Volcanoes may erupt tons of fine ash particles into the atmosphere. Gases such as sulfur dioxide may combine with water in the atmosphere to form tiny droplets of sulfuric acid. It is good that there are some aerosols in the atmosphere because they can act as surfaces on which water vapor condenses to form clouds and eventually produce rain. However, many aerosols, particularly most human-made ones, are pollutants. That is, they are a hazard to life forms.
Anthropogenic aerosols tend to produce negative radiative forcing.
We may consider that locally, the aerosol forcing can be large enough to more than offset the positive forcing due to greenhouse gases though anthropogenic aerosols are very short-lived in the atmosphere, hence their radiative forcing adjusts rapidly to increases or decreases in emissions. They do, however, represent one Of the few agents which serve to counteract the effect Of greenhouse gases. The most important natural perturbations result from explosive volcanic eruptions which affect concentrations in the lower stratosphere. The ozone story is a little more complicated than that of most of the other greenhouse gases since it plays a multiple role depending upon its location in the atmospheric system, and in its distribution in latitude. At the Earth's surface ozone is known to be toxic to living matter, including human beings, plants and in particular forests.