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Discussion papers
https://doi.org/10.5194/acp-2018-926
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-926
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Sep 2018

Research article | 25 Sep 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Upper tropospheric CH4 and CO affected by the South Asian summer monsoon during OMO

Laura Tomsche, Andrea Pozzer, Narendra Ojha, Uwe Parchatka, Jos Lelieveld, and Horst Fischer Laura Tomsche et al.
  • Department of Atmospheric Chemistry, Max-Planck-Institute for Chemistry, Mainz, 55128, Germany

Abstract. The Asian monsoon anticyclone (AMA) is a yearly recurring phenomenon in the northern hemispheric upper troposphere and lower stratosphere. It is connected to the South Asian summer monsoon, and the circulation extends approximately across 20°–120°E and 15°–40°N longitude-latitude. It has a clearly observable signature due to vertical transport of polluted air masses from the surface to the upper troposphere by the monsoon convection. However, the transport pathways and the fate of pollutants in the upper troposphere are not yet fully understood. As pollution emissions in South Asia are increasing, changes in the chemical composition of the AMA can be expected. We performed in situ measurements of carbon monoxide (CO) and methane (CH4) in the region of monsoon outflow and in background air in the upper troposphere (Mediterranean, Arabian Peninsula, Arabian Sea) by optical absorption spectroscopy on board the German High Altitude and Long range (HALO) research aircraft during the OMO (Oxidation Mechanism Observations) mission in summer 2015. We identified the transport pathways and the origin of the trace gases with back trajectories, calculated with the Lagrangian particle dispersion model FLEXPART, and we compared the in situ data with simulations of the atmospheric chemistry general circulation model EMAC. CH4 and CO mixing ratios were found to be enhanced within the AMA, on average by 72.1ppb and 20.1ppb, respectively, originating in the South Asian region (Indio-Gangetic Plain, North East India, Bangladesh and Bay of Bengal). It appears that CH4 is an ideal monsoon tracer in the upper troposphere due to its extended lifetime and the strong South Asian emissions. Furthermore, we used the measurements and model results to study the dynamics of the AMA, with an emphasis on the southern and western areas within the upper troposphere. For example, we distinguished four AMA modes based on different meteorological conditions. During one occasion we observed that under the influence of dwindling flow the transport barrier between the anticyclone and its surroundings weakened, expelling air masses from the AMA. The trace gases exhibited a distinct fingerprint of the AMA, and we also found that CH4 accumulated over the course of the OMO campaign.

Laura Tomsche et al.
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Short summary
The Asian monsoon anticyclone (AMA) is a yearly recurring phenomenon in the northern hemispheric upper troposphere (UT) and lower stratosphere. We performed in situ measurements of carbon monoxide (CO) and methane (CH4) in the monsoon outflow region and in background air in the UT (Mediterranean, Arabian Peninsula, Arabian Sea) by airborne optical absorption spectroscopy during the Oxidation Mechanism Observations mission (summer 2015). The trace gases increased within the AMA, especially CH4.
The Asian monsoon anticyclone (AMA) is a yearly recurring phenomenon in the northern hemispheric...
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