Abstract
Abstract. This work estimates the primary marine organic aerosol global emission source and its impact on cloud condensation nuclei (CCN) concentrations by implementing an organic sea spray source function into a series of global aerosol simulations. The source function assumes that a fraction of the sea spray emissions, depending on the local chlorophyll concentration, is organic matter in place of sea salt. Effect on CCN concentrations (at 0.2% supersaturation) is modeled using the Two-Moment Aerosol Sectional (TOMAS) microphysics algorithm coupled to the GISS II-prime general circulation model. The presence of organics affects CCN activity in competing ways: by reducing the amount of solute available in the particle and decreasing surface tension of CCN. To model surfactant effects, surface tension depression data from seawater samples taken near the Georgia coast were applied as a function of carbon concentrations. A global marine organic aerosol emission rate of 17.7 Tg C yr−1 is estimated from the simulations. Marine organics exert a localized influence on CCN(0.2%) concentrations, decreasing regional concentrations by no more than 5% and by less than 0.5% over most of the globe, assuming direct replacement of sea salt aerosol with organic aerosol. The decrease in CCN concentrations results from the fact that the decrease in particle solute concentration outweighs the organic surfactant effects. The low sensitivity of CCN(0.2%) to the marine organic emissions is likely due to the small compositional changes: the mass fraction of OA in accumulation mode aerosol increases by only ~15% in a biologically active region of the Southern Ocean. To test the sensitivity to uncertainty in the sea spray emissions process, we relax the assumption that sea spray aerosol number and mass remain fixed and instead can add to sea spray emissions rather than replace existing sea salt. In these simulations, we find that marine organic aerosol can increase CCN by up to 50% in the Southern Ocean and 3.7% globally during the austral summer. This vast difference in CCN impact highlights the need for further observational exploration of the sea spray aerosol emission process as well as evaluation and development of model parameterizations.
Highlights
Physical processes on the ocean surface result in formation of sea spray aerosol
While past studies have focused mainly on mass concentrations and global aerosol budgets, this study aims to extend the analysis to determine the impact of primary marine organic matter on global and regional cloud condensation nuclei (CCN) concentrations
The sea salt budget is dominated by the coarse mode and is largely unchanged since marine organic OC emissions were limited to submicron sizes
Summary
Sea spray aerosol in conjunction with dimethyl sulfide (DMS) emissions from phytoplankton is thought to dominate oceanic aerosol sources. It is recognized that the oceans are a significant source of primary and secondary organic matter (Meskhidze and Nenes, 2006; O’Dowd et al, 2004). Meskhidze and Nenes (2006) found that cloud effective radius was reduced by 30 % over a phytoplankton bloom in the Southern Ocean and attributed this decrease to marine isoprene emission and SOA formation. Oceanic phytoplankton emit several types of volatile organic compounds (VOCs) with isoprene being the major constituent, secondary organic aerosol (SOA) derived from marine sources of isoprene is thought to comprise less than 10 % of the observed total marine organic aerosol (Arnold et al, 2009; Claeys et al, 2010; Myriokefalitakis et al, 2010; Ovadnevaite et al, 2011).
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