Aerosol optical depth (AOD) retrieved by remote sensing from space is highly inhomogeneous, with the largest values in Asia and the tropical regions of Africa (see Figure 2). The estimated contributions from different aerosol types in selected regions are shown in pie charts. In general, there is large spatial and temporal variability in global aerosol composition. Remote sensing from both space and ground together with in situ observations have substantially advanced an understanding of geographical aerosol distribution, but there are still large uncertainties in the chemical composition and the anthropogenic contribution to the AOD.
MODIS aerosol optical depth [AOD (550 nm); dimensionless] averaged over the 10-year period 2001–2010 (Remer et al. 2008). Pie charts show how various aerosol types contribute to the total AOD for different regions, as estimated by a global aerosol model (Myhre et al. 2009). Aerosol types are Sul (sulphate), BC and OC from fossil fuel usage, Bio (OC and BC from biomass burning), Nitrate, Sea (sea salt), and Min (mineral dust). Gray areas indicate lack of MODIS data. Some aerosol types, e.g. sulphate, have enhanced contributions to AOD due to hygroscopic growth. The contribution from OC is likely underestimated as in most of the global aerosol models (Zhang et al. 2007).