Aérosols et climat

International audienc

I.Theory of the relaxation by collision of molecular multipole moments : impact approximation with long range electrostatic interactions

A theoretical treatment of the relaxation and transfer of molecular multipole moments due to intermolecular collisions is presented assuming the impact approximation. General expressions for the collision cross section are obtained for any multipole-multipole interaction, to first and second order of perturbation assuming a long range electrostatic potential. The generalization to short range potential is discussed. Effects of fine and hyperfine structure in Hund's cases a and b are sketched.Un traitement théorique de la relaxation et du transfert de moments multipolaires de molécules lors des collisions intermoléculaires est présenté, basé sur l'approximation d'impact. Les expressions générales des sections efficaces de collision relatives aux différentes interactions multipole-multipole sont obtenues au premier et second ordre des perturbations en considérant un potentiel électrostatique à longue distance. La generalisation à un potentiel à courte distance est discutée. Les effets des structures fines et hyperfines sont traités dans les cas de couplage de Hund a et b

Radionuclides in a cave sediment core from Ghetarul de sub Zgurasti (Romania)

Lake sediments had been cored in a typical karst region cave - Ghetarul de sub Zgurasti (Romania) - at a depth of 13 m. The site lies 250 m below ground and at 400 m from the cave 's entrance. The sediments proved to be contamined with ¹³⁷Cs and ¹³⁴Cs - artificial radionuclides originating from the nuclear tests in the early sixties and from the Chernobyl accident in the middle eighties. These radionuclides were first deposited at earth surface and have subsequently been transferred to the cave via seepage water, in a very effective manner, the chronology of the signal being not disturbed by the sedimentation process. The deposition flux of ²¹⁰Pb (a natural radionuclide originating from the atmospheric decay of Radon) reveals a supplementary production of this radionuclide inside the cave. The analysis of cross section in terms of sedimentation processes, provides coherent results, allowing estimate of sedimentation rates, diffusion coefficients and the history of the sediment. This history is in agreement with the climat this specific region had shared in the last decades. The soils cover of the limestone and the sediment core show no signs of marked anthropogenic metal contamination