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. The Asian monsoon anticyclone (AMA) is annual phenomenon in thenorthern hemispheric upper troposphere and lower stratosphere. It is part ofthe South Asian summer monsoon system, and it has a clearly observablesignature due to the vertical transport of polluted air masses from the surfaceto the upper troposphere by monsoon convection. We performed in situmeasurements of carbon monoxide (CO) and methane ( CH4 ) in the region ofmonsoon outflow and in background air in the upper troposphere(Mediterranean, Arabian Peninsula, and Arabian Sea) using optical absorptionspectroscopy on board the High Altitude and LOng range (HALO) researchaircraft during the OMO (Oxidation Mechanism Observations) mission in summer2015. We identified the transport pathways and the origin of the trace gaseswith back trajectories, which were calculated using the Lagrangian particle dispersionmodel FLEXPART, and we compared the in situ data with simulations of theatmospheric chemistry general circulation model EMAC. CH4 and CO mixingratios were found to be enhanced within the AMA, the in situ data increasedby 72.1 and 20.1 ppbv on average, respectively, and originated in the SouthAsian region (Indo-Gangetic Plain, northeastern India, Bangladesh, and the Bay ofBengal). It appears that CH4 is an ideal monsoon tracer in the uppertroposphere due to its extended lifetime and the strong South Asianemissions. Furthermore, we used the measurements and model results to studythe dynamics of the AMA over several weeks during the monsoon season, with anemphasis on the southern and western areas in the upper troposphere. Wedistinguished four AMA modes based on different meteorological conditions.On one occasion we observed that under the influence of dwindling flowthe transport barrier between the anticyclone and its surroundings weakened,expelling air masses from the AMA. The trace gases exhibited a distinctAMA fingerprint; we also found that CH4 accumulated over thecourse of the OMO campaign.

atmospheric chemistry and physics

Upper tropospheric CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and CO affected by the South Asian summer monsoon during the Oxidation Mechanism Observations mission