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Stratospheric aerosols: a key player in climate change

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Aerosols are an important player in the Earth’s climate: they diffuse sunlight and modify the physico-chemical, thermal and dynamic properties of the atmosphere as well as the radiation received at ground level. The origin of stratospheric aerosols is mainly volcanic, but increasingly, other actors such as forest and bush fires are taking their place in this theatre. Understanding the composition and microphysical characteristics of aerosols, and in particular the particle size, is a major issue for the study and modelling of climate change.
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BIRA-IASB has long experience in the characterisation of stratospheric aerosols from satellite data. BIRA-IASB researchers have published several reference data sets for various radiative and microphysical parameters of aerosols in the upper troposphere and stratosphere, derived from various Belgian (ORA), European (GOMOS) and American (SAGE II, POAM III) satellite experiments.

These climatologies cover extremely diverse situations in terms of volcanic activity (the primary source of aerosols in the stratosphere) and aerosol load over nearly 3 decades.

New sources of aerosols of different composition

Long time series are important for studying the evolution of aerosols as a function of volcanic activity but also of human activity. We know, for example, that the anticyclone linked to the Asian monsoons allows the formation of a layer of pollution at the tropopause and provides a pathway for the pollution to the stratosphere.

Moreover, forest and bush fires are becoming more and more widespread with climate change and are also a source of aerosols in the stratosphere: the intense heat produces pyrocumulus, a kind of bubble that rises quickly enough to cross the otherwise hardly penetrable layer of the tropopause.

These new sources of aerosols of different composition change the situation and require characterisation algorithms to be developed, particularly to determine the size properties of the particles.

The challenge of processing of satellite measurements

BIRA-IASB has played a pioneering role in the derivation of particle sizes from occultation instruments. Recently, other research groups have been investing in this field using other types of satellite instruments using limb measurements.

A challenge that has emerged relatively recently is that the processing of satellite measurements leads to different particle sizes for the two types of instruments. The exact cause of this difference is not yet elucidated, although it is clear that it is due partly to the different configuration of the instruments, partly to the inversion methods used.

Nevertheless, it forces researchers to put the problem of inversion of satellite data back on the trade. This problem is of major importance because the size of aerosol particles influences their chemical and radiative impact. A poor estimate can therefore affect the quality of the predictions provided by the models.

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Launched in 2002, Envisat is the largest Earth observation satellite ever built. Amongst the 10 instruments contributing to the monitoring of the atmosphere, land, oceans, and ice caps, the pioneering GOMOS made use of the light of about 300 stars to observe the concentration of different atmospheric species (and mainly ozone): a real challenge for both the instrument design and the data processing. Copyright ESA.
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