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Offhand it looks like there's barely any pollution in there, but since the tiles have all been made concrete there isn't any pollution absorption anymore (concrete absorbs 0, most other tiles have a non-zero amount) so it can't dissipate. Logic for making pollution spread IIRC uses the amount in the chunk to determine how much spreads, so with those tiny amounts almost none (if any) is going to move to the adjacent chunks where the grass can absorb it.

If you got rid of the concrete in the four middle squares of each roboport, you maybe wouldn't be able to see they're missing but you'd still have a non-zero pollution absorption in those chunks, so it'd go away eventually.

Because no grass?

Pollution can spread only if its above 15.0 but non of those chunks have 15.0+ pollution value
and because u have 0 grass on this chunk u can't remove pollution...

first number -> Pollution value
second number -> Pollution that is gone every minute

//Edit:
ohh i didnt see Silari answer xD because i skipped everything blow ur pics to answer...
thats why u put pictures into spoiler... lol...

To expand slightly on the two responses,

Its because putting down ANY tiles (stone brick, concrete, refined concrete, hazard concrete, hazard refined concrete) means that that particular tile within the chunk stops absorbing pollution. If you remove any means of pollution absorption (landfill in all the water, clear out all the trees, and pave the entire chunk with stone brick or other pavement) then there is simply no means by which the chunk can ever completely go pollution free.