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

Abstract

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 35 S as a tracer for air masses originating in the stratosphere and transported downward. We measure concentrations of 35 S 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 35 SO 2 and 35 SO 4 2À 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 SO 2 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 SO 2 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 35 S 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.

Detection of deep stratospheric intrusions by cosmogenic 35 S

“…In particular, a global chemistry-climate model showed strong contributions of stratospheric intrusions to MDA8 ground-level O 3 in Nevada (6,7,10). Our previous measurements showed high 35 S concentrations in the San Fernando Valley in California (a sampling site close to Nevada) (16) and at Mount Everest in the Himalayas (19), supporting the model results. These measurements imply that the spatial distribution of 35 S may provide invaluable information on regional variabilities of stratospheric intrusion strength and frequency to constrain model results.…”

“…other stratospheric signatures (e.g., high O 3 level and low humidity) were not observed in suspected aged stratospheric air masses (18,19). Measurements of 35 SO 4 2− during deep stratospheric intrusions, which directly entrain fresh stratospheric air to the boundary layer, have never been made.…”

“…Based on simple and unconstrained 1D box model calculations, slightly elevated 35 SO 4 2− concentrations in early studies were linked to the polar vortex activity (17), Santa Ana winds, and shallow stratospheretroposphere exchange events in southern California (18). Recent studies applied mesoscale meteorology models to investigate the possible downward transport processes of aged stratospheric air (19,20). However, the reliability of 35 S as a stratospheric tracer remains uncertain and debated, because the magnitudes of 35 SO 4 2− enhancements were relatively small and…”

Vertically uniform formation pathways of tropospheric sulfate aerosols in East China detected from triple stable oxygen and radiogenic sulfur isotopes

“…In this study, we report 35 S specific activities (SA; unit: 35 S atoms/nmol SO 4 2− ) to compare with Δ 17 O(SO 4 2− ) because SA accounts for nonradiogenic sulfur ([ 35 S] = 0 atoms m −3 ) emitted from the PBL, while 35 SO 4 2− concentrations only incorporate radiogenic sulfur from the higher atmosphere [ Brothers et al , ; Hill‐Falkenthal et al , ; Hill‐Falkenthal et al , ]. The 35 S SA in this study ranges from 2 to 35 atoms/nmol SO 4 2− , close to most samples collected in California [ Hill‐Falkenthal et al , ] but significantly lower than the pristine Antarctic and Tibetan Plateau [ Hill‐Falkenthal et al , ; Lin et al , ], which are less affected by emissions of nonradiogenic sulfur and much closer to the higher atmosphere ( 35 S source) (Figure ).…”

Chemical characteristics of submicron particles at the central Tibetan Plateau: insights from aerosol mass spectrometry

“…Recently, Gu et al (2016) examined the aerosol compositions using the global three-dimensional Goddard Earth Observing System chemical transport model (GEOS-Chem) and found elevated concentrations of sulfate, nitrate, ammonium, BC, and organic carbon over the TP. Further, observational and modeling studies have also shown that deep convection over the TP during daytime is one of the important routes for tropospheric and stratospheric exchange of aerosols (Cristofanelli et al, 2009Lin et al, 2016). Thus the enhanced aerosol concentrations during afternoon could possibly be attributed to the downward mixing of the aerosol layer from the upper troposphere during the growth of TP boundary layer.…”