Overview of the trace gas measurements on board the Citation aircraft during the intensive field phase of INDOEX

During the intensive field phase of the Indian Ocean Experiment (INDOEX), m easurements of the atmospheric chemical and aerosol composition over the In dian Ocean were performed from a Cessna Citation aircraft. Measurements wer e performed during February and March 1999 over the northern Indian Ocean f rom 70 degreesE to 80 degreesE, and from 8 degreesN tb 8 degreesS in the 0- 13 km altitude range. An overview of the trace gas-measurements is presente d. In the lowest 3 km the highest levels of pollution were found during Feb ruary 1999, mostly originating from northeastern India and southeastern Asi a. Lower levels of pollution were detected in March 1999, when the sampled air mostly originated from the Arabian Sea region. The mixing ratios of a n umber of trace compounds, indicative of biomass burning, were well correlat ed. The pollutant emission factors inferred from the measurements are consi stent with literature values for fire plumes, confirming that the residenti al use of biofuels in Asia is a major source of gaseous pollutants to the a tmosphere over the Indian Ocean, in accord with emission databases. The rem oval of reactive trace gases was studied over an extended area without inte rfering local emissions, and is shown to be governed by photochemical proce sses rather than by mixing and deposition. At intermediate altitudes of 3-8 km the mixing ratios of all trace gases other than ozone were generally lo wer, and the measurements suggest that the Photochemical processing of thes e air masses is much more extensive than in the 0-3 kin range. In the 8-13 km altitude range some evidence is obtained for the importance of convectiv e cloud systems in the transport of gaseous pollutants to the upper troposp here.