Provenance of Anthropogenic Pb and Atmospheric Dust to Northwestern North America

Abstract

Industrial activities release aerosols containing toxic metals into the atmosphere, where they are transported far from their sources, impacting ecosystems and human health. Concomitantly, long-range-transported mineral dust aerosols play a role in Earth's radiative balance and supply micronutrients to iron-limited ecosystems. To evaluate the sources of dust and pollutant aerosols to Alaska following the 2001 phase-out of leaded gasoline in China, we measured Pb-Sr-Nd isotopic compositions of particles collected in 2016 from snow pits across an elevational transect (2180-5240 m-a.s.l) in Denali National Park, USA. We also determined Pb flux and enrichment from 1991-2011 in the Denali ice core (3870 m-a.s.l). Chinese coal-burning and non-ferrous metal smelting account for up to 64% of Pb deposition at our sites, a value consistent across the western Arctic. Pb isotope ratios in the aerosols did not change between 2001 and 2016, despite the ban on lead additives. Emissions estimates demonstrate that industrial activities have more than compensated for the phase-out of leaded gasoline, with China emitting ∼37,000 metric tons year^-1 of Pb during 2013-2015, approximately 78% of the Pb from East Asia. The Pb flux to Alaska now equals that measured in southern Greenland during peak pollution from North America.

Keywords: Alaska; China; North Pacific; Sr-Nd-Pb isotopes; air pollution; ice core; lead emissions; lead flux.

Figure 1

Maps showing sites discussed in the text. (A) Overview map showing snow pit sampling locations: Denali (yellow star, this study), the St. Elias mountains (red star^,,), and Barrow, Alaska and the Chukchi Sea sector of the Arctic Ocean (triangles). (B) Digital elevation model showing locations of sediments collected to characterize local dust source compositions. Samples were collected from the three major drainages of southcentral Alaska: the Susitna, Knik, and Copper Rivers. (C) Schematic topographic profile showing snow pit elevations in Denali National Park. Mt. Hunter is also the site of the Denali deep ice core used to estimate Pb flux and enrichment.

Figure 2

Snow pit Sr-Nd-Pb isotope data from Denali and the St. Elias plotted as a function of elevation. (A) ⁸⁷Sr/⁸⁶Sr, (B) εNd, (C) ²⁰⁶Pb/²⁰⁷Pb, and (D) ²⁰⁸Pb/²⁰⁷Pb. The elevation dependence reflects aerosol contributions from different mixing ratios of sources at different elevations. Error bars are plotted where they are greater than symbol size.

Figure 3

Snow pit Sr-Nd isotope compositions from Alaska, the St. Elias, and the western Arctic compared to potential dust source regions.^,, St. Elias snow pit elevations are as follows: red star, 2620 m; orange star, 2800 m; yellow star, 4150 m a.s.l. Barrow and Arctic Ocean samples are from sea level. The dotted lines show calculated mixtures of southcentral Alaska dust with Region A and Region C desert sediments that could plausibly produce several of the observed snow pit dust compositions. Beijing loess data are shown for context but are not considered an independent source region. Source region data include the <5 μm fraction from refs (31) and (50)and the HOAc-residue fraction from ref (49). Error bars are generally smaller than symbols. Camp 14 data have been corrected to remove a volcanic contribution (see the SI).

Figure 4

²⁰⁶Pb/²⁰⁷Pb vs ²⁰⁸Pb/²⁰⁷Pb of Alaska and Arctic snow pit samples compared to potential dust^, and pollution^,,, sources. Denali Base Camp samples fall within the field of southcentral Alaska glaciogenic sediments (A), while higher-elevation samples follow a mixing line between a mixed-dust end-member and a pollution end-member (B). Barrow, Alaska and Arctic Ocean snow samples follow a similar trend, and Arctic samples are indistinguishable from Chinese aerosols. The mixed-dust end-member represents an ∼60/40 mixture of southcentral Alaska and Asian dust source sediments, based on the Sr-Nd isotope data. The pollution end-member, with ²⁰⁶Pb/²⁰⁷Pb of 1.1542 and ²⁰⁸Pb/²⁰⁷Pb of 2.4386, is calculated using ice core Pb enrichment data from Mt. Hunter, assuming the other Pb source is the mixed-dust end-member (see text). China unleaded fuels are shown for reference but are not considered a major Pb source. Error bars on samples from this study represent 2σ external errors and are smaller than the symbol size.

Figure 5

Atmospheric Pb emissions estimates for countries in Asia. (A) Gasoline, (B) coal-burning, and (C) smelting of Cu, Ni, Pb, and Zn ores, which comprise the three largest sources of Pb emissions over the period shown. (D) Total Pb emissions are calculated as the sum of leaded gasoline, coal-burning, and non-ferrous metal smelting emissions. *Excludes additional minor sources of Pb emissions, including iron and steel smelting, unleaded vehicle exhaust, cement production, and waste incineration. (E) Comparison of Chinese Pb emissions by source. Gasoline data, coal data up to 1989, and smelting data up to 2002 are from ref (39). Offsets in panel (C) likely reflect differences in how the underlying ore data were reported, as the same approach was used to calculate Pb emissions between the two studies; see Materials and Methods and the SI for details and data sources.

Figure 6

Bar plot showing median source proportions of pollution Pb calculated for each site. The major portion (21–64%) of pollution Pb deposited in high-elevation and high-latitude sites can be attributed to Chinese pollution sources, with lesser contributions (totaling 5–16%) from Japan, South Korea, and Russia. The Barrow and Arctic Ocean data are from ref (22).

Figure 7

Comparison of Denali high-elevation snow pit Pb isotope compositions from 2016 (yellow squares) with Eclipse Icefield ice core data (circles, color bar in years C.E.). Panels show the progressive shift toward higher ²⁰⁸Pb/²⁰⁷Pb values over the past 50 years including (A) 1970s, (B), 1980s, (C), 1990s, and (D) 2000–2001. Panel (D) shows the mixed-dust end-member (teal diamond) and pollution end-member (gray circle) inferred for the Denali samples. Error bars for the Denali samples are smaller than the symbol size. Eclipse error bars can be seen in Figure S7. Source data citations are the same as in Figure 4.