Observing, understanding and predicting the influence of aerosol in the atmosphere is particularly challenging given their transient chemical and physical state. Aerosol in the atmosphere arise from a number of natural and anthropogenic sources and may exist over a size distribution covering many orders of magnitude. Once in the atmosphere, they are subject to physical and chemical transformations that result in diverse compositions and properties. The lifetime of an aerosol in the atmosphere ranges from minutes to days or weeks and they may be lost from the atmosphere due to complete evaporation, gravitational sedimentation, impaction or rain-out.
Atmospheric aerosol interact with incoming solar radiation, and have both positive and negative radiative forcing effects. Absorbing particles, such as black carbon, have a warming effect as they absorb solar radiation. Transparent particles, for example sea spray, typically have a cooling effect as they scatter solar radiation away from the surface. The bigger atmospheric role of aerosol is their seeding of cloud droplets, and one of the biggest questions relating to climate change is the way in which anthropocentric emissions of aerosol have changed the interaction between clouds and aerosol.
Figures © 2014 James Davies