Abstract

Abstract. The global marine sources of organic carbon (OC) are estimated here using a physically-based parameterization for the emission of marine isoprene and primary organic matter. The marine isoprene emission model incorporates new physical parameters such as light sensitivity of phytoplankton isoprene production and dynamic euphotic depth to simulate hourly marine isoprene emissions totaling 0.92 Tg C yr−1. Sensitivity studies using different schemes for the euphotic zone depth and ocean phytoplankton speciation produce the upper and the lower range of marine-isoprene emissions of 0.31 to 1.09 Tg C yr−1, respectively. Established relationships between sea spray fractionation of water-insoluble organic carbon (WIOC) and chlorophyll-a concentration are used to estimate the total primary sources of marine sub- and super-micron OC of 2.9 and 19.4 Tg C yr−1, respectively. The consistent spatial and temporal resolution of the two emission types allow us, for the first time, to explore the relative contributions of sub- and super-micron organic matter and marine isoprene-derived secondary organic aerosol (SOA) to the total OC fraction of marine aerosol. Using a fixed 3% mass yield for the conversion of isoprene to SOA, our emission simulations show minor (<0.2%) contribution of marine isoprene to the total marine source of OC on a global scale. However, our model calculations also indicate that over the tropical oceanic regions (30° S to 30° N), marine isoprene SOA may contribute over 30% of the total monthly-averaged sub-micron OC fraction of marine aerosol. The estimated contribution of marine isoprene SOA to hourly-averaged sub-micron marine OC emission is even higher, approaching 50% over the vast regions of the oceans during the midday hours when isoprene emissions are highest. As it is widely believed that sub-micron OC has the potential to influence the cloud droplet activation of marine aerosols, our findings suggest that marine isoprene SOA could play critical role in modulating properties of shallow marine clouds and influencing the climate.

Highlights

  • Marine aerosols strongly affect properties and lifetime of stratiform clouds, influencing Earth’s radiation budget and climate

  • Out of multiple biogenic volatile organic compounds (BVOCs) emitted by phytoplankton, including dimethyl sulfide (DMS), halocarbons, isoprene, and monoterpenes (Shaw et al, 1983; Tokarczyk et al, 1994; Bonsang et al, 1992; Yassaa et al, 2008), in this study we focus on marine emissions of isoprene

  • It was shown that waterinsoluble organic carbon (WIOC) had a net production at the surface, while watersoluble organic carbon (WSOC) was predominantly of secondary origin

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Summary

Introduction

Marine aerosols strongly affect properties and lifetime of stratiform clouds, influencing Earth’s radiation budget and climate. It has been proposed that primary emissions of biogenic organic matter, bacterial and viral debris from wave breaking (Middlebrook et al, 1998; O’Dowd et al, 2004), and secondary organic aerosol (SOA) from phytoplankton-emitted biogenic volatile organic compounds (BVOCs) (O’Dowd et al, 2002; Meskhidze and Nenes, 2006; O’Dowd and de Leeuw, 2007) can act synergistically with the established mechanisms, leading to changes in marine aerosol chemical composition and number concentration. Despite the significant progress in the recent decade, the mechanism and the magnitude of the marine OC sources are still highly uncertain, and the role of oceanic biota in modifying chemical composition and size distribution of marine cloud condensation nuclei (CCN) remains one of the most intriguing questions in the climate studies.

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