Atmospheric-pressure microplasma jets (μAPPJs) are versatile sources of reactive species with diverse applications. However, understanding the plasma chemistry in these jets is challenging due to plasma-flow interactions in heterogeneous gas mixtures. Spatial metastable density profiles help to understand these physical and chemical mechanisms. This work focuses on controlling the shielding gas around a μAPPJ. We use a dielectric barrier discharge co-axial reactor where a co-flow shields the pure argon jet with different N 2 -O 2 gas mixtures. A voltage pulse (4 kV, 1 μs, 20 kHz) generates a first discharge at the pulse’s rising edge and a second discharge at the falling edge. Tunable diode laser absorption spectroscopy measures the local Ar(1s 5 ) density. A pure N 2 (100%N 2 -0%O 2 ) co-flow leads to less reproducible and lower peak Ar(1s 5 ) density (5.8 × 10 13 cm −3 ). Increasing the O 2 admixture in the co-flow yields narrower Ar(1s 5 ) absorbance profiles and increases the Ar(1s 5 ) density (6.9 × 10 13 - 9.1 × 10 13 cm −3 ). The position of the peak density is closer to the reactor for higher O 2 fractions. Absence of N 2 results in comparable Ar(1s 5 ) densities between the first and second discharges (maxima of 9.1 × 10 13 and 9.3 × 10 13 cm −3 , respectively). Local Ar(1s 5 ) density profiles from pure N 2 to pure O 2 shielding provide insights into physical and chemical processes. The spatially-resolved data may contribute to optimising argon μAPPJ reactors across the various applications and to validate numerical models.