  Metal surfaces with disorder or with nanostructure modifications are studied,
allowing for a localized charge layer (CL) in addition to continuous charges
(CC) in the bulk, both charges having a compressional or diffusive non-local
response. The notorious problem of "additional boundary conditions" is resolved
with the help of a Boltzmann equation that involves the scattering between the
two charge types. Depending on the strength of this scattering, the oscillating
charges can be dominantly CC or CL; the surface plasmon (SP) resonance acquires
then a relatively small linewidth, in agreement with a large set of data. With
a few parameters our model describes a large variety of SP dispersions
corresponding to observed data.
