Quantifying the distribution and origins of aerosol zinc across the Northern Hemisphere using stable zinc isotopes: A review.

Quantitatively assessing the origins of aerosol zinc (Zn) is crucial for understanding of the global atmospheric Zn cycle and for formulating targeted policies to mitigate anthropogenic Zn emissions. Zn isotope ratios (denoted as δ⁶⁶Zn) serve as powerful tools for constraining the origins of aerosol Zn. This review comprehensively compiles an δ⁶⁶Zn (relative to Lyon JMC Zn standard) dataset (n = 207) for multi-sized aerosols observed exclusively in the Northern Hemisphere, encompassing diverse atmospheric environments, including urban areas and remote deserts, glacier, and ocean. Reanalysis of the integrated dataset reveals both the spatial heterogeneity of aerosol δ⁶⁶Zn and its partitioning pattern across various size fractions. Spatially, aerosol δ⁶⁶Zn exhibits an upward trend from urban areas to ocean, deserts, and ultimately to glacier environments, accompanied by a shift in isotopic signatures from negative to very positive values. Notably, with the exception of desert aerosols, those from other atmospheric environments exhibit δ⁶⁶Zn that deviate significantly from the Bulk Silicate Earth (BSE) background value. In terms of particle size, δ⁶⁶Zn in size-fractionated aerosols displays an initial decrease followed by an increase as particle size decreases, with the 2.7 µm serving as a transitional boundary. Given the limited research on quantifying the origins of aerosol Zn, this review conducts a quantitative source allocation of aerosol Zn by incorporating a compiled δ⁶⁶Zn dataset into the MixSIAR model. Four primary endmembers are defined, each with respective δ⁶⁶Zn values: natural dust (+0.25 ± 0.15 ‰), metal smelting (-0.68 ± 0.07 ‰), coal combustion (+1.31 ± 0.11 ‰), and non-exhaust traffic emissions (+0.22 ± 0.07 ‰). Model calculations indicate that beyond urban areas-which are primary hotspots for anthropogenic Zn impacts-over half of atmospheric Zn in remote glacier and ocean regions originates from anthropogenic emissions. This underscores the pervasive influence of human activities on the atmospheric Zn cycle, even in geographically remote regions. To deepen understanding of the global Zn transport and cycle, future research should prioritize expanding isotope observations in the Southern Hemisphere.