The 17 O excess (Δ 17 O = δ 17 O−0.52 × δ 18 O) of sulfate and nitrate reflects the relative importance of their different production pathways in the atmosphere. A new record of sulfate and nitrate Δ 17 O spanning the last 2400 years from the West Antarctic Ice Sheet Divide ice core project shows significant changes in both sulfate and nitrate Δ 17 O in the most recent 200 years, indicating changes in their formation pathways. The sulfate Δ 17 O 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 O 3 , relative to that in the gas phase. Nitrate Δ 17 O 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 RO 2 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 O 3 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, O 3 , RO 2 , H 2 O 2 , and reactive halogens) concentrations. However, assuming OH, H 2 O 2 , and O 3 are the main oxidants contributing to sulfate formation, Monte Carlo box model simulations require a large (≥ 260%) increase in the O 3 / OH mole fraction ratio over the Southern Ocean in the early 19th century to match the sulfate Δ 17 O 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 Δ 17 O since the mid-19th century is most likely due to an increased importance of RO 2 over O 3 in NO x cycling and can be explained by a 60–90% decrease in the O 3 / RO 2 mole fraction ratio in the extratropical Southern Hemisphere NO x -source regions.