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2007 .

(10 publications)

A. Jones, J. M. Haywood, and O. Boucher. Aerosol forcing, climate response and climate sensitivity in the Hadley Centre climate model. Journal of Geophysical Research (Atmospheres), 112:20211, October 2007. [ bib | DOI | ADS link ]

An atmosphere/mixed-layer-ocean climate model is used to investigate the climate responses to forcing by 1860-2000 changes in anthropogenic sulfate, biomass-burning and black carbon aerosols, and how they compare with the effect of doubling CO2. While the patterns of temperature response from sulfate and black carbon aerosols are similar and reveal high sensitivity at high latitudes in the northern hemisphere, they are significantly different to that due to CO2 which shows high latitude sensitivity in both hemispheres, and to biomass-burning aerosols which shows a much more uniform temperature response. Climate sensitivity, the response of natural primary aerosols, and the degree to which forcings and responses are additive, are also investigated. The sum of the separate temperature and precipitation responses to each aerosol is found to be remarkably similar to that obtained if all aerosols are changed simultaneously.

P. Stier, J. H. Seinfeld, S. Kinne, and O. Boucher. Aerosol absorption and radiative forcing. Atmospheric Chemistry & Physics, 7:5237-5261, October 2007. [ bib | ADS link ]

We present a comprehensive examination of aerosol absorption with a focus on evaluating the sensitivity of the global distribution of aerosol absorption to key uncertainties in the process representation. For this purpose we extended the comprehensive aerosol-climate model ECHAM5-HAM by effective medium approximations for the calculation of aerosol effective refractive indices, updated black carbon refractive indices, new cloud radiative properties considering the effect of aerosol inclusions, as well as by modules for the calculation of long-wave aerosol radiative properties and instantaneous aerosol forcing. The evaluation of the simulated aerosol absorption optical depth with the AERONET sun-photometer network shows a good agreement in the large scale global patterns. On a regional basis it becomes evident that the update of the BC refractive indices to Bond and Bergstrom (2006) significantly improves the previous underestimation of the aerosol absorption optical depth. In the global annual-mean, absorption acts to reduce the short-wave anthropogenic aerosol top-of-atmosphere (TOA) radiative forcing clear-sky from -0.79 to -0.53 W m-2 (33%) and all-sky from -0.47 to -0.13 W m-2 (72%). Our results confirm that basic assumptions about the BC refractive index play a key role for aerosol absorption and radiative forcing. The effect of the usage of more accurate effective medium approximations is comparably small. We demonstrate that the diversity in the AeroCom land-surface albedo fields contributes to the uncertainty in the simulated anthropogenic aerosol radiative forcings: the usage of an upper versus lower bound of the AeroCom land albedos introduces a global annual-mean TOA forcing range of 0.19 W m-2 (36%) clear-sky and of 0.12 W m-2 (92%) all-sky. The consideration of black carbon inclusions on cloud radiative properties results in a small global annual-mean all-sky absorption of 0.05 W m-2 and a positive TOA forcing perturbation of 0.02 W m-2. The long-wave aerosol radiative effects are small for anthropogenic aerosols but become of relevance for the larger natural dust and sea-salt aerosols.

R. A. Betts, O. Boucher, M. Collins, P. M. Cox, P. D. Falloon, N. Gedney, D. L. Hemming, C. Huntingford, C. D. Jones, D. M. H. Sexton, and M. J. Webb. Projected increase in continental runoff due to plant responses to increasing carbon dioxide. Nature, 448:1037-1041, August 2007. [ bib | DOI | ADS link ]

