Vertical distribution of ozone over the Indian Ocean (15°N-20°S) during First Field Phase INDOEX-1998

The vertical distribution of ozone over the Indian Ocean has been measured during the First Field Phase (FFP) of Indian Ocean Experiment (INDOEX) from 15 degrees N to 20 degrees S in February and March 1998, Using various ozonesondes during onward (18 February through 13;March) and return (18-28 March) journeys. A latitudinal gradient in ozone mixing ratio from southern (pristine air) to northern (pollutant air) hemisphere is observed in the troposphere, In the northern hemisphere, ozone mixing ratio of the order of 50 ppbv is observed except within the region of 5-8 degrees N, where ozone mixing ratio shoots to 120 ppbv. These large values are likely attributable to continental flow with the advection of photochemically produced ozone. In addition, a layered structure was observed during both onward and return journeys, A pocket of low ozone values (<10 ppbv) is observed simultaneously in the upper troposphere and near the surface. Total ozone maps obtained from total:ozone mapping spectrometer-Earth Probe (TOMS-EP) over this region (30 degrees N-30 degrees S, 30 degrees E-150 degrees E) remained around 250 +/- 5 DUI except during 26-28 March 1998 when total ozone reached 270 DU. Marine and continental ozone profiles of Indian, East Asian, and South African stations are compared.