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‐O 3 events. Implications of these observations are discussed.