In addition to influencing climatic conditions directly through radiative forcing, increasing carbon dioxide concentration influences the climate system through its effects on plant physiology. Plant stomata generally open less widely under increased carbon dioxide concentration, which reduces transpiration and thus leaves more water at the land surface. This driver of change in the climate system, which we term `physiological forcing', has been detected in observational records of increasing average continental runoff over the twentieth century. Here we use an ensemble of experiments with a global climate model that includes a vegetation component to assess the contribution of physiological forcing to future changes in continental runoff, in the context of uncertainties in future precipitation. We find that the physiological effect of doubled carbon dioxide concentrations on plant transpiration increases simulated global mean runoff by 6 per cent relative to pre-industrial levels; an increase that is comparable to that simulated in response to radiatively forced climate change (11+/-6 per cent). Assessments of the effect of increasing carbon dioxide concentrations on the hydrological cycle that only consider radiative forcing will therefore tend to underestimate future increases in runoff and overestimate decreases. This suggests that freshwater resources may be less limited than previously assumed under scenarios of future global warming, although there is still an increased risk of drought. Moreover, our results highlight that the practice of assessing the climate-forcing potential of all greenhouse gases in terms of their radiative forcing potential relative to carbon dioxide does not accurately reflect the relative effects of different greenhouse gases on freshwater resources.

C. Textor, M. Schulz, S. Guibert, S. Kinne, Y. Balkanski, S. Bauer, T. Berntsen, T. Berglen, O. Boucher, M. Chin, F. Dentener, T. Diehl, J. Feichter, D. Fillmore, P. Ginoux, S. Gong, A. Grini, J. Hendricks, L. Horowitz, P. Huang, I. S. A. Isaksen, T. Iversen, S. Kloster, D. Koch, A. Kirkevâg, J. E. Kristjansson, M. Krol, A. Lauer, J. F. Lamarque, X. Liu, V. Montanaro, G. Myhre, J. E. Penner, G. Pitari, M. S. Reddy, Ø. Seland, P. Stier, T. Takemura, and X. Tie. The effect of harmonized emissions on aerosol properties in global models an AeroCom experiment. Atmospheric Chemistry & Physics, 7:4489-4501, August 2007. [ bib | ADS link ]

The effects of unified aerosol sources on global aerosol fields simulated by different models are examined in this paper. We compare results from two AeroCom experiments, one with different (ExpA) and one with unified emissions, injection heights, and particle sizes at the source (ExpB). Surprisingly, harmonization of aerosol sources has only a small impact on the simulated inter-model diversity of the global aerosol burden, and consequently global optical properties, as the results are largely controlled by model-specific transport, removal, chemistry (leading to the formation of secondary aerosols) and parameterizations of aerosol microphysics (e.g., the split between deposition pathways) and to a lesser extent by the spatial and temporal distributions of the (precursor) emissions. The burdens of black carbon and especially sea salt become more coherent in ExpB only, because the large ExpA diversities for these two species were caused by a few outliers. The experiment also showed that despite prescribing emission fluxes and size distributions, ambiguities in the implementation in individual models can lead to substantial differences. These results indicate the need for a better understanding of aerosol life cycles at process level (including spatial dispersal and interaction with meteorological parameters) in order to obtain more reliable results from global aerosol simulations. This is particularly important as such model results are used to assess the consequences of specific air pollution abatement strategies.

N. Huneeus and O. Boucher. One-dimensional variational retrieval of aerosol extinction coefficient from synthetic LIDAR and radiometric measurements. Journal of Geophysical Research (Atmospheres), 112:14303, July 2007. [ bib | DOI | ADS link ]

The Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP) [onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) platform] and the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument (onboard the AQUA platform) will provide simultaneous measurements as part of the ”AQUA-train,” thus offering a unique opportunity to improve our knowledge on aerosol properties and their spatial distribution. Here we investigate to which extent both the vertical distribution of the aerosol extinction coefficient and the aerosol bimodal size distribution can be retrieved from a synergetic use of the vertically-resolved lidar signal and the spectral radiance measurements. To this effect, a variational retrieval scheme based on a simplified radiative transfer model was developed. The extinction-coefficient profile for fine and coarse-mode aerosols was retrieved from synthetic observations of the profile of the attenuated backscatter lidar signal at two wavelengths and radiances at six wavelengths. Our method aims at minimizing a cost function which measures the departure of the solution to the observations. The adjoint method was applied to find the gradient of the cost function with respect to the input parameters. The retrieval scheme was tested under a realistic noise level and different microphysical perturbations. The retrieval of extinction-coefficient profiles, for fine and coarse particles, is successful if there is a predominance of fine particles. If coarse particles dominate over fine ones, the scheme retrieves the profile of the total extinction coefficient with a higher confidence than that of the fine mode. When perturbations on the aerosol microphysical properties are introduced, thus simulating a more challenging case with incomplete information of the aerosol model present in the atmosphere, the scheme shows a very good performance in terms of total extinction-coefficient retrieval but less success for individual modes. It retrieves the modal radii for both modes simultaneously but can not retrieve at the same time the refractive index and true-mode radii for both modes. Results also reveal that there is some prospect for improvement in the quality of the retrieval by either increasing the size of the predefined set of aerosol models or by including other sources of independent information such as Polarization and Directionality of Earth Reflectances (POLDER)-like measurements.

