lmd_Boucher2002_abstracts.html

2002 .

(5 publications)

E. Cosme, C. Genthon, P. Martinerie, O. Boucher, and M. Pham. The sulfur cycle at high-southern latitudes in the LMD-ZT General Circulation Model. Journal of Geophysical Research (Atmospheres), 107:4690, December 2002. [ bib | DOI | ADS link ]

This modeling study was motivated by the recent publication of year-round records of dimethylsulfide (DMS) and dimethylsulfoxide (DMSO) in Antarctica, completing the available series of sulfate and methanesulfonic acid (MSA). Sulfur chemistry has been incorporated in the Laboratoire de Météorologie Dynamique-Zoom Tracers (LMD-ZT) Atmospheric General Circulation Model (AGCM), with high-resolution and improved physics at high-southern latitudes. The model predicts the concentration of six major sulfur species through emissions, transport, wet and dry deposition, and chemistry in both gas and aqueous phases. Model results are broadly realistic when compared with measurements in air and snow or ice, as well as to results of other modeling studies, at high- and middle-southern latitudes. Atmospheric MSA concentrations are underestimated and DMSO concentrations are overestimated in summer, reflecting the lack of a DMSO heterogeneous sink leading to MSA. Experiments with various recently published estimates of the rate of this sink are reported. Although not corrected in this work, other defects are identified and discussed: DMS concentrations are underestimated in winter, MSA and non-sea-salt (nss) sulfate concentrations may be underestimated at the South Pole, the deposition scheme used in the model may not be adapted to polar regions, and the model does not adequately reproduces interannual variability. Oceanic DMS sources have a major contribution to the variability of sulfur in these regions. The model results suggest that in a large part of central Antarctica ground-level atmospheric DMS concentrations are larger in winter than in summer. At high-southern latitudes, high loads of DMS and DMSO are found and the main chemical sink of sulfur dioxide (SO₂) is aqueous oxidation by ozone (O₃), whereas oxidation by hydrogen peroxide (H₂O₂) dominates at the global scale. A comprehensive modeled sulfur budget of Antarctica is provided.

P. Formenti, O. Boucher, T. Reiner, D. Sprung, M. O. Andreae, M. Wendisch, H. Wex, D. Kindred, M. Tzortziou, A. Vasaras, and C. Zerefos. STAAARTE-MED 1998 summer airborne measurements over the Aegean Sea 2. Aerosol scattering and absorption, and radiative calculations. Journal of Geophysical Research (Atmospheres), 107:4551, November 2002. [ bib | DOI | ADS link ]

Chemical, physical, and optical measurements of aerosol particle properties within an aged biomass-burning plume were performed on board a research aircraft during a profile descent over a ground-based site in northeastern Greece (40deg24'N, 23deg57'E 170 m asl) where continuous measurements of the spectral downwelling solar irradiance (global, direct, and diffuse) are being made. The aerosol optical depth measured at the ground during the time of overflight was significantly enhanced (0.39 at a wavelength of 500 nm) due to a haze layer between 1 and 3.5 km altitude. The dry particle scattering coefficient within the layer was around 80 Mm^-1, and the particle absorption coefficient was around 15 Mm^-1, giving a single scattering albedo of 0.89 at 500 nm (dry state). The black carbon fraction is estimated to account for 6-9% of the total accumulation mode particle mass (1 μm diameter). The increase of the particle scattering coefficient with increasing relative humidity at 500 nm is of the order of 40% for a change in relative humidity from 30 to 80%. The dry, altitude-dependent, particle number size distribution is used as input parameter for radiative transfer calculations of the spectral short-wave, downwelling irradiance at the surface. The agreement between the calculated irradiances and the experimental results from the ground-based radiometer is within 10%, both for the direct and the diffuse components (at 415, 501, and 615 nm). Calculations of the net radiative forcing at the surface and at the top of the atmosphere (TOA) show that due to particle absorption the effect of aerosols is much stronger at the surface than at the TOA. Over sea the net short-wave radiative forcing (daytime average) between 280 nm and 4 μm is up to -64 W m^-2 at the surface and up to -22 W m^-2 at the TOA.

Chemical, physical, and optical measurements of aerosol particle properties within an aged biomass-burning plume were performed on board a research aircraft during a profile descent over a ground-based site in northeastern Greece (40deg24'N, 23deg57'E; 170 m asl) where continuous measurements of the spectral downwelling solar irradiance (global, direct, and diffuse) are being made. The aerosol optical depth measured at the ground during the time of overflight was significantly enhanced (0.39 at a wavelength of 500 nm) due to a haze layer between 1 and 3.5 km altitude. The dry particle scattering coefficient within the layer was around 80 Mm^-1, and the particle absorption coefficient was around 15 Mm^-1, giving a single scattering albedo of 0.89 at 500 nm (dry state). The black carbon fraction is estimated to account for 6-9% of the total accumulation mode particle mass (1 μm diameter). The increase of the particle scattering coefficient with increasing relative humidity at 500 nm is of the order of 40% for a change in relative humidity from 30 to 80%. The dry, altitude-dependent, particle number size distribution is used as input parameter for radiative transfer calculations of the spectral short-wave, downwelling irradiance at the surface. The agreement between the calculated irradiances and the experimental results from the ground-based radiometer is within 10%, both for the direct and the diffuse components (at 415, 501, and 615 nm). Calculations of the net radiative forcing at the surface and at the top of the atmosphere (TOA) show that due to particle absorption the effect of aerosols is much stronger at the surface than at the TOA. Over sea the net short-wave radiative forcing (daytime average) between 280 nm and 4 μm is up to -64 W m^-2 at the surface and up to -22 W m^-2 at the TOA.

Y. J. Kaufman, D. Tanré, and O. Boucher. A satellite view of aerosols in the climate system. Nature, 419:215-223, September 2002. [ bib | ADS link ]

Anthropogenic aerosols are intricately linked to the climate system and to the hydrologic cycle. The net effect of aerosols is to cool the climate system by reflecting sunlight. Depending on their composition, aerosols can also absorb sunlight in the atmosphere, further cooling the surface but warming the atmosphere in the process. These effects of aerosols on the temperature profile, along with the role of aerosols as cloud condensation nuclei, impact the hydrologic cycle, through changes in cloud cover, cloud properties and precipitation. Unravelling these feedbacks is particularly difficult because aerosols take a multitude of shapes and forms, ranging from desert dust to urban pollution, and because aerosol concentrations vary strongly over time and space. To accurately study aerosol distribution and composition therefore requires continuous observations from satellites, networks of ground-based instruments and dedicated field experiments. Increases in aerosol concentration and changes in their composition, driven by industrialization and an expanding population, may adversely affect the Earth's climate and water supply.

O. Boucher and M. Pham. History of sulfate aerosol radiative forcings. Geophysical Research Letters, 29:1308, May 2002. [ bib | DOI | ADS link ]

The history of the global sulfur cycle has been simulated using an emission inventory of SO₂ for 1990 and previously published historical trends in emission on a per country basis. The global- annual-mean radiative forcings due to sulfate aerosols increase (in absolute values) from near-zero and -0.17 Wm^-2 up to -0.4 and -1 Wm^-2 between 1850 and 1990, for the direct and indirect effects, respectively. The forcing efficiency (defined as the ratio of the radiative forcing to the anthropogenic sulfate burden) is fairly constant for the direct effect at -150 W(g sulfate)^-1 but decreases significantly for the indirect effect with increasing sulfate burden. The model results are compared with long-term observations for the period 1980 to 1998 in the U.S. and Europe.