Isotopic investigation of anthropogenic- and climate-driven changes in sulfate and nitrate aerosol production

Abstract

The oxygen triple-isotopic composition (Δ<super>17</super>O) of sulfate or nitrate provides insight into the relative importance of the different pathways that lead to their formation in the atmosphere, with implications for their radiative impact, lifetime, and aerosol chemistry. Measurements of sulfate and nitrate Δ<super>17</super>O from ice cores provide constraints on long-term (up to 100 kyr) changes in the formation pathways of sulfate and nitrate and have the potential to constrain changes in the abundances of the oxidants responsible for their formation. Quantitatively connecting changes in the oxygen isotopes of sulfate and nitrate to changes in atmospheric conditions remains a key challenge in the application of these measurements to paleo-chemistry. In this work, a new ice core record of sulfate and nitrate isotopes from WAIS Divide, Antarctica is presented, and global chemical transport models and box models of sulfate and nitrate formation are used to quantitatively interpret ice core records of nitrate and sulfate Δ<super>17</super>O over a variety of time scales. In Ch. 2, a global chemical transport model is used to simulate preindustrial to present-day changes in tropospheric oxidants and the Δ<super>17</super>O of sulfate due to changing emissions only, constrained by previously published Greenland and Antarctic ice core records. The sulfate Δ<super>17</super>O record in Greenland demonstrates the increasing importance of metal-catalyzed sulfate production by O₂ due to increases in anthropogenic emissions of transition metals. Antarctic sulfate Δ<super>17</super>O indicates extratropical Southern Hemisphere increases in both H₂O₂ and O₃ by 51% and 27%, respectively. This provides a constraint on the relative increases in the abundance of O₃ and H₂O₂ in the remote Southern Hemisphere due to anthropogenic activity. In Ch. 3, a new ice core record of both nitrate and sulfate isotopes from the West Antarctic Ice Sheet (WAIS) Divide ice core spanning the past 2400 years is presented. There is a large (1.1‰) step increase in sulfate Δ<super>17</super>O in the early 19th century, while nitrate Δ<super>17</super>O shows a more gradual downward trend of 5.6‰ between the mid-19th century and the present-day. Using other chemical measurements from the WAIS Divide ice core, global chemical transport models, and box models, we investigate the possible explanations for the variability observed in the new ice core record. The increase in sulfate Δ<super>17</super>O suggests an increase in aqueous-phase sulfate production by O₃ that is difficult to reconcile with our understanding of sulfate chemistry and suggests that other oxidants (e.g. hypohalous acids, HOCl and HOBr) may play an important role in extratropical Southern Hemisphere marine boundary layer sulfate formation. The decrease in nitrate Δ<super>17</super>O is consistent with an increase in the importance of RO₂ relative to O₃, suggesting a 50-80% decrease in the [O₃]/[RO₂] ratio in extratropical Southern Hemisphere NO_x-source regions resulting from anthropogenic activity. Existing measurements of sulfate Δ<super>17</super>O from the Vostok, Antarctica ice core spanning the last glacial-interglacial cycle are consistent with greater gas-phase sulfate formation by OH during the last glacial period (110 - 15 kya) than during either the Holocene (11.7 kya to present) or Eemian (130 - 114 kya) interglacial periods. However, the drivers for this change are unclear. In Ch. 4, a box model of sulfate formation with boundary conditions from the offline-coupled climate/biosphere/atmospheric chemistry modeling project ICE age Chemistry And Proxies (ICECAP) is used to quantitatively investigate the relative importance of of the different factors that impact sulfate formation. It is demonstrated that the increase in the concentration of OH in the extratropical Southern Hemisphere during the last glacial period is can explain most (up to two thirds) of the increase in gas-phase sulfate production. The reduction in the glacial cloud fraction leads to a further increase in marine boundary layer sulfate formation in the gas phase. The reduction in clouds has a secondary effect of allowing a greater fraction of SO₂ to reach the free troposphere, where it is oxidized by OH. This leads to a longer lifetime of sulfate in the glacial period (from 3 days to 4 days), enabling more efficient long-range transport of sulfate to remote areas such as Antarctica. This likely explains at least part of the observed increase in ice core sulfate concentrations during the glacial period and also has implications for the influence of sulfate aerosols on climate. An increase in metal-catalyzed oxidation by O₂ in clouds may also play a role in explaining the observed variability in sulfate Δ<super>17</super>O in the ice core record. However, the magnitude of the change in Fe(III) and Mn(II) due to increases in dust in the glacial period are thought to be too small to be significant.