Particulates in the Atmosphere
Liquid or solid particulates called aerosols can affect climate by altering the way electromagnetic energy flows through the atmosphere.
Particulates in the atmosphere can absorb and reflect various electromagnetic wavelengths. Where more particulates are present, like in smoggy urban areas, radiation is reflected and the region cools down. Where less particulates are present, more radiation is absorbed, so those regions heat up.
During the 1960's, there was intense emissions of sulfate and other aerosols into the atmosphere primarily from fossil fuel burning. Scientists observed a cooling effect on the bottom of the atmosphere near ground level. This led some to believe that the entire atmosphere was cooling because no measurements were available to observe any warming of the upper atmosphere at that time.
Since then, fuels have been burned more "cleanly" to reduce pollution and acid rain, and the surface cooling effect has been greatly reduced. For example, coal-fired power plants now have “scrubbers” which use calcium from limestone in the smokestack to convert sulfur dioxide into gypsum, a useful mineral used in wallboard.