On the Impacts of Phytoplankton-Derived Organic Matter on the Properties of the Primary Marine Aerosol – Part 1: Source fluxes
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Atmos. Chem. Phys., 10, 9295–9317, 2010 www.atmos-chem-phys.net/10/9295/2010/ Atmospheric doi:10.5194/acp-10-9295-2010 Chemistry © Author(s) 2010. CC Attribution 3.0 License. and Physics On the impacts of phytoplankton-derived organic matter on the properties of the primary marine aerosol – Part 1: Source fluxes E. Fuentes1, H. Coe1, D. Green2, G. de Leeuw3,4,5, and G. McFiggans1 1Centre for Atmospheric Sciences, School of Earth, Atmospheric and Environmental Sciences, Manchester M13 9PL, UK 2Scottish Association for Marine Science, Oban, UK 3Finnish Meteorological Institute, Climate Change Unit, Helsinki, Finland 4University of Helsinki, Department of Physics, Helsinki, Finland 5TNO, Environment, Health and Safety, Utrecht, The Netherlands Received: 15 March 2010 – Published in Atmos. Chem. Phys. Discuss.: 8 June 2010 Revised: 24 September 2010 – Accepted: 29 September 2010 – Published: 1 October 2010 Abstract. The effect of biogenic dissolved and colloidal or- ter and the algal exudate type. Diatomaceous exudate with ganic matter on the production of submicron primary sea- organic carbon concentration (OC<0.2 µm) >175 µM was re- spray aerosol was investigated via the simulation of bubble quired to observe a significant impact on the size distribu- bursting in seawater enriched with phytoplankton-released tion, which implies that effects are expected to be substantial organics. only in high biological activity areas abundant with diatom Seawater samples collected along a transect off the West algal populations. The laboratory findings were in agreement African coast during the RHaMBLe cruise (RRS Discovery with analogous bubble-bursting experiments conducted with cruise D319), conducted as part of the SOLAS UK program, unfiltered oceanic seawater collected during the RHaMBLe were analysed in order to identify the dominant oceanic al- cruise, which revealed a higher production of particles with gal species in a region of high biological activity. Cultures Dp0<100 nm at regions with high biological activity. These of microalgal strains representative of the species found in findings suggest that the increase in the atmospheric aerosol the collected seawater were grown in order to produce nat- modal sizes from winter to summer, reported by long-term ural bioexudate. Colloidal plus dissolved organic fraction observations in North Atlantic waters, is not directly due to in this material remaining after <0.2 µm filtration was em- an impact of the higher primary organic matter production ployed to prepare organic-enriched seawater proxies for the occurring during warm periods. laboratory production of marine aerosol using a plunging- A novel sub-micrometric size-resolved source flux func- waterjet system as an aerosol generator. Submicron size dis- tion, explicitly defined as a function of the diatomaceous tributions of aerosols generated from different organic mono- exudate concentration, was derived from the size distribu- layers and seawater proxies enriched with biogenic exudate tion measurements and the estimation of the fractional white- were measured and compared with blanks performed with cap coverage. According to the defined parameterisation, a artificial seawater devoid of marine organics. A shift of the 300 µM OC<0.2 µm concentration of diatomaceous exudate in aerosol submicron size distribution toward smaller sizes and seawater produces an overall increment in the total source an increase in the production of particles with dry diame- particle flux of ∼20% with respect to the organics-free sea- ter (Dp0)<100 nm was repeatedly observed with increasing water case. The effect increases with decreasing particle size amounts of diatomaceous bioexudate in the seawater prox- for Dp0<100 nm, resulting in multiplicative factors between ies used for aerosol generation. The effect was found to 1.02–2 with respect to the particle flux generated from sea- be sensitive to the organic carbon concentration in seawa- water devoid of marine organics. The total source flux de- rived from the presented parameterisation was compared to recent definitions of sea-spray source fluxes based on labora- Correspondence to: G. McFiggans tory and field observations in the literature. (g.mcfi[email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 9296 E. Fuentes et al: Part 1: Source fluxes 1 Introduction mary organic matter released by marine biota (O’Dowd et al., 2004) little is known on the influence of these organics on the production and the behaviour of the marine aerosol. The marine aerosol constitutes an important fraction of the Bubble bursting experiments with seawater have shown global natural aerosol flux. As a consequence, it