Retrieval of isoprene concentration from satellite data in the Southern Ocean

Marine isoprene (C₅H₈) is a volatile organic compound produced eminently by phytoplankton, which acts as a precursor of secondary organic aerosol upon emission to the atmosphere. In remote oceanic regions, like the Southern Ocean (SO), the emission of isoprene and other trace gases may be a determinant factor for cloud formation and brightness. Despite its relevance, there is still a huge quantitative uncertainty in isoprene emission not only at the regional scale, but also globally (0.1-11.6 Tg C year^-1).

In this work we propose the first satellite-based algorithm for the retrieval of isoprene concentration in surface waters of the Southern Ocean. To that aim, we matched up the tracks of three Southern Ocean cruises with remote sensing products of predictor variables. With these, we performed a set of statistical analysis involving correlation matrices and principal component analyses. Finally, we developed a multiple linear regression model as an algorithm for retrieval of isoprene concentrations in surface waters of the Southern Ocean. The proposed algorithm is based on remote sensed chlorophyll-a, sea surface temperature and photosynthetically active radiation. We found a regime shift on the relationship between isoprene concentration and the predictor variables when sea surface temperature is higher or lower than 3.5ºC.

A band of higher isoprene concentrations occurs at latitudes around 50ºS. Isoprene levels also peak in island-associated phytoplankton blooms and some coastal areas. To validate the algorithm we use a set of ca. 600 measurements, which is the biggest data base ever compiled for this region. We use the algorithm output to quantify the annual emission of isoprene from the SO and compare it with non-published estimates in the same region, and with global estimates.