Since the banning of freon as a coolant in automobile air conditioners, there has been some confusion over which coolants are suitable for use in which automobiles. Production of freon, a chlorofluorocarbon (CFC), was stopped in 1997 because it is an ozone depleting substance. But it is still being recycled and used in older air conditioners until it is phased out completely. Newer auto air conditioners are designed to use a hydrofluorocarbon called HP-134a. Although it is not an ozone depleting substance, it is a greenhouse gas-one that absorbs infrared radiation that is re-radiated from the earth’s surface when heated by the sun. And Chemical and Engineering News recently reported that a company was selling a hydrocarbon-based substance as a replacement for freon. (1).

There are two categories of greenhouse gases, natural occurring and synthetic. Because they absorb this radiation, they all contribute to some extent to the warming of the earth.

Naturally occurring gases:	Synthetic gases:
water vapor (H2O)	chlorofluorocarbons (CFCs)
carbon dioxide (CO2)	hydrochlorofluorocarbons (HCFCs)
nitrous oxide (N2O)	hydrofluorocarbons (HFCs)
methane (CH4)	perfluorocarbons (PFCs)
ozone (O3)	sulfur hexachloride (SF6)

Looking at the graph below, you might conclude that synthetic gases are not significant contributors to the greenhouse effect. After all, these gases together have constitute only 1.9% of the total while carbon dioxide alone constitutes 82.3%. But while synthetic gases are a very small fraction of the total, their concentrations are increasing. In addition, some of them have a very long life?for instance, SF₆ has a life of 3000 years.

*MMTCE= 29.7 Tg(Teragrams) or 29.7 x 10¹² g

Freon, which is a derivative of methane (CH₄), has a molecular formula of CCl₂F₂. Because it is recycled for use in older air-conditioners, it still escapes into the atmosphere despite efforts to prevent its escape. Freon has a long life so its concentration will decrease very slowly in the atmosphere. The replacement gas HP-134a is a derivative of ethane, CH₃CH₃, in which four of the hydrogen atoms are replaced with fluorine to give CH₂FCF₃. This gas also contributes to the greenhouse effect, and its concentration will increase with time as more is produced. However, it does not contribute to the depletion of ozone in the stratosphere since it does not have any chlorine.

According to the Chemistry and Engineering News article, some auto services are mixing these gases in air-conditioning systems, probably in part based on costs and ignorance. This is not a good practice since the newer air-conditioners are designed only for HP-134a. And hydrocarbons should never be used as a coolant unless the air-conditioner is especially designed for its use. In the future, manufacturers may use carbon dioxide as a coolant. This would not contribute to the greenhouse effect since carbon dioxide comes from the distillation of liquid air.

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