High Levels of Isoprene in the Marine Boundary Layer of the Arabian Sea during Spring Inter-Monsoon: Role of Phytoplankton Blooms

The Arabian Sea possesses an intense oxygen minimum zone because of high primary productivity. These important biogeochemical aspects of the Arabian Sea have led us to investigate air-sea exchanges over the region. The measurements of the isoprene mixing ratio in marine air and biological parameters in seawater were conducted during the inter-monsoon period of April-May 2017. The year 2017 was the second warmest year since 1880 for the global ocean with positive sea surface temperature (SST) anomalies over the Arabian Sea during the campaign. The overall variation of isoprene follows the distribution of chlorophyll-a (Chl-a) with lower (0.38 +/- 0.14 ppbv) and higher (0.75 +/- 0.17 ppbv) values over central and northern regions, respectively. The diurnal pattern of isoprene covaries with solar flux, equivalent potential temperature, and wind speed as the daytime mixing ratio was similar to 55% higher than its night-time value. Major enhancements (>0.6 ppbv) were associated with the blooms of Trichodesmium and Thalassiosira in oligotrophic conditions. High abundance of diatoms and cyanobacteria, intense solar flux, and high SST favored the production of isoprene from microbial sources. The estimated emission fluxes of isoprene were in the range of 1.5 x 10(7) to 1.2 x 10(8) molecules cm(-2) s(-1). Levels of isoprene in marine air and its emission fluxes were higher than the values reported for most of the other highly productive oceans. This study highlights implications of the "Arabian Sea Paradox" on regional atmospheric chemistry.