Trace gas composition in the Asian summer monsoon anticyclone: a case study based on aircraft observations and model simulations
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 Research Aircraft (HALO) in the frame of the Earth System Model Validation (ESMVal) campaign. Air masses with enhanced O...
| Main Authors: | , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-05-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/6091/2017/acp-17-6091-2017.pdf |
| Summary: | 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 Research Aircraft (HALO) in
the frame of the Earth System Model Validation (ESMVal) campaign. Air masses
with enhanced O<sub>3</sub> 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 the presumption that the anticyclone's interior is dominated by
recently uplifted air with low O<sub>3</sub> in the monsoon season. We also
observed enhanced CO and HCl in the ASMA, which are tracers for boundary layer
pollution and tropopause layer (TL) air or stratospheric in-mixing 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.
<br><br>
Lagrangian trajectory calculations using HYSPLIT show that HALO sampled
a filament of UT air three times, 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 part and
downstream of the eastern ASMA flank. 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 O<sub>3</sub>-rich
filament is diverted from the fringe towards the interior of the original
anticyclone, and is at least partially bound to become part of the new Iranian
eddy.
<br><br>
A simulation with the ECHAM/MESSy Atmospheric Chemistry (EMAC) model is
found to reproduce the observations reasonably well. It shows that
O<sub>3</sub>-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 O<sub>3</sub> in originally O<sub>3</sub>-poor, but CO-rich air. Simulated
monthly mean trace gas distributions show decreased O<sub>3</sub> in the ASMA
centre only at the 100 hPa level in July and August, but at lower altitudes
and in September the ASMA is dominated by increased O<sub>3</sub>. The combination
of entrainment from the tropopause region, photochemistry and dynamical
instabilities can explain the in situ observations, and might have a larger
impact on the highly variable trace gas composition of the anticyclone than
previously thought. |
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| ISSN: | 1680-7316 1680-7324 |