  In this paper we model the observed absorption measure distribution (AMD) in
Mrk 509, which spans three orders of magnitude in ionization level with a
single-zone absorber in pressure equilibrium. AMD is usually constructed from
observations of narrow absorption lines in radio-quiet active galaxies with
warm absorbers. We study the properties of the warm absorber in Mrk~509 using
recently published broad-band spectral energy distribution observed with
different instruments. This spectrum is an input in radiative transfer
computations with full photoionisation treatment using {\sc titan} code. We
show that the simplest way to fully reproduce the shape of AMD is to assume
that the warm absorber is a single zone under constant total pressure. With
this assumption we found theoretical AMD which matches the observed one
determined on the basis of 600 ks RGS XMM-Newton spectrum of Mrk~509. The
softness of the source spectrum and the important role of the free-free
emission breaks the usual degeneracy in the ionization state calculations, and
the explicit dependence of the depths of AMD dips on density open a new path to
density diagnostic for the warm absorber. In Mrk 509 the implied density is of
the order of $10^8$ cm$^{-3}$.
