Aerosols - distribution & optical properties
1. Major Aerosol Types & Global Distribution
Figure 7.13 (below) is a global map for major aerosol type - sulfate (SO4), organic carbon (OC), nitrate (NO3), ammonium (NH4), elemental carbon (EC, referring to a particular way of measuring black carbon - BC), and mineral dust or sea salt. It can be seen that the aerosol chemical compositions vary drastically between continents, or different areas on the same continents (e. g. urban or rural), as well as depending on how far away the locations are from major continents. Such chemical distribution of aerosol compositions are mainly caused by anthropogenic influence.
Figure 7.13 | Bar chart plots summarizing the mass concentration (μg m^–3) of seven major aerosol components for particles with diameter smaller than 10 μm, from various rural and urban sites (dots on the central world map) in six continental areas of the world with at least an entire year of data and two marine sites.
2. Global Aerosol Optical Depth & Extinction Coefficient
Different aerosol compositions and physical properties (e.g. size, shape, hygroscopicity) result in different aerosol bulk optical properties (e.g. aerosol optical depth – AOD, extinction coefficient, etc.). Changes in the electromagnetic field due to aerosols’ distinctive absorption and backscattering properties form the basis for remote-sensing monitoring of global aerosol distribution. Figure 7.14 (below) shows a global distribution of the 550 nm (green band) aerosol optical depth (AOD) measured by a satellite instrument called Moderate Resolution Imaging Spectrometer (MODIS) over the period 2003-2010, and four latitudinal vertical cross sections of the 532 nm aerosol extinction coefficient (km^-1) measured by another satellite instrument, Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) for the year 2010. As a gross first guess, one can think the ‘redder’ areas are associated with heavier aerosol loads due to higher extinction of light.