S. Verma, C. Venkataraman, O. Boucher, and S. Ramachandran. Source evaluation of aerosols measured during the Indian Ocean Experiment using combined chemical transport and back trajectory modeling. Journal of Geophysical Research (Atmospheres), 112:11210, June 2007. [ bib | DOI | ADS link ]

This work presents an analysis of aerosol measurements made during the Oceanographic Research Vessel Sagar Kanya cruise of January-March 1999, in the Indian Ocean Experiment intensive field phase (INDOEX-IFP), with regard to the aerosol chemical constituents and identification of source regions of their origin. This is done through a hybrid approach which uses an Eulerian forward transport calculation in a general circulation model (GCM) with region-tagged emissions along with an analysis of Lagrangian back trajectories and emission inventory information, for overlapping time periods. Back trajectory analysis showed that the ship was mainly influenced by air masses from the Indo-Gangetic plain, central India, or south India during the early part of its cruise with the GCM-predicted aerosol species composed of mainly sulfate and organic matter, whereas dust species dominated during its cruise in late February and early March over the Arabian Sea when the ship was influenced by air masses from Africa-west Asia or northwest India. However, a typical clean marine aerosol dominated by sea salt was encountered during February when the ship cruised in the tropical Indian Ocean and was mostly influenced by marine air masses. The high aerosol optical depth was due to roughly equal parts of organic matter and sulfate. Region-tagged GCM estimates showed the presence of distinct transport at surface and higher layers for, e.g., DOY 56-61 and 65-70, indicating strong signals of emissions of black carbon, organic matter, and sulfate originating in central and northwest India, whereas elevated transport channels of black carbon and organic matter from Africa-west Asia. This is consistent with the back trajectory analysis and in corroboration with INDOEX measurement studies which observed different aerosol properties from aircraft and ship attributed to different transport pathways in surface and elevated flows. However, back trajectory analysis is not sufficient to evaluate the major source regions contributing to the transported aerosol. The fractional contribution of a source region also depended upon the emission flux from the region and its proximity to the receptor domain.

M. S. Reddy and O. Boucher. Climate impact of black carbon emitted from energy consumption in the world's regions. Geophysical Research Letters, 34:11802, June 2007. [ bib | DOI | ADS link ]

We have used the Laboratoire de Météorologie Dynamique General Circulation Model (LMD GCM) to estimate the contribution of different regions to global black carbon (BC) atmospheric burden and direct radiative forcing (DRF). On the global scale, fossil fuels and biofuels account for 66% and 34% of energy-related BC emissions, respectively. East and South Asia together contribute more than 50% of the global surface, atmospheric, and top-of-atmosphere DRF by BC. The regional contributions to global mean forcings closely follow the respective contributions to atmospheric burden. The global warming potential (GWP) of BC for different regions ranges from 374 to 677 with a global mean of 480. Europe is the largest contributor (63%) to BC deposition at high latitudes. The indirect GWP due to the BC effect on snow albedo is estimated to be largest for Europe (possibly as large as 1200), suggesting that BC emission reductions from this region are more efficient to mitigate climate change.

