89th American Meteorological Society Annual Meeting

Monday, 12 January 2009
Effect of Marine Biogenic Organic Aerosols on Cloud Properties: Community Atmosphere Model (CAM) Study
Hall 5 (Phoenix Convention Center)
Jun Xu, North Carolina State University, Raleigh, NC; and N. Meskhidze, Y. Zhang, B. Gantt, S. Ghan, A. Nenes, X. Liu, R. Easter, and R. Zaveri
Marine organic matter (OM) of biogenic origin has been shown to be a significant contributor to sub-micron aerosol mass over the oceans. There is substantial body of evidence suggesting that organic aerosols can enter marine atmosphere through direct emissions of biogenic OM by the bubble bursting process. It has been recently proposed that marine biota can produce large variety of volatile organic compounds (VOCs), which can also contribute considerably to the organic fraction of marine aerosols via oxidation to form secondary organic aerosol (SOA). In the vicinity of biologically-rich waters, e.g., large phytoplankton bloom areas over the remote oceans or near the coastal upwelling regions, marine organic aerosols can significantly influence chemical composition of cloud condensation nuclei (CCN) and aerosol activation behavior. Large organic aerosol source may be missing from current global models, which can be partly explained by the omission of biogenic source of marine OM in these models. Therefore, there is a need for improved quantification of the effects of marine biota on aerosol chemical composition, cloud properties, and climate.

In this study, primary organic mass flux of sub-micron aerosols of marine origin will be parameterized as a function of simulated wind speed and remotely-sensed chlorophyll-a concentrations [Chl-a]. Size segregation of OM between water insoluble and soluble fractions will be prescribed based on laboratory and ambient measurements of physical and chemical properties of marine aerosol. Phytoplankton-produced isoprene is assumed to be a main source of marine SOA and a global marine isoprene emission algorithm is implemented to estimate the oceanic sources isoprene. In the estimation, we use Sea-viewing Wide Field-of-view Sensor (SeaWiFS) retrieved [Chl-a], species and environmental conditions-specific- (e.g., incoming solar radiation, temperature) isoprene emission rates measured in our laboratory, and solar fluxes at different water levels in the ocean determined by incoming surface radiation and SeaWiFS diffuse attenuation coefficient at 490 nm (k490) .

Oceanic source of primary and secondary aerosols will be implemented into the National Center of Atmospheric Research (NCAR)'s next-generation Community Atmosphere Model, with an aerosol module from the Pacific Northwest National Laboratory (PNNL)'s Model for Integrated Research on Atmospheric Global Exchanges (MIRAGE) version2 (referred to as CAM/MIRAGE hereafter). CAM/MIRAGE consists of a chemical transport model coupled online with a global climate model. It can explicitly treat aerosol-cloud interaction using physically-based formulations for aerosol activation and prediction of cloud droplet number. Marine biogenic aerosol mass will be added to the existing accumulation-mode organic carbon (OC) in the model. SOA produced from the oxidation of ocean-emitted isoprene will be added in the model by using a mass yield approach, and will be treated as water soluble OC in accordance to observations. CAM/MIRAGE simulations will be conducted to examine the extent to which oceanic sources of primary and secondary aerosols can affect the microphysics and optical properties of shallow marine clouds in different parts of the oceans for different seasons. Two physically-based aerosol activation parameterizations, one by Abdul-Razzak and Ghan and one by Fountoukis and Nenes (recently implemented in CAM/MIRAGE), will be used to examine marine organic aerosol-cloud interactions. These prognostic parameterizations are capable of handling the effects of organic species on activation behavior and droplet growth kinetics, while maintaining differences in their treatments of kinetic limitations in droplet growth and methods for maximum supersaturation calculations. The effects of marine biogenic OM on cloud properties will be evaluated against available remotely-sensed data from the Moderate Resolution Imaging Spectroradiometer (MODIS).

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