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 35SO42− 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.