Isotopic evidence for acidity-driven enhancement of sulfate formation after SO2 emission control

After the 1980s, atmospheric sulfate reduction is slower than the dramatic reductions in sulfur dioxide (SO2) emissions. However, a lack of observational evidence has hindered the identification of causal feedback mechanisms. Here, we report an increase in the oxygen isotopic composition of sulfate (Delta O-17(SO4)2-) in a Greenland ice core, implying an enhanced role of acidity-dependent in-cloud oxidation by ozone (up to 17 to 27%) in sulfate production since the 1960s. A global chemical transport model reproduces the magnitude of the increase in observed Delta O-17(SO4)2- with a 10 to 15% enhancement in the conversion efficiency from SO2 to sulfate in Eastern North America and Western Europe. With an expected continued decrease in atmospheric acidity, this feedback will continue in the future and partially hinder air quality improvements.

Automated summary

Since the 1980s, there has been a slower reduction in atmospheric sulfate compared to dramatic decreases in sulfur dioxide emissions. However, recent observations from a Greenland ice core show an increase in the oxygen isotopic composition of sulfate, indicating that acidity-dependent in-cloud oxidation by ozone has played a larger role in sulfate production since the 1960s. Global chemical transport models confirm an increase in conversion efficiency from SO2 to sulfate in Eastern North America and Western Europe, suggesting a continuing feedback mechanism that may hinder future air quality improvements.