Understanding the physicochemical properties of inorganic aerosol mixtures is important for accurately predicting their atmospheric behavior and climate impacts. Here, we apply in‑flight X‑ray photoelectron spectroscopy (XPS) to study particle- and gas‑phase chlorine in aerosols generated from nebulized mixed NaCl and (NH₄)₂SO₄ solutions. By detecting Cl 2p photoelectrons from the particle surface and gas‑phase hydrogen chloride (HCl), we provide experimental evidence of gaseous HCl formation during aerosol generation and drying. The dual‑phase capability of the in‑flight XPS approach enables clear identification of how relative humidity (RH) and aerosol mass concentration influence Cl gas-particle partitioning. We demonstrate that HCl formation is strongly dependent on the RH after drying, with increasing HCl evaporation as the RH decreases from 40 % to 10 %, while the extent of HCl formation remains largely independent of the aerosol mass concentration within the range 1700 µg/m³ – 6500 µg/m³. The results provide direct confirmation of key processes governing HCl partitioning in mixed inorganic aerosols and contribute to a better understanding of aerosol chemistry, relevant for atmospheric modeling and climate prediction. These findings further demonstrate the power of combining intense synchrotron radiation, XPS, and an in‑flight aerosol sample‑delivery system.