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

The Asian monsoon anticyclone (AMA) is a yearly recurring phenomen on 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 imulations of the atmospheric chemistry general circulation model EMAC. CH4 and CO mixing ratios were found to be enhanced within the AMA, on average by 72.1 ppbv and 20.1 ppbv, 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