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GCOS Essential Climate Variables (ECV) Definition: Aerosol - a colloidal system in which the dispersed phase is composed of either solid or liquid particles, and in which the dispersion medium is some gas, usually air. There is no clear-cut upper limit to the size of particles composing the dispersed phase in an aerosol, but as in all other colloidal systems, it is rather commonly set at 1 μm. Haze, most smokes, and some fogs and clouds may thus be regarded as aerosols. However, it is not good usage to apply the term to ordinary clouds with drops so large as to rule out the usual concept of colloidal stability. It is also poor usage to apply the term to the dispersed particles alone; an aerosol is a system of dispersed phase and dispersing medium taken together. Compare airborne particulates, particles, PM-2.5, PM-10. (AMS Glossary of Meteorology) Introduction: Aerosols are tiny particles suspended in the air. The majority are derived from natural phenomena such as volcanic eruptions, but it is estimated that some 10–20% are generated by human activities such as burning of fossil fuels. The majority of aerosols form a thin haze in the lower atmosphere and are regularly washed out by precipitation. The remainder are found in the stratosphere where they can remain for many months or years. Scientists have yet to quantify accurately the relative impacts on climate of natural aerosols and those of human origin, so there is still uncertainty whether aerosols are warming or cooling the Earth. Predicting the rate and nature of future climate change requires this clarification of the processes involved. As a consequence, the IPCC identifies further information on aerosols as a priority, highlighting a particular need for additional systematic, integrated and sustained observations which include the spatial distribution of greenhouse gases and aerosols. Satellite Observations: The Integrated Global Atmospheric Chemistry Observations (IGACO) Theme of the IGOS Partnership aims to provide a framework ensuring continuity and spatial comprehensiveness of the full spectrum of atmospheric chemistry observations, including the monitoring of atmospheric composition parameters related to climate change and environmental conditions. The IGACO Theme Report (available from www.igospartners.org) was finalised in May 2004 and provides a comprehensive overview of current and future satellite measurements for tropospheric and stratospheric aerosols. The report states, in particular, that “satellite observations of aerosol optical properties have progressed to a point where they range from pre-operational to operational, although there are demonstration-mode instruments on a number of research satellites”. Reliable information on aerosols is also required by applications outside the study of the climate system. For example, accurate and timely warnings of the presence of airborne dust and ash – such as that arising from desert dust clouds and volcanic eruptions – are important to the safety of airline operations. A worldwide volcanic ash monitoring system, which is dependent on satellite observations, is in place to provide real-time advice to pilots. Measuring the distribution of aerosols through the depth of the atmosphere is technically difficult, particularly in the troposphere. Previously, techniques using instruments such as AVHRR and ATSR were limited to producing estimates of vertically-integrated total amounts, mainly over oceanic regions. Measurements over land are difficult (due to persistent cloud cover and the high value, and variability, of land surface reflectance), but the new generation of multi-directional or polarimetric instruments – such as AATSR, MISR and APS (planned for NPOESS but recently demanifested) – can provide detailed information. Today, MODIS, MERIS, MISR, and POLDER-P offer better optical depth at different frequencies, enabling aerosol particle sizes, particularly over oceans, to be inferred. The development of active instruments such as ATLID and ALADIN, and laser altimeter sensors, including GLAS on ICESat, should yield much improved measurement capability. Since April 2006, CALIPSO has flown a 3-channel lidar (designed specifically to provide vertical profiles) and passive instruments, orbiting in formation with Aqua, Aura, PARASOL and CloudSat to obtain coincident observations of radiative fluxes and atmospheric state. This comprehensive set of measurements is essential for accurate quantification of global aerosol and cloud radiative effects.Limb-sounding instruments such as ACE-FTS, SCIAMACHY, GOMOS, and HIRDLS principally provide data on the upper troposphere and stratosphere with high vertical resolution, but horizontal resolution is relatively poor (typically of the order of a few hundred km). Current, long-term climatologies are based upon AVHRR/3 on the NOAA and MetOp series of low Earth orbit satellites. These observations will continue to provide estimates of total column aerosol amounts over the ocean. AVHRR/3 will be replaced by a more capable visible and infrared imager, called VIIRS, on the NPOESS series of satellites, starting with the preparatory NPP mission in 2010. VIIRS will acquire high resolution atmospheric imagery and generate a variety of applied products, including some that give information on atmospheric aerosols. The CEOS response to the GCOS Implementation Plan recognised that no operational aerosol instruments measuring particle composition and size/shape have been yet been flown and efforts should be made to rectify this. It encouraged re-planning of the aerosol measurements envisaged by APS/NPOESS and consideration of operational active sensing lidar (such as CALIPSO). CEOS committed to pursue the following action: “CEOS agencies will participate in re-planning the APS instrument removed from the planned payload of NPOESS”. (Satellite Missions) (from the CEOS web site) Data, Products, Metadata and Information Access
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