Major Ground Networks

AERONET (AErosol RObotic NETwork; http://aeronet.gsfc.nasa.gov/) is a federation of ground-based remote-sensing aerosol networks that was established in the mid-1990s by NASA and CNRS; it was expanded thereafter through the efforts of national agencies, universities, and individuals (Holben et al. 1998). Using standards for instruments, calibration techniques, processing, and data distribution, individual sites are connected into a network. The basic instrument is a CIMEL sun/sky photometer. Data are transmitted via satellite or internet to the central processing facility at NASA, where the retrieved products from almost all sites are available within hours. Later, quality-assured products are released after the instruments have been recalibrated. Retrieved products are aerosol column properties for the mid-visible AOD and the AnP; an inversion algorithm employing sky radiances (Dubovik and King 2000) provides the particle size distribution and data on particle absorption (with the caveat that estimates for ro0 are only reliable, if the mid-visible AOD value exceeds 0.3). Now more than 150 instruments are simultaneously in operation worldwide. Since many instruments have been moved during their lifetime, column data on annual cycles of aerosol optical properties are available for more than about 400 sites.

SKYNET (http://atmos.cr.chiba-u.ac.jp/) is an observation network that is based in Eastern Asia. It was established to further an understanding of the atmospheric interactions between particles, clouds, and radiation (Aoki 2006). Basic instrumental configurations include a PREDE sun/sky photometer at ten sites. The retrieved aerosol column properties are mid-visible AOD, AnP, and, by processing sky radiances in an inversion routine, ro0 (whose estimate can only be trusted at larger AOD values, as for AERONET). Data obtained are locally collected and then transferred to a central site at the Chiba University in Japan, where data are archived and made accessible to the public. Side-by-side measurements next to CIMEL instruments of AERONET demonstrated good agreement and comparability of different network data. Thus, even the relatively few SKYNET sites (7 sites in 2003 and 2004) complement AERONET statistics in a CIMEL-sparse region.

GAW (Global Atmospheric Watch) is a WHO/WMO-sponsored program combining 22 global and more than 300 regional sites into a measurement network. GAW is geared primarily toward the background monitoring of surface concentrations of trace gases and compositional properties of the near-surface aerosol. It extends a similar regional network monitoring by EMEP (Klein et al. 2007) in Europe and IMPROVE (Malm and Hand 2003) in the U.S. Some GAW stations operate a PRF sun photometer for estimates of aerosol column properties. Retrieved properties are mid-visible values for AOD and AnP. Observations from each site are sent to Davos, Switzerland, and released after quality assurance to the World Data Centre for Aerosol at Ispra (http:// wdca.jrc.it/). The PFR sun photometer differs in design and spectral capabilities from CIMEL (of AERONET) or PREDE (of SKYNET) photometers, but retrieved properties compare well in side-by-side sampling. The relatively few GAW sites (15 sites in 2003 and 2004) complement AERONET by providing aerosol statistics in remote regions.

EARLINET (European Aerosol Research LIdar NETwork, http://www. earlinet.org/) was established in 2000 to provide comprehensive, quantitative, and statistically significant information on the aerosol vertical distribution over Europe (Bosenberg et al. 2003). It involves a coordinated probing of the atmosphere by more than twenty lidar systems at sites that are primarily located in central and southern Europe. About half of the sites have Raman capabilities for more accurate estimates on aerosol extinction profiles. Time-coordinated sampling is conducted on a regular and event basis; a quality assurance program addresses both instrumentation and evaluation algorithms; and efforts are underway to harmonize data exchange with format and processing standards.

NIES (National Institute for Environmental Studies) has established a network of automated two wavelength dual-polarization lidars (http://www.nies. go.jp/gaiyo/index-e.html) in cooperation with various universities and research organizations. This network provides aerosol vertical distribution monitoring (Nishizawa et al. 2007) at about 15 sites in Eastern Asia.

MPL-Net (http://mplnet.gsfc.nasa.gov/) is a network of backscattering lidars, which are co-located with CIMEL sun/sky photometers at 11 AERONET sites (Welton et al. 2001). Such a co-location of aerosol instrumentations elevates the site importance. Thus, many sites of other (aerosol) lidar networks are (and will be) instrumented with co-located sun photometers.

EARLINET, NIES, and MPL-Net are major but certainly not the only existing regional (aerosol) lidar networks. Efforts are underway to merge sampling in individual regions into one global effort, under the GALION initiative, which is supported by the WMO.

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