WAIS Divide ice core suggests sustained changes in the atmospheric formation pathways of sulfate and nitrate since the 19th century in the extratropical Southern Hemisphere

Abstract. The ¹⁷O excess (Δ¹⁷O = δ¹⁷O−0.52 × δ¹⁸O) of sulfate and nitrate reflects the relative importance of their different production pathways in the atmosphere. A new record of sulfate and nitrate Δ¹⁷O spanning the last 2400 years from the West Antarctic Ice Sheet Divide ice core project shows significant changes in both sulfate and nitrate Δ¹⁷O in the most recent 200 years, indicating changes in their formation pathways. The sulfate Δ¹⁷O record exhibits a 1.1 ‰ increase in the early 19th century from (2.4 ± 0.2) ‰ to (3.5 ± 0.2) ‰, which suggests that an additional 12–18% of sulfate formation occurs via aqueous-phase production by O₃, relative to that in the gas phase. Nitrate Δ¹⁷O gradually decreases over the whole record, with a more rapid decrease between the mid-19th century and the present day of 5.6 ‰, indicating an increasing importance of RO₂ in NO_x cycling between the mid-19th century and the present day in the mid- to high-latitude Southern Hemisphere. The former has implications for the climate impacts of sulfate aerosol, while the latter has implications for the tropospheric O₃ production rate in remote low-NO_x environments. Using other ice core observations, we rule out drivers for these changes other than variability in extratropical oxidant (OH, O₃, RO₂, H₂O₂, and reactive halogens) concentrations. However, assuming OH, H₂O₂, and O₃ are the main oxidants contributing to sulfate formation, Monte Carlo box model simulations require a large (≥ 260%) increase in the O₃ / OH mole fraction ratio over the Southern Ocean in the early 19th century to match the sulfate Δ¹⁷O record. This unlikely scenario points to a~deficiency in our understanding of sulfur chemistry and suggests other oxidants may play an important role in sulfate formation in the mid- to high-latitude marine boundary layer. The observed decrease in nitrate Δ¹⁷O since the mid-19th century is most likely due to an increased importance of RO₂ over O₃ in NO_x cycling and can be explained by a 60–90% decrease in the O₃ / RO₂ mole fraction ratio in the extratropical Southern Hemisphere NO_x-source regions.