<p>This study investigated the concentration variation and chemical characteristics of particulate matter (PM₁₀ and PM_2.5) at three deep underground metro stations (Dongtan, Guseong and Seongnam Stations) on the GTX-A line in Korea. Real-time particle counters and filter-based samplers were used to assess spatial variations between platforms and concourses, as well as changes in concentration as a function of heating, ventilation, and air conditioning (HVAC) operation. The elemental composition of PM_2.5 was analyzed by X-ray fluorescence (XRF) and compared with previous findings to identify the unique characteristics in deep underground environments. The results showed that PM concentrations at Guseong and Dongtan Stations often exceeded the World Health Organization guidelines (15 μg m⁻³ for PM_2.5 and 45 μg m⁻³ for PM₁₀ (24-hour average)) by a factor of two to three, particularly during non-ventilated periods. Seongnam Station maintained lower PM levels due to the extended duration of HVAC operation. No significant difference in PM concentrations between platform and concourse areas suggests that physical separation alone does not ensure differentiation of air quality in deep stations. Iron (Fe) was the main metallic component of the particulate matter, while chromium (Cr) concentrations at Guseong were 48 and 38 times higher than those reported in cities such as New York and Barcelona, respectively. These findings underscore the need for station-specific air quality management strategies, including continuous ventilation, the adoption of low-wear brake materials, and the implementation of real-time monitoring systems for metal-containing particulates.</p>