Abstract
Abstract. We have combined the first satellite maps of the global distribution of phytoplankton functional type and new measurements of phytoplankton-specific isoprene productivities, with available remote marine isoprene observations and a global model, to evaluate our understanding of the marine isoprene source and its impacts on organic aerosol abundances. Using satellite products to scale up data on phytoplankton-specific isoprene productivity to the global oceans, we infer a mean "bottom-up" oceanic isoprene emission of 0.31±0.08 (1σ) Tg/yr. By minimising the mean bias between the model and isoprene observations in the marine atmosphere remote from the continents, we produce a "top-down" oceanic isoprene source estimate of 1.9 Tg/yr. We suggest our reliance on limited atmospheric isoprene data, difficulties in simulating in-situ isoprene production rates in laboratory phytoplankton cultures, and limited knowledge of isoprene production mechanisms across the broad range of phytoplankton communities in the oceans under different environmental conditions as contributors to this difference between the two estimates. Inclusion of secondary organic aerosol (SOA) production from oceanic isoprene in the model with a 2% yield produces small contributions (0.01–1.4%) to observed organic carbon (OC) aerosol mass at three remote marine sites in the Northern and Southern Hemispheres. Based on these findings we suggest an insignificant role for isoprene in modulating remote marine aerosol abundances, giving further support to a recently postulated primary OC source in the remote marine atmosphere.
Highlights
The globally extensive coverage of marine stratiform clouds means that they exert a significant influence on global climate
Despite a significant downward revision of their initial estimate of isoprene emission derived during the study (Wingenter, 2007), the authors maintain that a role for isoprene in the production of cloud condensation nuclei (CCN) cannot be ruled out, due to highly variable environmental conditions and variability in distributions of phytoplankton functional types (PFTs) encountered over the oceans (Meskhidze and Nenes, 2007)
Few isoprene observations have been made in regions remote from the continents, with most marine observations being made at continental coastal sites
Summary
The globally extensive coverage of marine stratiform clouds means that they exert a significant influence on global climate. Production of secondary organic aerosol (SOA) from oceanic emission of isoprene from phytoplankton, and its impact on CCN concentrations, has been postulated as a mechanism to explain an observed reduction in cloud droplet effective radius and increase in cloud droplet number over a large phytoplankton bloom in the remote Southern Ocean (SO) during Austral Summer (Meskhidze and Nenes, 2006). Despite a significant downward revision of their initial estimate of isoprene emission derived during the study (Wingenter, 2007), the authors maintain that a role for isoprene in the production of CCN cannot be ruled out, due to highly variable environmental conditions and variability in distributions of phytoplankton functional types (PFTs) encountered over the oceans (Meskhidze and Nenes, 2007). We identify key uncertainties and deficiencies in our understanding of the production, emission and fate of oceanic isoprene on the global scale
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