2.4.2.2 Surface-Based Retrievals
A significant advancement since the TAR is the continued deployment and development of surface based remote sensing sun-photometer sites such as AERONET (Holben et al., 1998), and the establishment of networks of aerosol lidar systems such as the European Aerosol Research Lidar Network (EARLINET, Matthias et al., 2004), the Asian Dust Network (ADNET, Murayama et al., 2001), and the Micro-Pulse Lidar Network (MPLNET, Welton et al., 2001).
The distribution of AERONET sites is also shown in Figure 2.11 (top panel). Currently there are approximately 150 sites operating at any one time, many of which are permanent to enable determination of climatological and interannual column-averaged monthly and seasonal means. In addition to measurements of τaer as a function of wavelength, new algorithms have been developed that measure sky radiance as a function of scattering angle (Nakajima et al., 1996; Dubovik and King, 2000). From these measurements, the column-averaged size distribution and, if the τaer is high enough (τaer > 0.5), the aerosol single scattering albedo, ωo, and refractive indices may be determined at particular wavelengths (Dubovik et al., 2000), allowing partitioning between scattering and absorption. While these inversion products have not been comprehensively validated, a number of studies show encouraging agreement for both the derived size distribution and ωo when compared against in situ measurements by instrumented aircraft for different aerosol species (e.g., Dubovik et al., 2002; Haywood et al., 2003a; Reid et al., 2003; Osborne et al., 2004). A climatology of the aerosol DRE based on the AERONET aerosols has also been derived (Zhou et al., 2005).
The MPLNET Lidar network currently consists of 11 lidars worldwide; 9 are co-located with AERONET sites and provide complementary vertical distributions of aerosol backscatter and extinction. Additional temporary MPLNET sites have supported major aerosol field campaigns (e.g., Campbell et al., 2003). The European-wide lidar network EARLINET currently has 15 aerosol lidars making routine retrievals of vertical profiles of aerosol extinction (Mathias et al., 2004), and ADNET is a network of 12 lidars making routine measurements in Asia that have been used to assess the vertical profiles of Asian dust and pollution events (e.g., Husar et al., 2001; Murayama et al., 2001).