Elevated mixing ratios of surface ozone over the Arabian Sea

Surface O-3 levels exceeding 70 nmol/mol, the highest observed to date over the Indian Ocean, were recorded in the Arabian Sea marine boundary layer during the Indian Ocean Experiment (INDOEX) in 1999. Contrasted with this, available O-3 observations from nearby coastal cities show generally lower values, rarely exceeding 50 nmol/mol, and averaging 25-35 nmol/mol during daytime, and 5-15 nmol/mol during nighttime. A 3D photochemical model simulates the high marine boundary layer O-3 values relatively well, but considerably overestimates the urban O-3 values. Surface wind fields show that the air masses with elevated O-3 frequently come from India, implying that rapid photochemical production is likely to be occurring in the continental outflow. Furthermore, downwards transport of O-3-rich air masses from the free troposphere may also be important in producing some of the observed high-Ga events. Implications of these observations are discussed.