CFCs are very stable and last for about 50 years in the atmosphere. As well as being greenhouse gases in the lower atmosphere, once in the stratosphere, CFCs damage the ozone layer. Thanks to the international agreement known as the Montreal Protocol, CFC production in developed countries has ceased. Atmospheric concentrations of many CFCs are now falling. In a curious turn of events, the warming effect of CFCs is offset by the fact that they destroy ozone, also a greenhouse gas, in the lower stratosphere.
Ozone is a component of photochemical smog.
There is strong evidence that ozone concentrations in the lower atmosphere are higher than in pre-industrial times, especially in the northern hemisphere. Atmospheric aerosol Aerosol are fine particles and droplets suspended in air. Some human activities increase the levels of aerosol in the atmosphere. The greatest source of aerosol is sulfur dioxide, a gas released by coal-burning power stations and during the processing of mineral ores. Once the sulfur dioxide is in the atmosphere, chemical reactions convert it into sulfate aerosol.