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2013
Collins, DB, Ault AP, Moffet RC, Ruppel MJ, Cuadra-Rodriguez LA, Guasco TL, Corrigan CE, Pedler BE, Azam F, Aluwihare LI, Bertram TH, Roberts GC, Grassian VH, Prather KA.  2013.  Impact of marine biogeochemistry on the chemical mixing state and cloud forming ability of nascent sea spray aerosol. Journal of Geophysical Research-Atmospheres. 118:8553-8565.   10.1002/jgrd.50598   AbstractWebsite

The composition and properties of sea spray aerosol, a major component of the atmosphere, are often controlled by marine biological activity; however, the scope of impacts that ocean chemistry has on the ability for sea spray aerosol to act as cloud condensation nuclei (CCN) is not well understood. In this study, we utilize a mesocosm experiment to investigate the impact of marine biogeochemical processes on the composition and mixing state of sea spray aerosol particles with diameters<0.2 mu m produced by controlled breaking waves in a unique ocean-atmosphere facility. An increase in relative abundance of a distinct, insoluble organic particle type was observed after concentrations of heterotrophic bacteria increased in the seawater, leading to an 86 +/- 5% reduction in the hygroscopicity parameter () at 0.2% supersaturation. Aerosol size distributions showed very little change and the submicron organic mass fraction increased by less than 15% throughout the experiment; as such, neither of these typical metrics can explain the observed reduction in hygroscopicity. Predictions of the hygroscopicity parameter that make the common assumption that all particles have the same bulk organic volume fractions lead to overpredictions of CCN concentrations by 25% in these experiments. Importantly, key changes in sea spray aerosol mixing state that ultimately influenced CCN activity were driven by bacteria-mediated alterations to the organic composition of seawater.

Siebert, H, Beals M, Bethke J, Bierwirth E, Conrath T, Dieckmann K, Ditas F, Ehrlich A, Farrell D, Hartmann S, Izaguirre MA, Katzwinkel J, Nuijens L, Roberts G, Schafer M, Shaw RA, Schmeissner T, Serikov I, Stevens B, Stratmann F, Wehner B, Wendisch M, Werner F, Wex H.  2013.  The fine-scale structure of the trade wind cumuli over Barbados - an introduction to the CARRIBA project. Atmospheric Chemistry and Physics. 13:10061-10077.   10.5194/acp-13-10061-2013   AbstractWebsite

The CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) project, focused on high resolution and collocated measurements of thermodynamic, turbulent, microphysical, and radiative properties of trade wind cumuli over Barbados, is introduced. The project is based on two one-month field campaigns in November 2010 (climatic wet season) and April 2011 (climatic dry season). Observations are based on helicopterborne and ground-based measurements in an area of 100 km(2) off the coast of Barbados. CARRIBA is accompanied by long-term observations at the Barbados Cloud Observatory located at the East coast of Barbados since early in 2010 and which provides a longer-term context for the CARRIBA measurements. The deployed instrumentation and sampling strategy are presented together with a classification of the meteorological conditions. The two campaigns were influenced by different air masses advected from the Caribbean area, the Atlantic Ocean, and the African continent which led to distinct aerosol conditions. Pristine conditions with low aerosol particle number concentrations of similar to 100 cm(3) were alternating with periods influenced by Saharan dust or aerosol from biomass burning resulting in comparably high number concentrations of similar to 500 cm(3). The biomass burning aerosol was originating from both the Caribbean area and Africa. The shallow cumulus clouds responded to the different aerosol conditions with a wide range of mean droplet sizes and number concentrations. Two days with different aerosol and cloud microphysical properties but almost identical meteorological conditions have been analyzed in detail. The differences in the droplet number concentration and droplet sizes appear not to show any significant change for turbulent cloud mixing, but the relative roles of droplet inertia and sedimentation in initiating coalescence, as well as the cloud reflectivity, do change substantially.