Acetylene C2H2 retrievals from MIPAS data and regions of enhanced upper tropospheric concentrations in August 2003
Earth Observation Science, Space Research Centre, University of Leicester, UK
Abstract. Acetylene (C2H2) volume mixing ratios (VMRs) have been successfully retrieved from MIPAS Level 1B radiances during August 2003. The data presented here contain most information between 300 hPa and 100 hPa based on the averaging kernels, with information also at lower altitude levels (up to 500 hPa) albeit with some influence from the 300 hPa level. In our C2H2 retrievals, data at altitude levels above 100 hPa must be treated with caution. Systematic errors are less than 10% at the upper levels but can reach higher levels at 300 hPa in the tropics due to water vapour influences. Random errors per point are less than 15% at lower pressure levels and are closer to 30% at 100 hPa.
Global distributions of both the absolute C2H2 and ratios to MOPITT 150 hPa retrievals of carbon monoxide (CO) confirm some significant features for this important hydrocarbon in a characteristic summer month (August 2003), showing tight correlations regionally but globally emphasising the differences between sources and lifetimes of CO and C2H2. The ratios to CO are estimated to be accurate to approximately 10%. A strong isolation of C2H2 within the Asian monsoon anticyclone is observed, evidencing convective transport into the upper troposphere, horizontal advection within the anticyclone at 200 hPa, distinct but measurable gradients at the westward edge of the vortex and formation of a secondary dynamical feature over the Asian Pacific. The data for C2H2 strongly support evidence for a strong isolated core to the anticyclone with distinct gradients surrounding this core. Within this region, there is a relatively lower correlation of C2H2 and CO suggesting difference in injection ratios or more likely due to expected chemical processing.
A second strong feature to the global distributions is observed in the enhancement and outflow of biomass burning from Africa at 200 hPa, both north-westward and eastward from 10° S. The easterly flow shows high C2H2 ratios to CO which have significantly decayed before reaching Australia. In the biomass burning regions, C2H2 and CO are relatively tightly correlated. C2H2 enhancements are observed to penetrate to lower altitudes in the African biomass outflow in this month compared to uplift observed in the Asian monsoon anticyclone region.
Overall, the data show the distinctive nature of C2H2 distributions, confirm in greater detail than previously possible features of hydrocarbon enhancements in the upper troposphere and highlight the future use of MIPAS hydrocarbon data for testing model transport and OH decay regimes in the middle to upper troposphere.