Revealing Vertical Distribution of Atmospheric Mercury Using Drone-Based Monitoring Technique: A Case Study in Vietnam

Unmanned aerial vehicles (UAVs) have emerged as effective tools for monitoring air pollution across varying altitudes, including assessing atmospheric mercury (Hg) levels. However, studies on the vertical distribution of atmospheric Hg (i.e., total gaseous mercury–TGM) concentrations remain limited, particularly in Southeast Asia. This study utilized a UAV equipped with a TGM sampling device to measure concentrations at different altitudes in Ben Cat City, an industrial area in Southern Vietnam. The purpose of this study is to examine the applicability of UAV in investigating the altitudinal distribution of TGM and to analyze specific case studies related to Hg emissions from stack. A total of 36 flight experiments were conducted (including 36 concurrently ground level measurements), including 50 m (20 flights), 200 m (7 flights), and 500 m (9 flights). TGM concentrations increase noticeably with altitude under stack emission conditions, while they remain relatively consistent at all altitudes during non-emission conditions. Under the emission conditions, three vertical distribution patterns were observed: (1) elevated TGM concentrations at higher altitudes compared to ground level; (2) lower TGM concentrations at higher altitudes relative to ground level; and (3) nearly equivalent TGM concentrations between ground level and higher altitudes, with differences less than 0.4 ng m−3. The observed distributions imply the important role of atmospheric dynamics in understanding the dispersion of pollutants and the impact of emissions. This study pioneers the use of UAVs in Vietnam for simultaneous TGM measurements across altitudes, highlights their potential for atmospheric Hg monitoring, and improves stack emission management.