Air conditioning eventually reach the upper atmosphere. Once the refrigerant reaches the stratosphere, UV radiation from the Sun homolytically cleaves the chlorine-carbon bond, yielding a chlorine radical. These chlorine radicals catalyze the breakdown of ozone into diatomic oxygen, depleting the ozone layer that shields the Earth's surface from strong UV radiation. Each chlorine radical remains active as a catalyst until it binds with another radical, forming a stable molecule and quenching the chain reaction. Prior to 1994, most automotive air conditioning systems used R-12 as a refrigerant. It was replaced with R-134a refrigerant, which has no ozone depletion potential. Old R-12 systems
S-Nitrosothiol exist for their synthesis. They can be synthesized from thiols using NaNO/H+, NO, NO, HNO, NOCl, RONO, NO, HNO, bovine aortic endothelial cells, among others. NaNO/H+ and "tert"-butyl nitrite (tBuONO) are commonly used. Once formed, these deeply colored compounds are often thermally unstable with respect to formation of the disulfide and nitric oxide: "S"-Nitrosothiols release NO upon treatment with acids: and they can transfer nitroso groups to other thiols: "S"-Nitrosothiols can be detected with UV-vis spectroscopy. S-Nitrosothiol "S"-Nitrosothiols, also known as thionitrites, are organic compounds or functional groups containing a nitroso group attached to the sulfur atom of a thiol.