Tropospheric distribution of ozone and its precursors over the tropical Indian Ocean

Abstract

To evaluate the atmospheric ozone measurements of the Indian Ocean during the dry winter season from January to March, an analysis of ozone and its precursors has been made using the Model of Ozone and its Related Tracers (MOZART-2), a global chemical transport model. The ozonesonde measurements were made on board the ship Sagar Kanya during the Indian Ocean Experiment 1999 (INDOEX-99), which covered the region from 11.3° N to 20.2° S and from 62.3° E to 75.8° E. These measurements reveal surface ozone values on the order of 20-50 ppb over the oceanic region and increasing concentrations in the midtroposphere in the range 50-100 ppb, followed by a steep gradient (>120 ppb) near the tropopause. The model qualitatively reproduces most of the broad features observed in the ozonesonde measurements. The low value of O₃ over the ocean is characterized by the low concentration of NO_x and CO as simulated by the model, but the relationship between O₃ and NO_x breaks down toward the continent where NO_x increases sharply and O3 does not follow the steep gradient. The variation in CO from ocean to continent is closely related to the O₃ variation. The model reproduces the latitudinal variation of CO in this region well but underestimates the CO concentration by 30-50%, probably because of underestimated emission values. The latitudinal distribution of ozone over the marine boundary layer shows a decrease from north of the equator to the south as the CO-rich air from the continent spreads toward the ocean, thereby reflecting the inflow of pollutants to the pristine marine region during the dry winter season.