GAW Aerosols Research

GAW Aerosol Programme

The Global Atmosphere Watch (GAW) aerosol programme strives "to determine the spatio-temporal distribution of aerosol properties related to climate forcing and air quality up to multidecadal time scales".  Aerosol Optical Depth (AOD) is one of the parameters measured in GAW.  More specific information is obtained by the GAW Atmospheric Lidar Observation Network (GALION) that provides the vertical aerosol distribution through advanced laser remote sensing in a network of ground-based stations.

Goals

The main goal of the GAW Aerosol Programme is to enhance the coverage, effectiveness, and application of long-term aerosol measurements within GAW and with cooperating networks worldwide, by

Airborne aerosols affect many aspects of human health and the environment. Aerosol mass and its toxicity are known to have links to chronic respiratory and acute cardio-vascular problems. Aerosols are also tightly linked to problems of visibility reduction, acid rain, and urban smog in many locations of the world.

Furthermore, aerosols influence the atmospheric energy budget through direct and indirect effects. The Intergovernmental Panel on Climate Change (IPCC) discusses these direct and indirect effects of aerosols (Figure 1).

• The direct effect is the mechanism by which aerosols scatter and absorb shortwave and longwave radiation, thereby altering the radiative balance of the Earth-atmosphere system. Sulphate, fossil fuel organic carbon, fossil fuel black carbon, biomass burning and mineral dust aerosols were all identified as having a significant anthropogenic component and exerting a significant direct radiative forcing.
• The indirect effect is the mechanism by which aerosols modify the microphysical and hence the radiative properties, amount and lifetime of clouds.
  • The ‘first indirect effect’, the ‘cloud albedo effect’, or the ‘Twomey effect' is the microphysically induced effect on the cloud droplet number concentration and hence the cloud droplet size, with the liquid water content held fixed.
  • The ‘second indirect effect’, the ‘cloud lifetime effect’ or the ‘Albrecht effect’ is the microphysically induced effect on the liquid water content, cloud height, and lifetime of clouds. 
  • The semi-direct effect is the mechanism by which absorption of shortwave radiation by tropospheric aerosols leads to heating of the troposphere that in turn changes the
    relative humidity and the stability of the troposphere and thereby influences cloud formation and lifetime.

More information may be found in the AR4 report of the IPCC (2007).

Figure 1 (click image to enlarge): Schematic diagram showing the various radiative mechanisms associated with cloud effects that have been identified as significant in relation to aerosols. The small black dots represent aerosol particles; the larger open circles cloud droplets. Straight lines represent the incident and reflected solar radiation, and wavy lines represent terrestrial radiation. The filled white circles indicate cloud droplet number concentration (CDNC). The unperturbed cloud contains larger cloud drops as only natural aerosols are available as cloud condensation nuclei, while the perturbed cloud contains a greater number of smaller cloud drops as both natural and anthropogenic aerosols are available as cloud condensation nuclei (CCN). The vertical grey dashes represent rainfall, and LWC refers to the liquid water content (Physical Science Basis, AR4 Report, IPCC, 2007).

Observations

GAW aerosol long-term observation core parameter srecommended as a priority for a global surface-based network are (GAW Report No. 153, revised in GAW Report 197, Table 2 ):
 • Physical Properties:
             • particle number concentration (size integrated)
             • particle number size distribution
             • particle mass concentration (two size fractions)
             • cloud condensation nuclei number concentration (at various super-saturations)
       

• Optical Properties:
             • light scattering coefficient (various wavelengths)
             • light hemispheric backscattering coefficient (various wavelengths)
             • light absorption coefficient (various wavelengths)

• Chemical Properties:
             • mass concentration of major chemical components (two size fractions)

• Column and Profile:
               • aerosol optical depth (various wavelengths)
               • vertical profile of aerosol backscattering coefficient
               • vertical profile of aerosol extinction coefficient

Additional parameters recommended for long-term or intermittent observation:
       • dependence of aerosol properties on relative humidity
       • detailed, size segregated chemical composition.

Ground-based Global Aerosol Optical Depth (AOD) Network

Aerosol optical depth (AOD) is a measure of the amount of light that aerosols scatter and absorb in the atmosphere (and generally prevent from reaching the surface). Radiometers — instruments that quantify the amount of electromagnetic radiation (light) — are among the most important tools available to measure AOD. These radiometer instruments measure AOD in an integrated way over the vertical or atmospheric column, hence providing horizontal distributions of AOD (NASA).

The objective of a global aerosol optical depth network is:

"to provide, on a multidecadal time scale, the spatio-temporal distribution of one of the five “core” aerosol properties recommended by the GAW Scientific Advisory Group for aerosols required for understanding climate forcing, air quality and hemispheric air pollution, transport and deposition (GAW Report No. 162)".

