Mercury Pollution in a Coastal City of Northern China Driven by Temperature Re-Emission, Coal Combustion, and Port Activities

This study investigates the dynamics and sources of atmospheric mercury in Qinhuangdao (QHD), a coastal urban area significantly impacted by both marine and terrestrial sources. Sampling of gaseous elemental mercury (GEM), fine particle-bound mercury (PBM2.5), and coarse particle-bound mercury (PBM2.5–10) was conducted from September 2022 to August 2023. The annual mean concentrations of GEM, PBM2.5, and PBM2.5–10 were 2.66, 1.01, and 0.73 ng m−3, respectively, with PBM levels among the highest reported for coastal cities in eastern China. GEM displayed a pronounced midday peak (12:00–14:00) with correlations to temperature (R2 = 0.25–0.65) and a significant winter association with SO2 (R2 = 0.52), suggesting the combined influence of surface re-emission and coal combustion. Seasonal variations in the GEM/CO ratio (spring: 7.12; winter: 2.62) further reflected the shift between natural and combustion-related sources. PBM2.5 exhibited elevated concentrations (1.0–1.4 ng m−3) under westerly winds (~3 m s−1), indicating inputs from traffic, shipping, and light industries, while PBM2.5–10 (0.5–1.1 μg m−3) was strongly linked to coal-handling activities at QHD port and soil resuspension. Backward trajectory analysis showed continental air masses dominated in winter (53–100%) and maritime air masses in summer (30–50%), whereas high Hg/Na ratios in PM2.5 (3.22 × 10−4) and PM2.5–10 (2.17 × 10−4), far exceeding typical marine aerosol values (10−7–10−5), indicated negligible marine contributions to PBM. These findings provide new insights into the processes driving mercury pollution in coastal urban environments and highlight the critical role of port-related activities in regional mercury management.