Trace gas composition in the Asian summer monsoon anticyclone: A case study based on aircraft observations and model simulations
Klaus-D. Gottschaldt1, Hans Schlager1, Robert Baumann1, Heiko Bozem2, Veronika Eyring1, Peter Hoor2, Patrick Jöckel1, Tina Jurkat1, Christiane Voigt1,2, Andreas Zahn3, and Helmut Ziereis11Deutsches Zentrum für Luft‐ und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany 2Johannes Gutenberg-Universität, Institut für Physik der Atmosphäre, Mainz, Germany 3Karlsruher Institut für Technologie (KIT), Institut für Meteorologie und Klimaforschung, Karlsruhe, Germany
Received: 09 Nov 2016 – Accepted for review: 25 Nov 2016 – Discussion started: 28 Nov 2016
Abstract. We present in-situ measurements of the trace gas composition of the upper tropospheric (UT) Asian summer monsoon anticyclone (ASMA) performed with the High Altitude and LOng range (HALO) research aircraft in the frame of the Earth System Model Validation (ESMVal) campaign. Air masses with enhanced O3 mixing ratios were encountered after entering the ASMA at its southern edge at about 150 hPa on 18 September 2012. This is in contrast to previous studies, reporting that the anticyclone's interior is dominated by recently uplifted air with low O3 in the monsoon season. We also observed enhanced CO and HCl in the ASMA, tracers for boundary layer pollution and tropopause layer (TL) air or stratospheric inmixing, respectively. In addition, reactive nitrogen was enhanced in the ASMA. Along the HALO flight track across the ASMA boundary, strong gradients of these tracers separate anticyclonic from outside air.
Lagrangian trajectory calculations using HYSPLIT show that HALO sampled three times a filament of UT air, which included air masses uplifted from the lower or mid troposphere north of the Bay of Bengal. The trace gas gradients between UT and uplifted air masses were preserved during transport within a belt of streamlines fringing the central part of the anticyclone (fringe), but are smaller than the gradients across the ASMA boundary. Our data represent the first in-situ observations across the southern and downstream the eastern ASMA flank, respectively. Back-trajectories starting at the flight track furthermore indicate that HALO transected the ASMA where it was just splitting into a Tibetan and an Iranian part. The O3-rich filament is diverted from the fringe towards the interior of the original anticyclone, and at least partially bound to become part of the new Iranian eddy.
A simulation with the ECHAM/MESSy Atmospheric Chemistry (EMAC) model is found to reproduce the observations reasonably well. It shows that O3-rich air is entrained by the outer streamlines of the anticyclone at its eastern flank. Back-trajectories and increased HCl mixing ratios indicate that the entrained air originates in the stratospherically influenced TL. Photochemical ageing of air masses in the ASMA additionally increases O3 in originally O3-poor, but CO-rich air.
Simulated monthly mean trace gas distributions show decreased O3 in the ASMA centre not in general, but only at the 100 hPa level in July and August, as also reported by previous studies. However, at lower altitudes and in September the ASMA is dominated by increased O3, indicating that the above processes are more important for the ASMA trace gas budgets than previously thought.
Gottschaldt, K.-D., Schlager, H., Baumann, R., Bozem, H., Eyring, V., Hoor, P., Jöckel, P., Jurkat, T., Voigt, C., Zahn, A., and Ziereis, H.: Trace gas composition in the Asian summer monsoon anticyclone: A case study based on aircraft observations and model simulations, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-997, in review, 2016.