Export of pollutants to the UT over Southern Asia during the summer monsoon: results from CARIBIC

Between April and December 2008 the CARIBIC (Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrumented Container) passenger aircraft conducted monthly measurement flights between Frankfurt, Germany and Chennai, India. These flights covered the period of the Asian summer monsoon (June-Sept), allowing for the detailed chemical characterization of the air inside the monsoon anticyclone. The persistent deep convection and associated circulation patterns of the monsoon make it an important pathway for polluted boundary layer air to reach the upper troposphere/lowermost stratosphere (UT/LS). From here pollutants can be advected to other regions, or further uplifted into the stratosphere. While there exists an increasing number of satellite observations of trace gases in the UT/LS within the monsoon anticyclone, in situ composition measurements are scarce, and CARIBIC has made the first measurements within the anticyclone for many atmospheric constituents.

Real time analyses included ozone (O3), carbon monoxide (CO), reactive nitrogen (NOy), water vapor and aerosols, and these were complemented by the collection of whole air samples, which provide information about a large suite of greenhouse gases, non-methane hydrocarbons (NMHCs) and halocarbons. Trace gases and aerosols were consistently enhanced in the UT during the monsoon as a result of increased convective transport. Concentrations of NMHCs and halocarbons were used to investigate pollutant sources influencing composition, and indicated a strong influence of biofuel burning as well as urban/industrial emissions, presumed to arise mainly from sources in India. The influence of marine boundary layer air is also apparent through elevated levels of compounds of marine origin (e.g. CHBr3, CH3I). In addition to pollutant characterization, transport of pollutants in the monsoon anticyclone and implications for downwind regions are also discussed.