Martian Air Pressure, How much variance is there? Options

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Just thinking when looking at the Victoria pics...how much does the average air pressure increase with respect to the surrounding area at the bottom of the crater? Also, where is the lowest point on Mars (which also presumably has the highest air pressure)? I can't remember if it's in Hellas or Coprates Chasma.

Rationale here is that, given MRO's astonishing capabilities, these low areas should be given priority for long-term monitoring; if transient surface water exists anywhere, it's probably way down in a crater or a canyon.