  The knowledge of scintillation quenching of $\alpha$-particles plays a
paramount role in understanding $\alpha$-induced backgrounds and improving the
sensitivity of liquid argon-based direct detection of dark matter experiments.
We performed a relative measurement of scintillation quenching in the MeV
energy region using radioactive isotopes ($^{222}$Rn, $^{218}$Po and $^{214}$Po
isotopes) present in trace amounts in the DEAP-3600 detector and quantified the
uncertainty of extrapolating the quenching factor to the low-energy region.
