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The term surface abundance, the abundance of something at something's surface, is often used for stars, to indicate the abundance of some element or compound at the surface, which is the part of the star producing spectral lines, thus where the abundance of elements can be measured.
For example, the solar lithium problem is the issue that the measured surface abundance of lithium on the Sun is much lower than would be expected given matter which is presumed to make up the Sun. Solutions to the problem include theories that the lithium has been mixed in the Sun in such a manner that the surface abundance is lowered.