The global long-term groundbased AOD activities are illustrated in Figure 2 by a Global map of long-term AOD network sites as of March 2004. The Global ground-based AOD network has the following characteristics:
- Ten independent networks;
- 90 stations with a continuous record for the past 4 years and temporal data coverage of
50% were identified and documented;
- Half of the stations fall within the AERONET project, the other half is maintained mainly by WMO Members and Asian SKYNET;
- Hemispheric coverage corresponds roughly to the landmass distribution, (⅓SH, ⅔NH),
with Australia, Europe and North America accounting for more than 50% of stations;
- Major gaps exist in Africa, India, Latin America and the polar regions  (GAW Report No. 162).

Figure 2 (click image to enlarge): Global map of long-term AOD network sites (GAW Report No. 162).

LIDAR Networks

LIDAR (Light Detection And Ranging) is the visible light analog of radar. Very short laser pulses of light are sent into the atmosphere, are scattered back to the lidar by gases and aerosols in the air, and from the time out to these scatterers and the time to return back to the lidar, the position, concentration and some information on the properties of the scatters are determined. In contrast to radiometers, LIDARs can measure the vertical distribution of aerosols (e.g. Figure 4 below). In the most common configuration of lidars in Europe in the EARLINET component of GALION (GAW Aerosol Lidar Observation Network), light at 355, 532 and 1064 nm (ultraviolet, green and infrared) wavelengths is emitted vertically.  LIDARs can also be carried by satellites.

Figure 3 (click image to enlarge): Distribution of stations as available through the cooperation between existing LIDAR networks. The different networks are indicated by the dot colour: Asian Dust Network (AD-Net) violet, American LIdar Network (ALINE) yellow, CISLiNet green, European Aerosol Research Lidar Network (EARLINET) red, Micro-Pulse Lidar Network (MPLNET) brown, Network for the Detection of Atmospheric Composition Change (NDACC) white, Regional East Aerosol Lidar Mesonet (REALM) blue (GAW Report No. 178).

The aerosol properties observed include the identification of aerosol layers, profiles of optical properties with known and specified precision (backscatter and extinction coefficients at selected wavelengths, lidar ratio, Ångström coefficients), aerosol type (e.g. dust, maritime, fire smoke, urban haze), and microphysical properties (e.g., volume and surface concentrations, size distribution parameters, refractive index).  Observations are planned to be made with sufficient coverage, resolution, and accuracy to establish comprehensive aerosol climatology, to evaluate model performance, to assist and complement space-borne observations, and to provide input to forecast models of "chemical weather".

For more information about some GALION partners:

• Asian Dust Network, AD-Net
• CIS-LiNet
• European Aerosol Research Lidar Network, EARLINET
• Micro-Pulse Lidar Network, MPLNET
• Network for the Detection of Atmospheric Composition Change, NDACC

Satellite measurements

There are two ways by which satellites perform measurements of aerosol properties:

(1) Spaceborne lidar observations

In 2006, NASA launched the Clouds and Aerosol Lidar for Pathfinder Spaceborne Observations (CALIPSO) mission. Satellite instruments such as CALIPSO can measure the distribution of aerosols in a vertical slice of the atmosphere. Over the first year of observation, many of the stations who intend to contribute to GALION have been making targeted observations on CALIPSO overpass times. It is clear that the quality of GALION ground based measurements are crucial to the validation of the spaceborne instrument (GAW Report No. 178).

(2) Column measurements made from satellite passive remote sensors

In a similar way, the AOD retrievals from instruments such as the Moderate Resolution Imaging Spectrometer (MODIS) on the Terra and Aqua satellites help synthesize a global picture between the temporally continuous daytime AOD measurements made by sunphotometers (radiometers). GALION will provide the same advantage when used in a synergistic observational mode with these satellite sensors to determine the height as which aerosols reside in the atmosphere (GAW Report No. 178).

Satellites provide a critical global perspective for understanding how aerosols affect Earth’s climate, but they are far from the only source of information. Networks of ground-based sensors are used to validate satellite measurements and offer some of the most accurate measurements of AOD available (NASA).

Volcanic ash detection - The eruption of the Eyjafjallajökull volcano (Iceland)

The eruption of the Eyjafjallajökull volcano in April 2010 raised the interest in the transport of volcanic emissions. During the eruption 22 fixed stations of the Europe lidar network EARLINET (see web page for details) and several other stations in Russia could detect different layers of the volcanic ash plume over Europe.

The two figures below show the backscatter for parallel and cross-polarized light at 1064nm from the Paliseau, France, station of GALION.  The cross-polarized signal allows discrimination between normal pollution which tends to be small spherical particles and the ash which, though small, is irregular in shape.

Figure 4 (click image to enlarge): Left: The cross polarized signal on April 16, 2010, showing the main part of the ash cloud as irregular particles originally at 6 km at 16UTC descending and thinning out to 3 km by the end of the day.  Right: the ash layers above Paliseau France on April 17, 2010, showing descending layers from 3 to 2 km and a more diffuse layer of dust up to 7 km. The features at 9-10 km are cirrus clouds. 

Paul Scherrer Institut communication with media on Volcanic Ash

Publications

GAW report 178. Plan for the implementation of the GAW Aerosol Lidar Observation Network GALION (Hamburg, Germany, 27-29 March 2007) (WMO TD No. 1443), 52 pgs, November 2008