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The Moon possesses our nearest example of a surface-bounded exosphere, the most common type of atmosphere in the solar system. Argon is the most abundant species in the lunar exosphere and was recently measured there in great detail by the Lunar Atmosphere and Dust Environment Explorer (LADEE). The results included the discovery of a persistent argon enhancement over the western maria - the 'bulge' - prompting competing proposed explanations. Using an empirically motivated model of the Moon’s argon exosphere to test these hypotheses, we perform the first simulations with: spatially varying surface interactions; a source that reflects the lunar near-surface potassium distribution; and temporally varying cold trap areas. We find that the near-sunrise peak in argon exospheric density is the same for both the highland and mare regions, and is well reproduced by simple surface interactions with a constant desorption energy. Only a localised source is able to reproduce the observed overdensity, and must be either highly localised or have a very high equilibrium source (and loss) rate. We also demonstrate that large, seasonally varying cold traps could explain the long-term variation in the global argon density observed by LADEE.