are very uncertain and may be the explanation for the broad feature. However, with the present data we cannot prove that hypothesis.

Finally, we note that the [[Si]{}1394,1403Å]{} doublet in PG0843+516 is very poorly fit by our atmosphere model (Fig.\[f-fit1\]). A weaker additional [[Si]{}1394,1403Å]{} absorption is also seen in the spectrum of SDSS1228+1040 (Fig.\[f-fit2\]). We interpret this as evidence for absorption by hot gas close to the white dwarf, see the discussion in Sect.\[s-hotgas\].

  Element                                                                     
  ---------- -------------------- --------------------- --------------------- --------------------
  C            $1.66\times10^{5}$    $4.65\times10^{4}$    $1.25\times10^{5}$   $4.57\times10^{5}$
  O            $9.27\times10^{7}$    $3.78\times10^{7}$    $2.70\times10^{8}$   $5.61\times10^{8}$
  Mg           $4.47\times10^{7}$    $2.66\times10^{7}$    $3.21\times10^{7}$   $1.47\times10^{8}$
  Al           $2.09\times10^{6}$                          $1.18\times10^{7}$   $3.08\times10^{6}$
  Si           $4.77\times10^{7}$    $3.64\times10^{6}$    $4.80\times10^{7}$   $9.93\times10^{7}$
  P            $2.44\times10^{6}$                         $<5.24\times10^{5}$   $7.57\times10^{5}$
  S            $3.92\times10^{7}$                         $<9.46\times10^{6}$   $2.64\times10^{6}$
  Ca                                 $4.84\times10^{6}$    $1.57\times10^{7}$   $8.10\times10^{6}$
  Cr           $3.81\times10^{7}$   $<3.85\times10^{7}$   $<2.29\times10^{7}$   $1.37\times10^{7}$
  Mn                                                                            $1.06\times10^{7}$
  Fe           $7.11\times10^{8}$    $9.50\times10^{7}$    $1.72\times10^{8}$   $6.45\times10^{8}$
  Ni           $1.66\times10^{7}$                         $<9.98\times10^{6}$   $4.71\times10^{6}$
  $\Sigma$     $1.02\times10^{9}$    $1.68\times10^{8}$    $5.61\times10^{8}$   $1.50\times10^{9}$

### Diffusion and stratified atmosphere models {#s-diffusion}

In the absence of a convection zone there is no deep homogenous reservoir in our DAZ sample, and therefore there is no straightforward definition of diffusion time scales. Adopting the usual definition, i.e. dividing the mass of some element above a layer in the envelope or atmosphere of the star by the diffusion flux, results in 