COMPOSITION OF THE TROPOSPHERE OVER THE INDIAN-OCEAN DURING THE MONSOONAL TRANSITION

The atmosphere over the equatorial Indian Ocean is a unique environmen t in which to study the chemical and radiative effects of an intense s ource of anthropogenic emissions from the northern hemisphere directly coupled to the relatively pristine background conditions present in t he southern hemisphere. As an initial investigation into the role of t he intertropical convergence zone (ITCZ) on interhemispheric transport of pollutants, a number of trace atmospheric species were measured ab oard the National Oceanic and Atmospheric Administration (NOAA) R/V Ma lcolm Baldrige between Durban, South Africa, and Colombo, Sri Lanka, f rom March 12 to April 22, 1995. Sharp increases in the concentrations of carbon monoxide (CO), carbon dioxide (CO2), and aerosols were assoc iated with four distinct meteorological regimes transected by the crui se track from 33 degrees S to 9 degrees N. Across the ITCZ, aerosol co ncentrations, including non-sea-salt sulfate, nitrate and ammonium, in creased by a factor of 4. Surface ozone measurements showed a latitudi nal gradient with a minimum near the equator and a strong diurnal vari ation in the equatorial regions. The latitudinal profile of gas-phase reactive nitrogen paralleled ozone and was higher in the remote southe rn hemisphere than in the remote northern hemisphere. Evidence of dire ct anthropogenic impact on the region was observed more than 1500 km f rom the southern tip of India. Back trajectories, calculated with NOAA 's medium range forecast data using the Hybrid Single-Particle Lagrang ian Integrated Trajectory (HY-SPLIT) program, identified the origin of the air mass regimes characterized by the trace gas and aerosol data. Continental emissions in the northern hemisphere were shown to have a major impact on the radiative properties and oxidizing capacity of th e marine atmosphere.