Aerosol and Climate Change: Direct and Indirect Aerosol Effects on Climate

Airborne aerosols exhibit overall number concentrations usually varying from a few hundreds per cubic centimeter of air in the most remote areas of the Earth to more than 104 cm-3 in the most polluted urban areas over the size range from 10-2 to more than 50 pm. Secondary aerosols are formed in the atmosphere through chemical reactions involving sulfur dioxide, nitrogen oxides, biogenic volatile organic compounds (VOCs), and other chemical species emitted from both natural and anthropogenic activities. This chapter discusses the theoretical aspects associated with the aerosol optical characteristics and the surface reflectance properties. Airborne aerosols can also exert important indirect effects on the terrestrial climate system through their interactions with surrounding clouds. In fact, aerosols act as cloud condensation nuclei (CCNs) and change the cloud droplet concentration and size-distribution parameters, thus modifying the cloud amount and, hence, influencing the radiation balance and hydrology through their impact on cloud microphysical processes.