APRIL 2005: Quantifying the marine isoprene source using MODIS chlorophyll observations

Phytoplankton are responsible for ~50% of global photosynthesis and oxygen production. They are also known to produce a number of reactive trace gases and aerosols that impact atmospheric chemistry and climate change. Palmer and Shaw [2005] estimated marine emissions of isoprene from phytoplankton. They used published laboratory data relating isoprene production rates from phytoplankton to chlorophyll content, together with MODIS chlorophyll content data to estimate ocean fluxes of isoprene. A steady-state water column model was used including isoprene losses to chemistry, bacteria, and air-sea exchange. Physical mixing is a negligible sink. Air-sea exchange is the dominant isoprene sink in the surface oceans, with bacteria the second largest, but much less important, sink.

Figure: Marine isoprene fluxes (10⁸ molecules cm^-2 s^-1) for January and April 2001, shown on a 36 km² grid. Missing polar values reflect seasonal ice movement where satellite observations are unavailable.

Flux estimates range 10⁷-10⁹ molecules cm^-2s^-1, with considerable spatial and temporal variability (see Figure showing the NH and SH phytoplankton blooms), resulting in a global annual total of 0.1 Tg C/yr. These fluxes only represent 1% of the OH sink the remote marine boundary layer. Application of our approach to other reactive compounds may improve a priori flux estimates for coupled atmosphere-ocean biogeochemistry inverse model studies.

The 2001 isoprene flux data can be downloaded directly.