Inorganic aerosols account for a large portion of PM_2.5 mass, and are composed of a variety of species including metal ions and ions formed from secondary formation. In addition, three water-soluble gases: nitric acid, ammonia and hydrochloric acid, can have a dramatic impact on the inorganic PM concentrations. Therefore, the abundance, diurnal pattern, and composition of PM_2.5 inorganic aerosols and water-soluble gases were investigated in an urban (Yuen Long) and suburban (HKUST) site in Hong Kong in the study. The measurement was conducted with the help of the Monitor for Aerosols and GAses (MARGA), an online measurement system with hourly time resolution for analysis of SO₄^2-, NO₃^-, Cl^-, NH₄⁺, Ca²⁺, Mg²⁺, K⁺ (aerosol) and HNO₃, NH₃, HCl, SO₂, and HONO (gas) at a very low detection limit. Through studying inorganic aerosols and associated gases together with meteorological parameters and air mass trajectories, chemical analysis of inorganic aerosols can be explored.