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Content archived on 2024-05-21

Mineral dust and tropospheric chemistry


Problems to be solved
This project focuses on the transformation of atmospheric pollutants from Europe in the presence of mineral dust over S. Europe and Africa. Mineral dust is the single most abundant aerosol. It is injected into the atmosphere by the action of surface winds on dry soils from cultivated regions as well as arid regions, especially over Southern Europe, North Africa and in Asia. Estimations of the global source strength vary from » 200 to 5000 Mt /yr, representing about 50 % of the total production of tropospheric aerosols by natural and anthropogenic sources together. At the global scale, the most important regions of dust emission are located near industrial regions with fast growing anthropogenic emissions of precursors of ozone and aerosols. Examples are the Sahel/Sahara (close to the Mediterranean Area) and the S. E. Asia deserts (close to Japan and to Eastern China). Reactions on the surface of aerosol particles (i.e. heterogeneous reactions) taking place in the troposphere can affect the radiative budget and alter the atmospheric content of key atmospheric species. Predicting the impact of changing anthropogenic emissions on atmospheric composition and climate, in Southern Europe and the Mediterranean Basin, requires an understanding of dust/pollution interactions. A few model studies have investigated atmospheric chemistry when dust and anthropogenic emissions interact showing a substantial effect on nitrate, sulphate and ozone. There is however a chronically shortage of laboratory data to back-up the (heterogeneous) reaction schemes used in present modelling studies. Furthermore, there is a lack of specific field data to show unambiguously the effect of dust/gas interactions We propose a multi-disciplinary approach that involves a high degree of synergy between laboratory studies, two field experiments and a modelling strategy. This project will eventually perform a study of how the gas/dust interactions studied affect the IPCC (2000) estimates of radiative forcing by ozone and anthropogenic aerosols.
Scientific objectives and approach
The project main objectives are: first, to quantify the impact of mineral dust on tropospheric photochemical cycles leading to ozone production and destruction, second to quantify the specific direct radiative effect of secondary aerosol (e.g. sulphate and organics) in the present of mineral dust. This results in an enhancement in our understanding and predictive capabilities regarding atmospheric composition change and climate. The multi-disciplinary approach of the project enables us to determine the efficiency and mechanism of the interaction of mineral dust with a number of important trace gases using laboratory experiments. Two field experiments of 4 weeks each allow characterizing both the gas phase and the aerosol state before and after an air parcel encounters Saharan dust. A simulation by a chemical transport model including detailed dust / gas interactions using the meteotological fields of the campaigns of observation will help to analyse and quantify the influence of mineral dust on photochemical oxidant cycles and ozone concentrations. One aspect is the divergence in modelled and measured NOy / NOz concentrations. Furthermore, we quantify the degree to which radiative cooling by secondary aerosols (sulphate, organics) aerosols is modified if the aerosol is internally/externally mixed with mineral dust. Particular attention is put on the influence of reactions on mineral dust on ozone concentrations and how it affects radiation balance. We further assess this impact for future conditions using the IPCC scenario.
Expected impacts
This project has relevance for the understanding of the factors that control the acidity of deposition, radiative effects of pollutants, or the oxidizing capacity of the atmosphere, and that are related to the transport and interaction of mineral dust with atmospheric pollutants. These factors are thought to affect particularly Southern Europe. Southern Europe and Northern Africa are growing areas in terms of industry and air pollution. They are also strongly influenced by the dust emission from the nearby Sahara. It is documented that the acidity of rain in southern Europe is considerably modified by the capacity of mineral dust to neutralize it. This in turns depends on the chemical history of the mineral dust and specifically on its calcite content, which can be severely depleted after transport through polluted regions and interaction with H2SO4 and HNO3. It is also likely that through such interactions the export of pollutants from S. Europe and N. Africa and their effect on the global atmosphere is modified. In particular the atmospheric residence time of pollution and the radiative effect of pollution aerosols are expected to be modified by the degree of mixing of those aerosols with dust, complicating e.g. estimation of the future impact on climate of increasing emissions of SO2 by developing countries. The approach proposed is one in which modelling studies laboratory and field experiments closely interact with one another, in order to provide convergence and closure on a number of issues related to pollution/dust interactions.

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Participants (7)