From the Environmental News Network:

Global dimming is a less well-known but real phenomenon resulting from atmospheric pollution. The burning of fossil fuels by industry and internal combustion engines, in addition to releasing the carbon dioxide that collects and traps the sun’s heat within our atmosphere, causes the emission of so-called particulate pollution—composed primarily of sulphur dioxide, soot and ash. When these particulates enter the atmosphere they absorb solar energy and reflect sunlight otherwise bound for the Earth’s surface back into space. Read more.