Observations of vertical distribution of tropospheric ozone over Indian Ocean and its comparison with continental profiles during INDOEX FFP-1998 andIFP-1999

The vertical distribution of ozone over the Indian Ocean was measured durin g the first field phase (FFP) of Indian Ocean experiment from 15 degreesN t o 20 degreesS in February/March 1998. Similar but exhaustive observation wa s taken during INDOEX-IFP in January/March 1999. A pocket of low ozone (sim ilar to 10 ppbv) was observed near the surface in addition to high ozone co ncentration observed at 8-12 km within the region of 5-15 degreesS during b oth INDOEX FFP-98 and INDOEX IFP-99. However, the north-south gradient in o zone concentration and the layered structure at 5-8 km as observed in INDOE X FFP-98 are not prominent during INDOEX IFP-99. East-west cross-section of ozone concentration in the troposphere along 20 degreesS and 15 degreesN may be characterized as the background value of pr istine and continental air on the northern and southern side of the equator , respectively. Though back trajectory analysis indicates that flow of air masses is mostly from the Indian subcontinent as well as south-east Asian r egion, it is difficult to distinguish the degree of relative contribution o f continental flow to ozone concentration over the Indian Ocean. The compar ison between marine and continental ozone profiles suggests that the northe rn side of the Inter Tropical Convergence Zone (ITCZ) resembles the contine ntal profiles as observed over the Indian subcontinent and African region, rather than east Asian region.