Optical aerosol properties of megacities: inland and coastal cities comparison

Measurements of aerosol optical depths allow the determination of microphysical and radiative characteristics of atmospheric aerosols and specially that of Megacities, which contribute to the deterioration of air quality live, increase health effects, and anthropogenic climate change. This paper analyzes the aerosol optical properties of ten megacities classified on inland and costal sites. The annual average of aerosol optical depths are around 0.5 and peaks can exceed 4 especially in summer for East Asia (Beijing and Bangkok) where the involvement of the anthropogenic aerosol is more important. Single scattering albedo is often greater than 0.8 and sometimes show wide variations between 0.6 and 0.98. The refractive index is constant and stands at 1.47 for the real part and 16 10 −3 for its imaginary part. The PSDs are 0.16 μm for the fine mode and 2.3 μm for the coarse particle mode with a 3 μm magnification trend for the coastal sites. The volume concentrations are on average close to 0.1 μm 3 /μm 2 for large particles and 0.04 μm 3 /μm 2 for fines with peaks observed at Ilorin for large and at Beijing for fines. Radiative forcing are always negative (cooling trends), relatively low at the top of the atmosphere, larger at surface, and relatively higher at coastal sites. For the vertical atmospheric column, anthropogenic radiative forcing is always positive (warming trends) estimated average of + 14 W/m 2 and natural registers three times increase for coastal sites. In reality, the coastal distinction is not at the origin of this increase since the maxima recorded are also included in the inland sites (Riyadh and Ilorin).