A numerical evaluation of global oceanic emissions of α-pinene and isoprene

Satellite remote sensing has indicated high aerosol optical depth over the Southern Ocean, however, the exact species and sources of these aerosols remain controversial. In-situ measurements indicate significant amounts of organic carbon aerosols within marine boundary layer, especially in the regions with enhanced oceanic biological activity. The high concentrations (up to ~ 150 pptv) of α-pinene and isoprene observed during a recent shipboard measurement of Organics over the Ocean Modifying Particles in both Hemispheres (OOMPH) summer cruise strongly suggests an oceanic source of these species. Here, we present a numerical evaluation of global oceanic emissions of α-pinene and isoprene based on both ‘bottom-up’ and ‘top-down’ methods. We infer that the global ‘bottom-up’ oceanic emissions of α-pinene and isoprene are 0.013 TgC yr-1 and 0.32 TgC yr-1, respectively. By constraining global chemistry model simulations with shipboard measurements of α-pinene and isoprene concentrations over the Southern Ocean region, we derive a ‘top-down’ global oceanic α-pinene source of 35.1 TgC yr-1 and isoprene source of 2.5 TgC yr-1. The comparisons between GEOS-Chem simulations and the OOMPH measurements indicate that the ‘bottom-up’ oceanic emission can hardly explain the observed α-pinene and isoprene concentrations along the ship route. Our derived ‘bottom-up’ and ‘top-down’ oceanic isoprene emissions are close to those in previous studies. The global oceanic α-pinene source, quantified for the first time in this study, is significant (compared with the terrestrial source) based on ‘top-down’ method, but is negligible based on ‘bottom-up’ approach. The large offset in the derived oceanic organic emissions (especially for α-pinene) is likely due to the incomplete understanding of the in-situ phytoplankton communities and their range of emission potentials as well as the uncertainties in the ‘top-down’ approach. Our research highlights the importance to carry out further research (especially measurements) to resolve the large offset in the derived oceanic organic emission based on two different approaches.