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page last modified:
23 January 2004
   
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Aerosols and the Earth's climate

Aerosols play an important role in the global climate, the radiative forcing of the climate, and the Earth's radiative balance.

Direct scattering and absorbing interaction between atmospheric aerosols and incoming solar radiation may influence the radiative forcing and explain the difference between observed and modeled temperature trends. Energy balance models have shown the aerosols effect on cooling. They act by modifying the local and planetary albedo and by absorbing the upward terrestrial thermal radiation. The aerosols scattering and absorption features depend on their chemical and physical properties.

Aerosols influence also indirectly the radiation balance through another key processes: by acting as cloud condensation nuclei and thus by affecting dramatically the optical properties of clouds.
 

Aerosols as strong events

Increasing interest is focused on absorbing aerosols, such as anthropogenic sulfate. The importance of the aerosol types such as black carbon from urban and industrial activities and agricultural dust is widely assessed. Particular interest is dedicated to smoke produced by biomass burning. Carbonaceous aerosols generated by biomass combustion consist of a mixture of material with varying radiative properties, both absorbing (fraction of elemental or graphitic carbon) and relatively non absorbing (group of volatile organic or inorganic components).

Also the presence of mineral dust coming from arid and semi-arid regions (desert dust) and volcanic aerosols and ash can be considered important events with an influence on the atmospheric conditions.

Aerosols play an important role in the troposphere, influencing also the presence of several gases, such as sulphur dioxide.
 

Remote Sensing of aerosols

Satellite monitoring of aerosol properties using passive techniques is widely considered to be a crucial tool for the study of climatic effects of atmospheric particles. Only space-based observation can provide the required global coverage information on spatial distribution and temporal variation of the aerosol field.

The GOME instrument, flying on board the second European Remote Sensing satellite (ERS 2), is a spectrometer that measures radiation reflected from the Earth in the spectral range 240-793 nm and in nadir-viewing observation. Each spectrum refers to a footprint of 320x40 km2.

Aerosol algorithms are developed and applied to GOME data to obtain atmospheric aerosol columns over sea, under cloud-free conditions, and selecting measurements in the atmospheric windows free of molecular absorption.

These algorithms can be applied also to nadir-viewing data of the SCIAMACHY instrument that flies on board ENVISAT. For our purpose the main differences are the higher spectral range (240-2380 nm) and better spatial resolution (60x30 km2) of SCIAMACHY with respect to GOME.

The information that is obtained are Aerosol Optical Depth (AOD) at 500 nm and aerosol type. For fixed aerosol components, the non-dimensional quantity AOD is proportional to the material density present in the atmospheric column.
 

Example: desert dust transport over the Atlantic Ocean

The following global maps show a typical event of desert dust transport over the Atlantic Ocean at Sahara desert latitude. The maps are obtained considering one month of GOME data measurements and refer to June 1997. The first regards the frequency of desert dust aerosol events and the second map the mean desert Aerosol Optical Depth at 500 nm.

 

GOME desert aerosol - frequency

 

GOME desert aerosol - AOD