| Zusammenfassung: | Surface measurements of ozone (O3), carbon monoxide (CO), methane (CH4) and sulfur hexafluoride (SF6) were made over the Arabian Sea and the Indian Ocean during the INDOEX field experiment in January–March 1999 covering the latitude range from 17°N to 20°S. Observations show decreasing trends in all these trace gases from the Indian coastal region to the open Indian Ocean. Ozone with a lifetime of few days, shortest among the species presented here, shows large variations while SF6, which has relatively longer lifetime, shows smaller variations. Ozone also shows clearly the effect of ITCZ and its movement. These observations show higher ozone near the Indian coastal region in January 1999 than in March 1999 and also compared to the observed ozone during the INDOEX-1998 cruise. This is believed to be due to strong anti-cyclonic conditions in January 1999 bringing more polluted air from the Indian continent. A decrease in the latitudinal gradient in CH4 was observed in 1999 compared to that in 1998 in the same region, indicating slower growth rate in 1998–1999 period.
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