  Iron-arsenide superconductor Na$_{1-\delta}$FeAs is highly reactive with the
environment. Due to the high mobility of Na ions, this reaction affects the
entire bulk of the crystals and leads an to effective stoichiometry change.
Here we use this effect to study the doping evolution of normal and
superconducting properties of \emph{the same} single crystals. Controlled
reaction with air increases the superconducting transition temperature, $T_c$,
from the initial value of 12 K to 27 K as probed by transport and magnetic
measurements. Similar effects are observed in samples reacted with Apiezon
N-grease, which slows down the reaction rate and results in more homogeneous
samples. In both cases the temperature dependent resistivity, $\rho_a(T)$,
shows a dramatic change with exposure time. In freshly prepared samples,
$\rho_a(T)$ reveals clear features at the tetragonal-to-orthorhombic ($T_s$
$\approx$ 60 K) and antiferromagnetic ($T_m$=45 K) transitions and
superconductivity with onset $T_{c,ons}$=16 K and offset $T_{c,off}$=12 K. The
exposed samples show $T-$linear variation of $\rho_a(T)$ above $T_c,ons$=30 K
($T_{c,off}$=26 K), suggesting bulk character of the observed doping evolution
and implying the existence of a quantum critical point at the optimal doping.
The resistivity for different doping levels is affected below $\sim$200 K
suggesting the existence of a characteristic energy scale that terminates the
$T-$linear regime, which could be identified with a pseudogap.
