Abstract
Isoprene contributes to the formation of ozone and secondary organic aerosol in the atmosphere, and thus influences cloud albedo and climate. Isoprene is ubiquitous in the surface open ocean where it is produced by phytoplankton, however emissions from the global ocean are poorly constrained, in part due to a lack of knowledge of oceanic sink or degradation terms. Here, we present analyses of ship-based seawater incubation experiments with samples from the Mediterranean, Atlantic, tropical Pacific and circum-Antarctic and Subantarctic oceans to determine chemical and biological isoprene consumption in the surface ocean. We find the total isoprene loss to be comprised of a constant chemical loss rate of 0.05 ± 0.01 d−1 and a biological consumption rate that varied between 0 and 0.59 d−1 (median 0.03 d−1) and was correlated with chlorophyll-a concentration. We suggest that isoprene consumption rates in the surface ocean are of similar magnitude or greater than ventilation rates to the atmosphere, especially in chlorophyll-a rich waters.
Highlights
Isoprene contributes to the formation of ozone and secondary organic aerosol in the atmosphere, and influences cloud albedo and climate
Most of the focus of isoprene cycling studies had been on the production term, considering specific production rates by phytoplankton as though they were constitutive and shaped by phylogeny[41], with an occasional emphasis on how they are tuned by acclimation to environmental conditions[45,47,50]
As a matter of fact, whilst isoprene production is grosso modo related to phytoplankton biomass and primary production (Fig. 4), the resulting isoprene concentration does not necessarily follow indicators of phytoplankton biomass such as chla but it is further influenced by environmental factors such as SST12–14,51
Summary
Isoprene contributes to the formation of ozone and secondary organic aerosol in the atmosphere, and influences cloud albedo and climate. Isoprene is ubiquitous in the surface open ocean where it is produced by phytoplankton, emissions from the global ocean are poorly constrained, in part due to a lack of knowledge of oceanic sink or degradation terms. We suggest that isoprene consumption rates in the surface ocean are of similar magnitude or greater than ventilation rates to the atmosphere, especially in chlorophyll-a rich waters. Estimations of the global ocean emission of isoprene have been attempted either by top-down (balancing modelled emissions to atmospheric observations) or bottom-up (modelling oceanic isoprene concentration and air–sea flux) approaches, and they diverge by one or two orders of magnitude[14] (maximum range: 0.1–12 TgC year−1). The occurrence of isoprene-degrading bacteria in seawater has been demonstrated[20,21] and a significant microbial sink has been suggested[22,23,24], but it has not been experimentally confirmed, let alone measured, in natural conditions including natural concentrations
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