Abstract
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 NOx 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-NOx 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 NOx cycling and can be explained by a 60–90% decrease in the O3 / RO2 mole fraction ratio in the extratropical Southern Hemisphere NOx-source regions.
Highlights
The formation pathways of tropospheric sulfate (SO42−) and nitrate (NO3−) impact atmospheric chemistry and climate in a number of ways
We find that the West Antarctic Ice Sheet (WAIS) Divide 17O(SO42−) and [H2O2] records (Fig. 3) are not correlated
Based on comparison to other ice core observations, we demonstrate that the long-term increase in δ15N(NO3−) and decrease in 17O(NO3−) can be explained by the impact of the long-term decrease in the snow accumulation rate on the postdepositional loss of snowpack nitrate
Summary
The formation pathways of tropospheric sulfate (SO42−) and nitrate (NO3−) impact atmospheric chemistry and climate in a number of ways. The triple oxygen isotopes of sulfate and nitrate from ice cores have been suggested as a potential constraint on local to regional paleo-oxidant changes because the sulfate and nitrate preserve the isotopic composition of the oxidants involved in their formation. Their interpretation is complicated by other factors influencing sulfate and nitrate chemistry and uncertainty in the spatial scale reflected by measurements at a single location. We compare these isotope records to other ice core chemical records and consider possible explanations for the observed variability in the 17O record
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