Airborne Measurements of Sulfur and Reactive Nitrogen Species in the Asian Summer Monsoon Anticyclone

We present first measurements of SO₂, H₂SO₄ , NO, HNO₃, and NOy in the upper troposphere and lower stratosphere (UTLS) in the center of the Asian Summer Monsoon Anticyclone (ASMA) using the Geophysica research aircraft during the StratoClim campaign in Kathmandu, Nepal, in July/August 2017. The ASMA forms in the UTLS during summer annually over Southeast Asia resulting in a huge confined air mass with a distinct chemical and aerosol composition. The experimental techniques used include chemical ionization mass spectrometry (SO₂, HNO₃), analysis of the ambient ion composition (H₂SO₄), and chemiluminescence (NO, NOy). NOy was measured in the gas- and condensed phase. Two main convective outflow layers with enhanced trace gas mixing ratios were observed at altitudes between 12-14 km and 16.5-18 km. In these outflow layers, SO₂ and NO mixing ratios of 200-300 ppt and 0.9 - 1.3 ppb, respectively, were detected. Interestingly, increased mixing ratios of SO₂ and H₂SO₄ were also found above the cold point tropopause resulting from overshooting convection. For conditions with very low temperatures at the tropopause, we observed depleted mixing ratios of HNO₃ in the gas-phase and HNO₃ uptake in aerosol and ice particles.