Jason Hill-Falkenthal

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 S-35 as a tracer for air masses originating in the stratosphere and transported downward. We measure concentrations of S-35 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 (SO2)-S-35 and (SO42-)-S-35 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 O-3 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 O-3 levels may shift the oxidation pathway of SO2 in the troposphere, which may be constrained by further intensive sampling and measurements. Comparison with surface O-3 measurements and traditional meteorological tracing methods shows that S-35 is a potentially unique and sensitive tracer to quantify the contribution of stratospheric air to surface O-3 levels in fresh or aged STT plumes.

Cosmogenic 35S measurements in the Tibetan Plateau to quantify glacier snowmelt

The cosmogenic radionuclide ³⁵S (t₁/₂ ~ 87 days) is a unique tracer for high-altitude air mass and has been used extensively to understand stratospheric air mass mixing. In this paper, we investigate if ³⁵S can be utilized as an independent tracer to quantify glacier melt. We report the first measurements of ³⁵S in samples collected from the Tibetan Plateau during 2009–2012 with an aim to interpret ³⁵S in atmospheric particles and their deposition over glacier and snowmelts. Our measurements show that ³⁵S activity in the aerosol phase varies from 116 ± 13 to 2229 ± 52 atoms/m³ resulting in higher values during winter–spring and lower values during summer–autumn. This seasonality is likely due to higher mixing of ³⁵S-rich stratospheric air masses during winter–spring and ³⁵S-poor air masses from the Bay of Bengal during the Asian summer monsoon. The average ³⁵S activity in the Zhadang glacier was found to be 3–8 times higher relative to the nearby lake water. The main source of ³⁵S activity in the Zhadang glacier is atmospheric deposition, whereas both atmospheric deposition and glacier snowmelt are the primary sources in the Nam Co Lake. The focus of this study is to quantitatively determine the spatial and temporal variations in glacier snowmelt. In the future, extensive sampling of aerosols and snow is required for determining ³⁵S in combination with stable oxygen isotopes in sulfate to better understand the glacier melt process and hydrological cycle on the Tibetan Plateau.