ice crystals growing in supercooled clouds were studied in a Nakaya chamber to determine their ability to include foreign particles in their structure. In a series of experiments on wash-out ratios of condensing water and growing ice, ice crystals proved to be 16 times more efficient than water in capturing atmospheric particles.

The existence of Facy's “dust-free space” around evaporating surfaces was confirmed. The dust-free space around supercooled droplets is considered, in connection with Stefan flow, to be an important mechanism in bringing particles toward ice surfaces, and may account for the observed wash-out efficiency of ice crystals.

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Ottavio Vittori, Claudio Tomasi, and Rodolfo Guzzi

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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 (λ), 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 (λ) 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.

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Rodolfo Guzzi, Claudio Tomasi, and Ottavio Vittori

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The optical thickness a(λ) of atmospheric particulate matter has been investigated in a near infrared wavelength interval. The instrument (a monochromator with a Newtonian telescope) was located at 1100 m MSL on the Appennine chain.

We found an increase of a(λ) by comparison with the Bouger-Lambert law with increasing solar elevation. The distribution of a(λ) as a function of wavelength, computed at different times, shows that particulate matter either forms locally in the morning or arrives from over the Po Valley.

According to the theory of Deirmendjian condensation of water also occurs on the atmospheric particles.

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Rodolfo Guzzi, Ottavio Vittori, and Claudio Tomasi

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Results of measurements of the monochromatic optical thickness (in the 0.65–13.5 µm wavelength region) of some thin clouds are reported. The observed wavelength patterns are discussed in terms of the optical properties of cloud particles (large ice crystals and water droplets).

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Claudio Tomasi, Rodolfo Guzzi, and Ottavio Vittori

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The e-type effect on the water vapor continuum, suggested by Bignell's laboratory experiments, is investigated in the atmospheric extinction spectrum of the sun between 8 and 13 µm.

A classification of atmospheres in terms of precipitable water w, surface air temperature T 0, water vapor pressure e 0, as well as horizontal visual range V 0, proved to be suitable in distinguishing the e-type effect. A procedure for determining the two terms arising from wing and e-effects on the water vapor continuum absorption coefficient, gives results in satisfactory agreement with laboratory experiments.

The observed water vapor continuum absorption in the 8–13 µm window is also compared with the extinction due to particles present in the local atmosphere.

The data obtained from the measurements allowed us to calculate also the weak absorption coefficients for water vapor in the 0.65–4 µm wavelength interval. They are reported in the Appendix.

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Claudio Tomasi, Rodolfo Guzzi, and Ottavio Vittori

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The transformation of airborne SO2 into SO4 taking place in a polluted urban atmosphere has been investigated in terms of the equilibrium of the system composed of SO2, liquid water and NH3.

The procedure used for selecting, compiling and assembling the field data set is described. Ten classes of atmospheric liquid water content were set up on the basis of average spectra of airborne particulate matter and local visibilities; 174 suitable selected days were available for investigation.

The behavior of the above–mentioned system resulting from our computations fits satisfactorily the Junge and Ryan laboratory model. The observed seasonal trend in the yield of the system in transforming atmospheric SO2 can be explained in terms of a seasonal change in both the catalytic efficiency of the airborne material (different sources of pollutants) and the efficiency Of the scavenging mechanisms which include airborne particles in the droplets.

The pH of the liquid water droplets computed on the basis of the SO2 concentration agrees fairly well with the pH observed in actual urban hazes and fogs.

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