Sulfur isotopes quantify the impact of anthropogenic activities on industrial-era Arctic sulfate in a Greenland ice core

Abstract

Anthropogenic sulfate aerosols are estimated to have offset sixty percent of greenhouse-gas-induced warming in the Arctic, a region warming four times faster than the rest of the world. However, sulfate radiative forcing estimates remain uncertain because the relative contributions from anthropogenic versus natural sources to total sulfate aerosols are unknown. Here we measure sulfur isotopes of sulfate in a Summit, Greenland ice core from 1850 to 2006 CE to quantify the contribution of anthropogenic sulfur emissions to ice core sulfate. We use a Keeling Plot to determine the anthropogenic sulfur isotopic signature (δ34Santhro = +2.9 ± 0.3 ‰), and compare this result to a compilation of sulfur isotope measurements of oil and coal. Using δ34Santhro, we quantify anthropogenic sulfate concentration separated from natural sulfate. Anthropogenic sulfate concentration increases to 68 ± 7% of non-sea-salt sulfate (65.1 ± 20.2 µg kg-1) during peak anthropogenic emissions from 1960 to 1990 and decreases to 45 ± 11% of non-sea-salt sulfate (25.4 ± 12.8 µg kg-1) from 1996 to 2006. These observations provide the first long-term record of anthropogenic sulfate distinguished from natural sources (e.g., volcanoes, dimethyl sulfide), and can be used to evaluate model characterization of anthropogenic sulfate aerosol fraction and radiative forcing over the industrial era.

Primary Sulfate Is the Dominant Source of Particulate Sulfate during Winter in Fairbanks, Alaska

“…δ 34 S emission is assumed to be the same as primary sulfate (δ 34 S primary = +4.7(±0.6)‰) since sulfur isotope fractionation of fuel oil during high-temperature combustion is expected to be minimal . This is consistent with previous studies that assume primary sulfate from combustion retains the δ 34 S(SO 4 2– ) signature of the sulfur source. − We assume that Fairbanks is a closed system, where long-range transport of biogenic and volcanic sulfur can be neglected. This assumption is further supported by prior literature showing that the pollution layer is often confined lower than 20 meters in Fairbanks with the highest PM 2.5 concentrations below 3 meters. ,,, On-road mobile sampling performed by Robinson et al (2023) found the lowest PM 2.5 concentrations at the top of hills and asserted that residential neighborhoods were unequivocally the dominant PM source. δ 34 S false( normalSO 4 2 − false) false( ‰ false) = f primary × δ 34 S primary + false( 1 − normalf normalprimary false) × false( δ 34 normalS normalemission − ( f H 2 O 2 · ε H 2 O 2 + f O 3 · ε …”

Industrial-era decline in Arctic methanesulfonic acid is offset by increased biogenic sulfate aerosol

“…We measure concentrations of MSA and sulfate (SO 4 2– ) and sulfur isotopes (δ 34 S) of sulfate in ice core samples from 1200 to 2006 CE ( Materials and Methods ). Sulfur isotopes are used to estimate the relative contribution from each of the main sources of Arctic sulfate: DMS, volcanic, and anthropogenic emissions ( 30 – 35 ). The influence of sea salt is subtracted from the sulfate concentration and sulfur isotope measurements ( Materials and Methods ).…”

“…where f anthro is the fraction of anthropogenic sulfate in the sample and δ 34 S anthro is the anthropogenic sulfur isotopic source signature, which is estimated to be +2.9 ± 0.3‰ [ SI Appendix, Supplementary Text S3 , ( 35 )]. We assume that source signatures (δ 34 S bio , δ 34 S volc , and δ 34 S anthro ) are relatively constant over time ( SI Appendix, Supplementary Text S3 ).…”

“…The isotopic source signatures of volcanic, DMS-derived biogenic, and anthropogenic sulfur are determined via two methods described in detail in Jongebloed et al ( 34 , 35 ) and SI Appendix, Supplementary Text S3 . The first method is a Keeling Plot, which is the regression of δ 34 S(nssSO 4 2– ) measurements against the reciprocal of the concentration measurements (1/[nssSO 4 2– ]) ( 36 – 38 ) and is used to determine δ 34 S volc and δ 34 S anthro ( SI Appendix , Fig.…”

“…Other sulfate sources to the Arctic and global atmosphere include volcanic degassing, sea salt, and anthropogenic sources, including fossil fuel combustion and metal smelting ( 29 ). Here, we leverage the distinct isotopic composition of DMS-derived biogenic sulfur compared to that of other sources ( 30 – 35 ) to quantify the contribution of DMS to total sulfate in a central Greenland ice core covering 1200 to 2006 CE. We compare this DMS-derived biogenic sulfate to measurements of MSA from the same ice core samples to assess MSA’s utility as a proxy for DMS emissions and to examine trends in DMS-derived biogenic sulfur aerosol over this time period.…”