  Several states of proton-unbound isotopes $^{30}$Ar and $^{29}$Cl were
investigated by measuring their in-flight decay products,
$^{28}$S+proton+proton and $^{28}$S+proton, respectively. A refined analysis of
$^{28}$S-proton angular correlations indicates that the ground state of
$^{30}$Ar is located at $2.45^{+0.05}_{-0.10}$ MeV above the two-proton
emission threshold. The theoretical investigation of the $^{30}$Ar ground state
decay demonstrates that its mechanism has the transition dynamics with a
surprisingly strong sensitivity of the correlation patterns of the decay
products to the two-proton decay energy of the $^{30}$Ar ground state and the
one-proton decay energy as well as the one-proton decay width of the $^{29}$Cl
ground state. The comparison of the experimental $^{28}$S-proton angular
correlations with those resulting from Monte Carlo simulations of the detector
response illustrates that other observed $^{30}$Ar excited states decay by
sequential emission of protons via intermediate resonances in $^{29}$Cl. Based
on the findings, the decay schemes of the observed states in $^{30}$Ar and
$^{29}$Cl were constructed. For calibration purposes and for checking the
performance of the experimental setup, decays of the previously-known states of
a two-proton emitter $^{19}$Mg were remeasured. Evidences for one new excited
state in $^{19}$Mg and two unknown states in $^{18}$Na were found.
