Aerosols—suspensions of liquid drops or solid particles in gas—can be human made or natural. Atmospheric aerosols include water droplets, wind-blown mineral dust, pollen and other biological particles, and carbonaceous particles such as soot. The diversity of particle shapes and sizes is vast. Some drops are nearly spherical, but bits of mineral dust have irregular shapes. Some aerosols are molecular in scale; others are hundreds of micrometers in size.
Atmospheric particles absorb and scatter sunlight and thereby affect the balance between incoming and outgoing radiative energy in the Earth–atmosphere system. Aerosols are but one of several components affecting that energy balance; greenhouse gases and ozone, for example, are other, more commonly known substances. The quantitative measure for how a given material influences the energy balance is called radiative forcing. Positive forcing values indicate a heating of the system, whereas negative values indicate cooling. For greenhouse gases, the forcing value is well...
