Distribution of ozone in the marine boundary layer of Arabian Sea prior to monsoon: Prevailing airmass and effect of aerosols

• Low surface O3 concentration over Arabian Sea in the premonsoon season.
• Association of surface O3 with aerosols.
• Surface O3 depletion due to chlorine chemistry.

Surface ozone (O3) measurements were carried out in the marine environment of the Arabian Sea (AS) during the premonsoon months, April–May 2006, as part of the Integrated Campaign for Aerosols, gases and Radiation Budget. The O3 mixing ratio over the AS varied in the range ∼3–22 ppb with a mean of 13.5 ± 2 ppb. Comparatively high mixing ratios were observed over the southern AS and close to the coast. The spatial pattern did not show any evidence of transport from nearby landmass or in situ photochemistry. Longitudinally separated narrow regions of low and high O3 were seen over the southern AS. The role of aerosols in modifying the O3 concentration was examined based on the co-located measurement of aerosol mass loading, number density, size distribution and optical depth. The O3 mixing ratio showed positive correlation with aerosol loading. Over high O3 regions, large particle concentration showed significant enhancement. The role of chloride ion in depleting O3 was also investigated. The observed spatial features were compared with those measured during the earlier cruises conducted in different seasons and over various oceanic regions. A comparison has been made with the measurements over the Bay of Bengal during the same cruise.