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Claudio Tomasi

Abstract

Estimates of the particulate matter optical thickness at 0.55 μm wavelength are obtained from measurements of attenuation of solar radiation through hazy atmospheres at three stations in the Po Valley. Simultaneous observations of visibility at ground level are used to obtain careful evaluations of the volume extinction coefficient produced by the particulate matter at 0.55 μm wavelength. The ratio between the optical thickness and the volume extinction coefficient is taken to measure the scale height H for particulate extinction. Values of H between 0.5 and 1.3 km are found in cold atmospheres characterized by temperature inversions whereas values of more than 2 km are found on summer days in the presence of strong convective motions due to intense ground heating by the sun. These marked variations of height H with season are closely related to the evolving features of the boundary layer. The daily time-patterns of H increase throughout the morning in an approximately linear fashion with average rates ranging between 30 and 180 m h−1 and show almost constant values in the afternoon.

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Claudio Tomasi

Abstract

In the relationship proposed by Reitan (1963) between the total precipitable water and the dew-point temperature at ground level, the intercept is found to be a function of the scale height of the atmospheric water vapor and of the ground temperature. The scale height is closely related to the features of the vertical moisture profile in the low troposphere. The ground temperature is responsible for diurnal and seasonal variations of the intercept which are found to be of little importance. The slope coefficient is found to have a constant value of 0.068°C −1.

On this basis, a method is proposed which determines the nocturnal time pattern of the intercept, by linear interpolation in time between the daytime estimates obtained from spectral hygrometer measurements. Thus, the nocturnal time pattern of the total precipitable water can be calculated from Reitan's relationship using these interpolated values of the intercept together with the nocturnal measurements of the dew-point temperature at ground level. Satisfactory results are found by applying this method to spectral hygrometer measurements made on several days.

Estimates of the total precipitable water at two stations of the Po Valley also have been obtained from Reitan's relationship by using local measurements of the dew-point temperature at ground level and suitable values of the intercept. These are determined by linear interpolation in time of the estimates of the intercept given by the radiosonde data taken from a remote station of the Po Valley. The comparison between calculated and measured values of the total precipitable water gives a satisfactory agreement for both autumn and winter periods, whereas considerable discrepancies are found on various summer days.

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Claudio Tomasi

Abstract

The total atmospheric precipitable water W has been measured with an infrared hygrometer for different atmospheric conditions and various seasons. Related to the simultaneous dew-point temperature Td at the surface, these data indicate that correct estimates of W can be given by the relationship lnW = a + b Td [first proposed by Reitan (1963)] only if properly chosen values for the intercept parameter a are used corresponding to the various periods of each day.

The examination of both spectral hygrometer and radiosonde data taken in atmospheres characterized by marked temperature inversions shows that the parameter a is related closely to the ratio β between the surface absolute humidity and W. On the other hand, this ratio appears to depend substantially on the general form of the vertical moisture profile of the low atmosphere. Thus empirical relationships are proposed which give the possibility of inferring from empirical values of β estimates of the partial precipitable water content within low atmospheric layers and of the absolute humidity at some standard levels, when radiosonde data are not available.

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Claudio Tomasi
,
Ezio Caroli
, and
Vito Vitale

Abstract

The volume extinction coefficient produced by aerosol particles at wavelength λ is generally proportional to λ−α as found by Ånström. On the other hand, particle size spectra frequently give a particle number density approximately proportional to the inverse of the radius power with exponent ν, as found by Junge. On the basis of computations made using a very accurate Mie extinction algorithm, a study of the relationship curve between exponents α and ν is made for aerosol particle polydispersions of different radius intervals and for different refractive index values. Moreover, a set of relationship curves is obtained for realistic particulate extinction models based on different size distribution curves and refractive index models. The results show that the linear relationship α = ν − 2 is not valid in most cases. In particular, exponent α assumes appreciably lower values than ν − 2 in the range ν > 3 for all the particulate extinction models.

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Rodolfo Guzzi
,
Ottavio Vittori
, and
Claudio Tomasi

Abstract

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

Abstract

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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Ottavio Vittori
,
Claudio Tomasi
, and
Rodolfo Guzzi

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 (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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Claudio Tomasi
,
Rodolfo Guzzi
, and
Ottavio Vittori

Abstract

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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Rodolfo Guzzi
,
Claudio Tomasi
, and
Ottavio Vittori

Abstract

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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Claudio Tomasi
,
Boyan Petkov
,
Elena Benedetti
,
Luca Valenziano
,
Angelo Lupi
,
Vito Vitale
, and
Ubaldo Bonafé

Abstract

Two-channel sun photometers can be easily employed at Antarctic sites, where harsh environmental conditions prevail, to carry out measurements of precipitable water W. In the very dry air conditions observed in the Antarctic atmosphere, water vapor does not produce strong absorption features along the sun path. Therefore, these instruments need to be calibrated using analytical forms different from the square root regime, which can be determined by simulating the output voltages measured at Antarctic sites, for the spectral near-IR curves of extraterrestrial solar irradiance, instrumental responsivity parameters, and atmospheric transmittance, relative to various measurement periods. For this purpose, average models of the Antarctic atmosphere from the ground level up to the 30-km altitude were considered for different solar zenith angles and relative humidity conditions. The ratios between the output voltages simulated in the band and window channels were plotted as a function of total water vapor content Cw , for each site and each period, to define the best-fit calibration curves, which were subsequently normalized to the field measurements to take into account the aging effects on the filter transmission characteristics. Each of the five calibration curves was found to present a slope coefficient decreasing gradually with Cw from values higher than 0.8 to about 0.6. Using these curves, measurements of W were obtained, which differ appreciably at both sea level and high-altitude sites from those given by the square root calibration curves, avoiding large overestimation errors of 10%–40% at the high-altitude sites and underestimation errors of 5%–15% at the sea level site.

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