HR: 1340h
AN: A33B-0897    [Abstracts]
TI: Boron Isotopic Composition of Atmospheric Precipitations and Liquid-Vapor Fractionations
AU: Rose-Koga, E F
EM: e.koga@opgc.univ-bpclermont.fr
AF: LMV, CNRS UMR 6524, Clermont-Ferrand, 63038 France
AU: Sheppard, S M
EM: s.sheppard@ens-lyon.fr
AF: ENSL, CNRS UMR 5570, Lyon, 69364 France
AU: Chaussidon, M
EM: chocho@crpg.cnrs-nancy.fr
AF: CRPG, CNRS, Vandoeuvre, 54501 France
AU: * Carignan, J
EM: carignan@crpg.cnrs-nancy.fr
AF: CRPG, CNRS, Vandoeuvre, 54501 France
AB: B-isotope compositions (δ¹¹B), measured by ion microprobe with a precision of ±0.5‰ or better, and B concentrations of rains and snows were studied to characterize the sources and fractionation processes taking place during the B atmospheric cycle. The B-isotopic compositions of rains and snows from coastal and continental sites show a large range of variations, from -1.5 to +26.0‰ and from -10.2 to +34.4‰, respectively. B concentrations vary between 0.1 and 3.0 ppb, all samples combined. Laboratory seawater (sw) evaporation experiments were performed at 11, 20 and 30°C and pHs of 6, 8.5 and 9.5 to measure the sw-vapor B isotopic fractionation. The analyzed vapor has two sources: (1) the evaporation of the main sw reservoir, and (2) the evaporation to dryness of sw droplets or aerosol. Water-vapor fractionations are ~+28‰ for sw, and ~+28 to ~+48‰ for sw aerosol, with the fractionation increasing with increase in the % of evaporation, reflecting concomitant evolution of speciation and solute interaction effects in the aerosol. The volatility of B in sw at 20-30°C increases with decrease in pH, reflecting the increasing concentration of B as the B(OH)₃ species. The vapor is always depleted in ¹¹B relative to the liquid at surface and atmospheric temperature. An empirical rain water-vapor B isotopic fractionation, derived from the relationship δD = 2.4 δ¹¹B - 108 followed by selected precipitations, gave 33‰. The large variations in the ¹¹B /¹⁰B ratio of atmospheric B, either as vapor or as precipitation, are related to the sw B reservoir via a combination of processes: (1) the sw-vapor fractionation (Δ_{sw-vap~+28‰), (2) the rain-vapor fractionation (Δ_rain-vap ~+33‰), (3) evolution of the δ¹¹B value of the atmospheric vapour reservoir via evaporation-condensation-precipitation processes that can be modelled as a Rayleigh distillation process, (4) any contribution of vapor from the evaporation of sw aerosols, that can have quite variable δ¹¹B values, and (5) any contribution from particulate matter (principally sea-salt and/or continental dust). Processes (1) and (2) are basic to all precipitations, and (3) to (5) either singly or in combination may be significant. These results can account for the large range of δ¹¹B of world-wide precipitations from +45 to -13‰, with sw at +39.5‰.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0312 Air/sea constituent fluxes (3339, 4504)
DE: 0322 Constituent sources and sinks
DE: 0330 Geochemical cycles (1030)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005