Different response characteristics of ambient hazardous trace metals and health impacts to global emission reduction

Airborne hazardous trace metals pose significant risks to human health. However, the response characteristics of ambient trace metals to emission reductions remain poorly understood. The COVID-19 pandemic offered a unique opportunity to investigate these response mechanisms and optimize emission control strategies. In this study, we employed the Global Modeling System for Chemistry (GEOS-Chem) chemical transport model to predict global variations in atmospheric concentrations of nine hazardous trace metals (As, Cd, Cr, Cu, Mn, Ni, Pb, V, and Zn) and assess their responses to COVID-19 lockdown measures. Our results revealed that global average concentrations of As, Cd, Cr, Cu, Mn, Ni, and V decreased by 1 %-7 %, whereas Pb and Zn levels increased by 0.4 % and 2 %, respectively. The rise in Pb and Zn concentrations during lockdowns was primarily linked to sustained coal combustion and non-ferrous smelting activities, which remained essential for residential energy demands. Spatially, India, Western Europe (WE), and North America (NA) experienced the most pronounced declines in trace metal levels, while Sub-Saharan Africa (SS) and Australia showed minimal sensitivity to lockdown-induced emission reductions. Based on the scenario analysis, we found the concentrations of trace metals displayed linear response to emission reduction. Combined with the health risk assessment, we demonstrated the reduced emissions of Pb and As during the lockdown period yielded the greatest health benefits - Pb reductions were associated with lower non-carcinogenic risks, while As declines contributed most significantly to reduced carcinogenic risks. Targeting fossil fuel combustion should be prioritized in Pb and As mitigation strategies.