Variations of ozone in the marine boundary layer over the Arabian Sea and the Indian Ocean during the 1998 and 1999 INDOEX campaigns

Abstract

Extensive measurements of surface ozone (O₃) have been undertaken to study its transport from the continent to the marine region during the field cruises of Indian Ocean Experiment (INDOEX) in the winters of 1998 and 1999. The role of Intertropical Convergence Zone (ITCZ) in the distribution of O₃ is investigated. These measurements show a large difference in ozone features during 1998 and 1999. Ozone mixing ratios, its latitudinal gradients, and amplitudes of diurnal variations are higher during INDOEX 1999 than in INDOEX 1998 over the Arabian Sea. The INDOEX 1999 period was characterized by many anticyclones and stronger inversions over the Indian region. The observed maximum latitudinal gradient and amplitude of diurnal variation are 7.5 ppbv lat^-1 and 54%, respectively, during INDOEX 1999, which are more than the earlier levels observed over this region. There is almost no longitudinal gradient along the 20°S latitudinal transect over the Indian Ocean. The marine type of diurnal variations observed in the regions where gradients in ozone are very small suggest that the theory recently given by de Laat and Lelieveld [2000] could not fully explain these variations. Using exponential decrease of ozone with latitude, we have estimated an e-fold time of 4.8 days for ozone, which is lower than that for aerosol (7 days) measured independently for this region. This indicates that in addition to mixing and deposition, which are common to both these species, chemical loss of ozone is also important over the marine environments of this region. Ozone levels over the Arabian Sea (~65 ppbv) are found to be more than four times the levels of ozone over the remote Indian Ocean (~14 ppbv), south of the ITCZ. These results suggest that anthropogenic activities in the Indian subcontinent can significantly affect its adjoining marine environments.