Resolving the impact of stratosphere-to-troposphere transport on the sulfur cycle and surface ozone over the Tibetan Plateau using a cosmogenic ³⁵S tracer

The Himalayas were recently identified as a global hot spot for deep stratosphere-to-troposphere transport (STT) in spring. Although the STT in this region may play a vital role in tropospheric chemistry, the hydrological cycle and aquatic ecosystems in Asia, there is no direct measurement of a chemical stratospheric tracer to verify and evaluate its possible impacts. Here we use cosmogenic ³⁵S as a tracer for air masses originating in the stratosphere and transported downward. We measure concentrations of ³⁵S in fresh surface snow and river runoff samples collected from Mount Everest in April 2013 to be more than 10 times higher than previously reported by any surface measurement, in support of the Himalayas as a gateway of springtime STT. In light of this result, measurements of ³⁵SO₂ and ³⁵SO₄^2- at Nam Co in spring 2011 are reanalyzed to investigate the magnitudes of stratospheric air masses from the Himalayas to the tropospheric sulfur cycle and surface O₃ level over the Tibetan Plateau. A simple one-box model reveals that the oxidative lifetime of SO₂ is reduced in aged STT plumes. Triple oxygen isotopic measurements of sulfate samples suggest that enhanced O₃ levels may shift the oxidation pathway of SO₂ in the troposphere, which may be constrained by further intensive sampling and measurements. Comparison with surface O₃ measurements and traditional meteorological tracing methods shows that ³⁵S is a potentially unique and sensitive tracer to quantify the contribution of stratospheric air to surface O₃ levels in fresh or aged STT plumes.