Showing papers by "Yves Balkanski published in 2004"

Organic aerosol and global climate modelling: a review

Abstract: The present paper reviews existing knowledge with regard to Organic Aerosol (OA) of importance for global climate modelling and defines critical gaps needed to reduce the involved uncertainties. All pieces required for the representation of OA in a global climate model are sketched out with special attention to Secondary Organic Aerosol (SOA): The emission estimates of primary carbonaceous particles and SOA precursor gases are summarized. The up-to-date understanding of the chemical formation and transformation of condensable organic material is outlined. Knowledge on the hygroscopicity of OA and measurements of optical properties of the organic aerosol constituents are summarized. The mechanisms of interactions of OA with clouds and dry and wet removal processes parameterisations in global models are outlined. This information is synthesized to provide a continuous analysis of the flow from the emitted material to the atmosphere up to the point of the climate impact of the produced organic aerosol. The sources of uncertainties at each step of this process are highlighted as areas that require further studies.

The aerosol-climate model ECHAM5-HAM

Abstract: The aerosol-climate modelling system ECHAM5-HAM is introduced. It is based on a flexible microphysical approach and, as the number of externally imposed parameters is minimised, allows the application in a wide range of climate regimes. ECHAM5-HAM predicts the evolution of an ensemble of microphysically interacting internally- and externally-mixed aerosol populations as well as their size-distribution and composition. The size-distribution is represented by a superposition of log-normal modes. In the current setup, the major global aerosol compounds sulfate (SU), black carbon (BC), particulate organic matter (POM), sea salt (SS), and mineral dust (DU) are included. The simulated global annual mean aerosol burdens (lifetimes) for the year 2000 are for SO4: 0.80 Tg(S) (3.9 days), for BC: 0.11 Tg (5.4 days), for POM: 0.99 Tg (5.4 days), for SS: 10.5 Tg (0.8 days), and for DU: 8.28 Tg (4.6 days). An extensive evaluation with in-situ and remote sensing measurements underscores that the model results are generally in good agreement with observations of the global aerosol system. The simulated global annual mean aerosol optical depth (AOD) is with 0.14 in excellent agreement with an estimate derived from AERONET measurements (0.14) and a composite derived from MODIS-MISR satellite retrievals (0.16). Regionally, the deviations are not negligible. However, the main patterns of AOD attributable to anthropogenic activity are reproduced.

Global modeling of heterogeneous chemistry on mineral aerosol surfaces: Influence on tropospheric ozone chemistry and comparison to observations

Abstract: [1] Mineral aerosols can affect gas phase chemistry in the troposphere by providing reactive sites for heterogeneous reactions. We present here a global modeling study of the influence of mineral dust on the tropospheric photochemical cycle. This work is part of the Mineral Dust and Tropospheric Chemistry (MINATROC) project, which focussed on measurement campaigns, laboratory experiments, and integrative modeling. The laboratory experiments provide uptake coefficients for chemical species on mineral aerosol surfaces, which are used to compute the heterogeneous reaction rates in the model. The field measurements at Mount Cimone, northern Italy, provide trace gas and aerosol measurements during a Saharan dust episode and are used to evaluate the model. The simulations include the reactions between mineral dust aerosols and the gas-phase species O3, HNO3, NO3, and N2O5. Under the conditions for the year 2000 the model simulates a decrease in global tropospheric ozone mass by about 5% due to the heterogeneous reactions on dust aerosols. The most important heterogeneous reaction is the uptake of HNO3 on the dust surface, whereby the direct uptake of ozone on dust is not important in atmospheric chemistry. The comparison of the model results to observations indicates that the model simulates well the aerosol mass transported into the Mediterranean during the dust events and the arrival of all major dust events that were observed during a 7 month period. The decrease in ozone concentration during dust events is better simulated by the model when the heterogeneous reactions are included.

Aerosol-ozone correlations during dust transport episodes

Abstract: . Its location in the Mediterranean region and its physical characteristics render Mt. Cimone (44°11′ N, 10°42′ E), the highest peak of the Italian northern Apennines (2165 m asl), particularly suitable to study the transport of air masses from the north African desert area to Europe. During these northward transports 12 dust events were registered in measurements of the aerosol concentration at the station during the period June–December 2000, allowing the study of the impact of mineral dust transports on free tropospheric ozone concentrations, which were also measured at Mt. Cimone. Three-dimensional backward trajectories were used to determine the air mass origin, while TOMS Aerosol Index data for the Mt. Cimone area were used to confirm the presence of absorbing aerosol over the measurement site. A trajectory statistical analysis allowed identifying the main source areas of ozone and aerosols. The analysis of these back trajectories showed that central Europe and north and central Italy are the major pollution source areas for ozone and fine aerosol, whereas the north African desert regions were the most important source areas for coarse aerosol and low ozone concentrations. During dust events, the Mt. Cimone mean volume concentration for coarse particles was 6.18 µm3/cm3 compared to 0.63 µm3/cm3 in dust-free conditions, while the ozone concentrations were 4% to 21% lower than the monthly mean background values. Our observations show that surface ozone concentrations were lower than the background values in air masses coming from north Africa, and when these air masses were also rich in coarse particles, the lowest ozone values were registered. Moreover, preliminary results on the possible impact of the dust events on PM10 and ozone values measured in Italian urban and rural areas showed that during the greater number of the considered dust events, significant PM10 increases and ozone decreases have occurred in the Po valley.

Sea-salt aerosol source functions and emissions

Abstract: Sea spray aerosols are important for a wide variety of processes. Part of the current interest is their role in climate (Penner et al., 2001). Sea spray aerosol contributes to atmospheric cooling because they scatter incoming solar radiation. It is a natural component of the climate system and therefore can not be regarded as a forcing component. However, it is often neglected in global climate models and may be responsible for feedback effects. Latham and Smith (1990) suggested that a changing climate would alter surface winds and thus sea spray emissions. Although the sea spray aerosol number concentrations are not very high compared to those of anthropogenic aerosols such as ammonium sulphates, their role is significant because the oceans cover 70% of the Earth, whereas anthropogenic aerosols are rather locally produced. Sea-salt is the dominant submicrometer scatterer in most ocean regions and dominates the marine boundary layer particulate mass concentration in remote oceanic regions, with a significant fraction occurring in the submicrometer size range (IPCC., 2001). Sea-salt contributes 44% to the global aerosol optical depth. Estimates for top-of-atmosphere, global-annual radiative forcing due to sea-salt are -1.51 and -5.03 Wm2 for low and high emission values, respectively (IPCC., 2001). Sea spray not only affects climate by scattering of solar radiation, it also acts as cloud condensation nuclei and thus contributes to the indirect aerosol effect (IAE).

Global Emissions of Mineral Aerosol: Formulation and Validation using Satellite Imagery

Abstract: The most abundant aerosol components present in the atmosphere are dust and sea salt (Andreae, 1995). Li et al. (1996) show that dust dominates the aerosol light-scattering over the tropical and sub-tropical Atlantic. Satellite retrievals also illustrate the importance of dust over large regions from arid deserts to remote oceanic regions downwind of West Africa, Asia and the Persian Gulf (Husar et al., 1997; Deuze et al., 2000; Tanre et al., 2001). Furthermore, land modification, agricultural practices and the migration of desert fringes appear to have contributed to the increase in the dust transport over the Atlantic from the 1960s to the 1980s. These perturbations to the dust cycle brought by human activity are thought to account for 15 to 50% of the atmospheric dust load (Tegen and Fung, 1995; Tegen, personnal communication, 2002).