signif- high organic mass fractions in the primary aerosol submicron icantly contributes to the Earth’s radiative budget, atmo- size region (Keene et al., 2007; Facchini et al., 2008). The spheric albedo and biogeochemical processes (O’Dowd et submicron organic matter was found to be almost entirely al., 2004; O’Dowd and de Leeuw, 2007). Marine aerosols water insoluble and consisted of colloids and aggregates ex- can affect the atmospheric radiative balance both directly uded by phytoplankton (Facchini et al., 2008). The distribu- by scattering and absorbing solar radiation and indirectly by tion of the insoluble organic matter and sea-salt content in modifying the microphysics, radiative properties, and life- the submicron size range observed in bubble bursting exper- time of clouds (Murphy et al., 1998). It has been postulated iments was similar to that measured in atmospheric marine that the sea-spray production would be involved in a cloud- aerosol samples collected during periods of high biological climate feedback process if the increment of wind speed as activity (O’Dowd et al., 2004; Facchini et al., 2008). a result of global warming on oceanic regions would lead to Under breaking wave conditions, the primary mechanism an increase in the sea-spray particle production (Latham and of transport of biological surfactants to the ocean surface is Smith, 1990). by bubble scavenging. When the bubbles reach the water sur- Sea spray primary particles are produced as a result of face, the organics absorbed on their surface are ejected into breaking wave processes occurring on oceans surfaces. Wind the atmosphere along with dissolved inorganic constituents −1 generated waves breaking at wind speeds higher than 4 ms of seawater. Thereby, the seawater composition, bubble spec- produce sea-spray droplets with sizes from ≤0.01 µm to trum, bubble hydrodynamics and the formation and chemical ≥25 µm (Clarke et al., 2006; Lewis and Schwartz, 2004). composition of the aerosol are closely interrelated and inter- Breaking waves dissipate up to 40% of their energy and up dependent (Duce and Hoffman, 1976; Tseng et al., 1992). to 50% of the energy loss is expended in entraining air in The generation of sea-spray particles by bubble bursting the water bulk and creating a dense plume of bubbles (Rapp has been simulated in the laboratory in order to study the and Melville, 1990). The bubbles formed rise and burst upon primary aerosol production. Martensson˚ et al. (2003) and reaching the surface, thereby producing the so-called film Tyree et al. (2007) conducted aerosol size distribution mea- and jet drops. The sea spray total number concentration is surements by bubbling artificial seawater through different normally dominated by particles in the submicron size range porous media. Sellegri et al. (2006) compared the bubble from 5 to 300 nm, with a decline in the particle number with spectrum and the submicron aerosol size distributions gen- increasing size (Fitzgerald, 1991). erated by bubble bursting using a water recirculation system Atmospheric marine aerosols consist of primary aerosol, and aeration through glass frits of different porosity. Some which comprises organic material and sea-salt and is pro- of these investigations also studied the effect of salinity, wa- duced on the ocean surface by bubble-bursting and tearing ter temperature (Martensson˚ et al., 2003; Tyree et al., 2007; from breaking waves (Anguelova and Webster, 1999), and Sellegri et al., 2006) and organic matter by using organic secondary aerosol, formed by non sea-salt sulphate and or- proxies, such as oleic acid (Tyree et al., 2007) and sodium ganic species originated via gas-to-particle conversion pro- dodecyl sulphate (SDS) (Sellegri et al., 2006). The study of cesses such as homogeneous nucleation and condensation the effect of surfactants on the size distribution performed by onto pre-existing particles (O’Dowd et al., 1997). Chemi- Sellegri et al. (2006) and Tyree et al. (2007) indicated a shift cal analysis of atmospheric sea-spray particles collected in of the distributions toward smaller sizes with respect to the field experiments has provided evidence for the presence of baseline case without organic surrogate. Tyree et al. (2007) significant concentrations of organic matter in the submicron reported that this effect was accompanied by an increase in size range (Hoffman and Duce, 1977; Novakov et al., 1997; the particle production at constant temperature with respect Middlebrook et al., 1998; Putaud et al., 2000; Cavalli et al., to the experiment without surfactants. The chemical compo- 2004). The contribution of the organic fraction in the ma- sition of the phytoplankton-released organic matter consists rine aerosol has been found to be seasonal, accounting for up of a complex mixture of dissolved and particulate species,