An aerosol is a mixture of fine solid particles or liquid droplets in air.
Aerosols can be natural or anthropogenic.
Examples of natural aerosols are fog, forest exudates and geyser steam.
Examples of artificial aerosols are haze, dust, particulate air pollutants and smoke.
Aerosols affect climate in multiple ways. The effect of aerosols on climate is normally quantified in terms of aerosol radiative forcing.
Aerosol radiative forcing is defined as the effect of anthropogenic aerosols on the radiative fluxes at the top of the atmosphere (TOA) and at the surface and on the absorption of radiation within the atmosphere.
The effect of the total (anthropogenic + natural) aerosols is called aerosol radiative effect or total aerosol forcing.
Aerosol absorbs or scatters radiation in the atmosphere (so-called direct effect). Aerosols, except dust, interfere mainly with solar radiation.
Some aerosols act as cloud condensation nuclei (CCN), thus affecting cloud albedo and lifetime (so-called indirect effect).
Some of the aerosols can absorb sunlight efficiently and heat the atmosphere. This heating can burn cloud (so-called semi-direct effect).
Dark color aerosols can be deposited on sea ice, snow packs and glaciers, thus darkening the snow and ice surfaces, and enhancing the absorption of sunlight (so-called surface darkening effect).
Indirectly, they modulate Earth’s energy balance by altering cloud properties—in particular, cloud droplet size—by serving as cloud condensation nuclei, which further influence the cloud fraction, height and lifetime.
All these aspects result in modification of the planetary albedo and hydrological cycle.
Some important environmental issues, such as haze, acid rain and tropospheric ozone pollution, are also closely correlated with aerosol pollution.