Prefer a replacement that releases little reactive chlorine or bromine into the stratosphere: ideally omit those elements, or give the parent gas a short troposphere removal time compared with transport to the stratosphere. A carbon–hydrogen bond can permit rapid attack by the hydroxyl radical; other readily oxidized functionality can also shorten the atmospheric residence time. The degradation products must themselves avoid transporting long-lived reactive halogen to altitude. Merely retaining fewer chlorine atoms is less effective than preventing their delivery altogether.
Both chlorine and bromine participate in catalytic ozone destruction. Bromine is much more effective per delivered atom, by a factor of roughly 60 in the usual lower-stratospheric comparison; the precise effectiveness depends on chemical partitioning and location. Fluorine is largely trapped in very stable hydrogen fluoride and does not sustain an analogous ozone-destroying cycle. Thus short-lived gases without chlorine or bromine have the smallest direct ozone-depletion potential. Fluorination alone does not ensure a small climate impact.[ref]
A small global warming potential requires both a small greenhouse gas radiative efficiency per unit emitted mass and a short atmospheric lifetime. Weak absorption in the terrestrial atmospheric window is desirable, particularly where absorption by existing gases is weak; strong carbon–fluorine vibrational bands can make even an ozone-safe substitute an efficient greenhouse gas. Rapid removal by lower-atmospheric chemistry limits the integrated burden, provided it does not generate abundant, long-lived radiatively active products. The atmospheric residence time and the absorption spectrum must therefore be assessed separately.
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