Acetylene C₂H ₂ retrievals from MIPAS data and regions of enhanced upper tropospheric concentrations in August 2003

Abstract. Acetylene (C₂H₂) volume mixing ratios (VMRs) have been successfully retrieved from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) Level 1B radiances during August 2003, providing the first global map of such data and ratios to CO in the literature. The data presented here contain most information between 300 hPa and 100 hPa with systematic errors less than 10% at the upper levels. Random errors per point are less than 15% at lower levels and are closer to 30% at 100 hPa.

Global distributions of the C₂H₂ and C₂H₂/CO ratio confirm significant features associated with both the Asian monsoon anticyclone and biomass burning for this important hydrocarbon in a characteristic summer month (August 2003), showing tight correlations regionally, particularly at lower to medium values, but globally emphasising the differences between sources and lifetimes of CO and C₂H₂. The correlations are seen to be particularly disturbed in the regions of highest C₂H₂ concentrations, indicating variability in the surface emissions or fast processing.

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 gradients at the westward edge of the vortex and formation of a secondary dynamical feature from the eastward extension of the anticyclone outflow over the Asian Pacific. Ratios of C₂H₂/CO are consistent with the evidence from the cross-sections that the C₂H₂ is uplifted rapidly in convection.

Observations are presented of enhanced C₂H₂ associated with the injection from biomass burning into the upper troposphere and the outflow from Africa at 200 hPa into both the Atlantic and Indian Oceans. In the biomass burning regions, C₂H₂ and CO are well correlated, but the uplift is less marked and peaks at lower altitudes compared to the strong effects observed in the Asian monsoon anticyclone. Ratios of C₂H₂/CO clearly decay along transport pathways for the outflow, indicating photochemical ageing of the plumes.

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.