When it comes the average lifetime of chemicals in the atmosphere, is the mean lifetime ~1.4427 times the half-life? As with radioactivity?

There are so many websites giving numbers for ‘average’ lifetimes of global-warming or ozone-depleting chemicals in the atmosphere, yet they are rarely explicit in their definition(s)…

For instance, methane is usually stated to last 12.4 years…. But is this half-life (of a large amount) or mean lifetime (of a single molecule)?