Long-term mercury isotope evidence for a shift toward background-dominated urban atmospheric mercury in North China under sustained emission controls

Abstract. Evaluating the effectiveness of the Minamata Convention requires a clear understanding of how emission controls reshape atmospheric mercury (Hg) budgets. Here, we present a multi-year investigation of gaseous elemental Hg (GEM) concentrations and isotope compositions in urban Tianjin, China, spanning three distinct periods: a pre-control phase (2018), the COVID-19 lockdown (2021–2022), and a post-pandemic phase under strengthened controls (2024–2025). By integrating long-term monitoring with isotope-based source apportionment, we capture changes in Hg sources and processes that are not evident from concentration data alone. GEM concentrations declined sharply from pre-control levels (~4.6 ng m⁻³) to regional background values (~1.5 ng m⁻³) during the COVID-19 lockdown, with no rebound following the resumption of socioeconomic activities. This sustained decline was accompanied by a pronounced isotopic transition, from negative δ²⁰²Hg and near-zero Δ¹⁹⁹Hg and Δ²⁰⁰Hg values characteristic of primary anthropogenic emissions to near-zero to positive δ²⁰²Hg and negative Δ¹⁹⁹Hg and Δ²⁰⁰Hg values indicative of the regionally well-mixed background Hg pool. Comparisons with other cities in China and South Asia further demonstrate that effective emission controls drive convergence toward background-like GEM concentrations and isotopic signatures. Isotopic mixing models indicate that the collapse of primary anthropogenic emissions accounted for nearly all of the observed concentration decline since the 2020s. Together, our results reveal a fundamental regime shift in urban atmospheric Hg cycling from local primary emission-dominated to background-dominated conditions modulated by secondary surface processes.