Himalayas as a global hot spot of springtime stratospheric intrusions: Insight from isotopic signatures in sulfate aerosols

Downward transport of stratospheric air into the troposphere (identified as stratospheric intrusions) could potentially modify the radiation budget and chemical of the Earth's surface atmosphere. As the highest and largest plateau on earth, the Tibetan Plateau including the Himalayas couples to global climate, and has attracted widespread attention due to rapid warming and cryospheric shrinking. Previous studies recognized strong stratospheric intrusions in the Himalayas but are poorly understood due to limited direct evidences and the complexity of the meteorological dynamics of the third pole. Cosmogenic ³⁵S is a radioactive isotope predominately produced in the lower stratosphere and has been demonstrated as a sensitive chemical tracer to detect stratospherically sourced air mass in the planetary boundary layer. Here, we report 6-month (April–September 2018) observation of ³⁵S in atmospheric sulfate aerosols (³⁵SO₄²⁻) collected from a remote site in the Himalayas to reveal the stratospheric intrusion phenomenon as well as its potential impacts in this region. Throughout the sampling campaign, the ³⁵SO₄²⁻ concentrations show an average of 1,070 ± 980 atoms/m³. In springtime, the average is 1,620 ± 730 atoms/m³, significantly higher than the global existing data measured so far. The significant enrichments of ³⁵SO₄²⁻ measured in this study verified the hypothesis that the Himalayas is a global hot spot of stratospheric intrusions, especially during the springtime as a consequence of its unique geology and atmospheric couplings. In combined with the ancillary evidences, e.g., oxygen-17 anomaly in sulfate and modeling results, we found that the stratospheric intrusions have a profound impact on the surface ozone concentrations over the study region, and potentially have the ability to constrain how the mechanisms of sulfate oxidation are affected by a change in plateau atmospheric properties and conditions. This study provides new observational constraints on stratospheric intrusions in the Himalayas, which would further provide additional information for a deeper understanding on the environment and climatic changes over the Tibetan Plateau.