J. G. L. Rae, C. E. Johnson, N. Bellouin, O. Boucher, J. M. Haywood, and A. Jones. Sensitivity of global sulphate aerosol production to changes in oxidant concentrations and climate. Journal of Geophysical Research (Atmospheres), 112:10312, May 2007. [ bib | DOI | ADS link ]

The oxidation of SO2 to sulphate aerosol is an important process to include in climate models, and uncertainties caused by ignoring feedback mechanisms affecting the oxidants concerned need to be investigated. Here we present the results of an investigation into the sensitivity of sulphate concentrations to oxidant changes (from changes in climate and in emissions of oxidant precursors) and to changes in climate, in a version of HadGAM1 (the atmosphere-only version of HadGEM1) with an improved sulphur cycle scheme. We find that, when oxidants alone are changed, the global total sulphate burden decreases by approximately 3%, due mainly to a reduction in the OH burden. When climate alone is changed, our results show that the global total sulphate burden increases by approximately 9%; we conclude that this is probably attributable to reduced precipitation in regions of high sulphate abundance. When both oxidants and climate are changed simultaneously, we find that the effects of the two changes combine approximately linearly.

G. Myhre, N. Bellouin, T. F. Berglen, T. K. Berntsen, O. Boucher, A. Grini, I. S. A. Isaksen, M. Johnsrud, M. I. Mishchenko, F. Stordal, and D. Tanré. Comparison of the radiative properties and direct radiative effect of aerosols from a global aerosol model and remote sensing data over ocean. Tellus Series B Chemical and Physical Meteorology B, 59:115-129, February 2007. [ bib | DOI | ADS link ]

A global aerosol transport model (Oslo CTM2) with main aerosol components included is compared to five satellite retrievals of aerosol optical depth (AOD) and one data set of the satellite-derived radiative effect of aerosols. The model is driven with meteorological data for the period November 1996 to June 1997 which is the time period investigated in this study. The modelled AOD is within the range of the AOD from the various satellite retrievals over oceanic regions. The direct radiative effect of the aerosols as well as the atmospheric absorption by aerosols are in both cases found to be of the order of 20 Wm-2 in certain regions in both the satellite-derived and the modelled estimates as a mean over the period studied. Satellite and model data exhibit similar patterns of aerosol optical depth, radiative effect of aerosols, and atmospheric absorption of the aerosols. Recently published results show that global aerosol models have a tendency to underestimate the magnitude of the clear-sky direct radiative effect of aerosols over ocean compared to satellite-derived estimates. However, this is only to a small extent the case with the Oslo CTM2. The global mean direct radiative effect of aerosols over ocean is modelled with the Oslo CTM2 to be -5.5 Wm-2 and the atmospheric aerosol absorption 1.5 Wm-2.

S. Verma, O. Boucher, M. S. Reddy, H. C. Upadhyaya, P. Le van, F. S. Binkowski, and O. P. Sharma. Modeling and analysis of aerosol processes in an interactive chemistry general circulation model. Journal of Geophysical Research (Atmospheres), 112:3207, February 2007. [ bib | DOI | ADS link ]

An ”online” aerosol dynamics and chemistry module is included in the Laboratoire de Météorologie Dynamique general circulation model (LMDZ), so that the chemical species are advected at each dynamical time step and evolve through chemical and physical processes that have been parameterized consistently with the meteorology. These processes include anthropogenic and biogenic emissions, over 50 gas/aqueous phase chemical reactions, transport due to advection, vertical diffusion and convection, dry deposition and wet scavenging. We have introduced a size-resolved representation of aerosols which undergo various processes such as coagulation, nucleation and dry and wet scavenging. The model considers 16 prognostic tracers: water vapor, liquid water, dimethyl sulfide (DMS), hydrogen sulfide (H2S), dimethyl sulphoxide (DMSO), methanesulphonic acid (MSA), sulfur dioxide (SO2), nitrogen oxides (NOX), carbon monoxide (CO), nitric acid (HNO3), ozone (O3), hydrogen peroxide (H2O2), sulfate mass and number for Aitken and accumulation modes. The scheme accounts for two-way interactions between tropospheric chemistry and aerosols. The oxidants and chemical species fields that represent the sulfate aerosol formation are evolved interactively with the model dynamics. A detailed description on the coupled climate-chemistry interactive module is presented with the evaluation of chemical species in winter and summer seasons. Aqueous phase reactions in cloud accounted for 71% of sulfate production rate, while only 45% of the sulfate burden in the troposphere is derived from in-cloud oxidation.

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