The miniaturized atmospheric pressure glow discharge (APGD) generated between a solid electrode and a flowing small size liquid cathode (dimension 2 mm) was investigated here using optical emission spectroscopy. The discharge was studied in an open air atmosphere, and the spectral characteristics of the plasma source was examined. Analysed APGD was operated at a discharge voltage of 1,100–1,700 V, a discharge current of 20 mA and gaps between a solid anode and a liquid cathode in the range from 0.5 to 3.5 mm. The emission intensities of the main species were measured as a function of various experimental conditions, including the solution flow rate, the gap between the electrodes, and the concentration of hydrochloric acid. The excitation temperature, the vibrational temperatures calculated from N2, OH, and NO bands, and the rotational temperatures determined from band of OH, N2 and NO, were found to be dependent on these experimental parameters. The electron number density was determined from the Stark broadening of Hβ line. Additionally, the ionization temperature and degree were calculated using the Saha–Boltzmann equation, with the ion to atom ratio for magnesium (MgII/MgI). The results demonstrated that Texc(H), Tvib(N2), Tvib(OH), Tvib(NO) and Trot(OH) were well comparable (~3,800–4,200 K) for selected plasma generation conditions (gap ≥2.5 mm, HCl concentration ≥0.1 mol L−1), while the rotational temperatures determined from band of N2 (~1,700–2,100 K) and band of NO (~3,000 K) were considerably lower. The electron number density was evaluated to be (3.4–6.8) × 1020 m−3 and the ionization temperature varied, throughout in the 4,900–5,200 K range.