Abstract
An overall number of 447 spectral series of aerosol optical depth were determined in the 0.4-3.7 mm wavelength range by examining the IR-RAD sun-radiometer measurements carried out at Sagres (Portugal) on six clear-sky days, during the CLEARCOLUMN (ACE-2) experiment in June and July 1997. These spectral series were then analysed with the King inversion method to defi ne the size-distribution curves of columnar aerosol particle total number and volume, assuming values of both real and imaginary parts of the particulate refractive index obtained on the six days by combining our measurements with simultaneous sky-brightness measurements taken by the Leipzig University group. For these results, we then calculated the daily time-patterns of the average single scattering albedo of the columnar aerosols, fi nding instantaneous values ranging between 0.70 and 0.96 on those days, with daily mean values varying from 0.83 to 0.95. Furthermore, for each spectral series of aerosol optical depth, we determined the instantaneous change DF^ induced by the columnar aerosols on the upwelling solar radiation fl ux leaving the atmosphere, over oceanic areas presenting low surface albedo. The 24-h average values of DF^ obtained on the six days were found to increase as a function of the daily mean value of aerosol optical depth at the 0.55 mm wavelength, following relationship curves whose positive slope coeffi cients decrease gradually with the single scattering albedo of the columnar aerosols. The said curves can be used to achieve reliable estimates of change DF^ directly from daily ground-based multispectral measurements of aerosol optical depth and skybrightness at different angular distances from the Sun.An overall number of 447 spectral series of aerosol optical depth were determined in the 0.4-3.7 mm wavelength range by examining the IR-RAD sun-radiometer measurements carried out at Sagres (Portugal) on six clear-sky days, during the CLEARCOLUMN (ACE-2) experiment in June and July 1997. These spectral series were then analysed with the King inversion method to defi ne the size-distribution curves of columnar aerosol particle total number and volume, assuming values of both real and imaginary parts of the particulate refractive index obtained on the six days by combining our measurements with simultaneous sky-brightness measurements taken by the Leipzig University group. For these results, we then calculated the daily time-patterns of the average single scattering albedo of the columnar aerosols, fi nding instantaneous values ranging between 0.70 and 0.96 on those days, with daily mean values varying from 0.83 to 0.95. Furthermore, for each spectral series of aerosol optical depth, we determined the instantaneous change DF^ induced by the columnar aerosols on the upwelling solar radiation flux leaving the atmosphere, over oceanic areas presenting low surface albedo. The 24-h average values of DF^ obtained on the six days were found to increase as a function of the daily mean value of aerosol optical depth at the 0.55 mm wavelength, following relationship curves whose positive slope coeffi cients decrease gradually with the single scattering albedo of the columnar aerosols. The said curves can be used to achieve reliable estimates of change DF^ directly from daily ground-based multispectral measurements of aerosol optical depth and skybrightness at different angular distances from the Sun.
Highlights
It is well known that airborne aerosols act on the Earth’s climatic system in two ways, directly through scattering and absorption of both solar and terrestrial radiation, and indirectly by Claudio Tomasi, Vito Vitale, Angelo Lupi, Alessandra Cacciari, Stefano Marani and Ubaldo Bonafé aerosol-induced forcing and columnar particulate content parameters
The present analysis provided results which point out that the variations DFo caused by aerosols in the radiation balance of the surface-atmosphere system can be realistically interpreted in terms of the aerosol optical depth and the single scattering albedo of the columnar particle polydispersion, the surface albedo being the third most important parameter to be taken into account in similar studies
The comparison shows that a substantial agreement exists between our evaluations and those of Russell et al (1999b), the present daily average estimates of DFo turn out to be overestimated with respect to those of the TARFOX measurements, by no more than 15% for values of d (550 nm) > 0.15 and by percentages varying between 8% and 62% in the other four cases
Summary
For these reasons, several regional experiments have been carried out over recent years in different areas of the planet to study the direct effects produced by aerosols in scattering and absorbing the incoming solar radiation: among them, the most important in oceanic areas have been ASTEX/MAGE in the Eastern Atlantic (Huebert et al, 1996), ACE-1 in the Southern Pacific (Bates et al, 1998), INDOEX in the Indian Ocean (Jayaraman et al, 1998), TARFOX in the eastern coastal region of U.S.A. This paper briefly presents: i) the procedure followed to determine the AOD spectral series at various times on six clearsky days; ii) the inversion procedure adopted to infer the columnar particle number and the volume distribution curves as a function of radius, determining the radiative properties of the columnar aerosol particles in terms of the complex refractive index of particulate matter, and iii) the calculations of the changes in the upwelling solar radiation flux at the atmospheric top-level produced over an oceanic region by columnar particle polydispersions of marine, continental and mixed origins, as observed on six selected days of the CLEARCOLUMN campaign
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