Publications

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2016
Wex, H, Dieckmann K, Roberts GC, Conrath T, Izaguirre MA, Hartmann S, Herenz P, Schafer M, Ditas F, Schmeissner T, Henning S, Wehner B, Siebert H, Stratmann F.  2016.  Aerosol arriving on the Caribbean island of Barbados: physical properties and origin. Atmospheric Chemistry and Physics. 16:14107-14130.   10.5194/acp-16-14107-2016   AbstractWebsite

The marine aerosol arriving at Barbados (Ragged Point) was characterized during two 3-week long measurement periods in November 2010 and April 2011, in the context of the measurement campaign CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados). Through a comparison between ground-based and airborne measurements it was shown that the former are representative of the marine boundary layer at least up to cloud base. In general, total particle number concentrations (N-total) ranged from as low as 100 up to 800 cm(-3), while number concentrations for cloud condensation nuclei (N-CCN) at a supersaturation of 0.26% ranged from some 10 to 600 cm(-3). N-total and N-CCN depended on the air mass origin. Three distinct types of air masses were found. One type showed elevated values for both N-total and N-CCN and could be attributed to long-range transport from Africa, by which biomass burning particles from the Sahel region and/or mineral dust particles from the Sahara were advected. The second and third type both had values for N-CCN below 200 cm(-3) and a clear minimum in the particle number size distribution (NSD) around 70 to 80 nm (Hoppel minimum). While for one of these two types the accumulation mode was dominating (albeit less so than for air masses advected from Africa), the Aitken mode dominated the other and contributed more than 50% of all particles. These Aitken mode particles likely were formed by new particle formation no more than 3 days prior to the measurements. Hygroscopicity of particles in the CCN size range was determined from CCN measurements to be kappa = 0.66 on average, which suggests that these particles contain mainly sulfate and do not show a strong influence from organic material, which might generally be the case for the months during which measurements were made. The average kappa could be used to derive N-CCN from measured number size distributions, showing that this is a valid approach to obtain N-CCN. Although the total particulate mass sampled on filters was found to be dominated by Na+ and Cl-, this was found to be contributed by a small number of large particles (> 500 nm, mostly even in the super-micron size range). Based on a three-modal fit, a sea spray mode observed in the NSDs was found to contribute 90% to the total particulate mass but only 4 to 10% to N-total and up to 15% to N-CCN. This is in accordance with finding no correlation between N-total and wind speed.

2008
Roberts, GC, Ramana MV, Corrigan C, Kim D, Ramanathan V.  2008.  Simultaneous observations of aerosol-cloud-albedo interactions with three stacked unmanned aerial vehicles. Proceedings of the National Academy of Sciences of the United States of America. 105:7370-7375.   10.1073/pnas.0710308105   AbstractWebsite

Aerosol impacts on climate change are still poorly understood, in part, because the few observations and methods for detecting their effects are not well established. For the first time, the enhancement in cloud albedo is directly measured on a cloud-by-cloud basis and linked to increasing aerosol concentrations by using multiple autonomous unmanned aerial vehicles to simultaneously observe the cloud microphysics, vertical aerosol distribution, and associated solar radiative fluxes. In the presence of long-range transport of dust and anthropogenic pollution, the trade cumuli have higher droplet concentrations and are on average brighter. Our observations suggest a higher sensitivity of radiative forcing by trade cumuli to increases in cloud droplet concentrations. than previously reported owing to a constrained droplet radius such that increases in droplet concentrations also increase cloud liquid water content. This aerosol-cloud forcing efficiency is as much as -60 W m(-2) per 100% percent cloud fraction for a doubling of droplet concentrations and associated increase of liquid water content. Finally, we develop a strategy for detecting aerosol-cloud interactions based on a nondimensional scaling analysis that relates the contribution of single clouds to albedo measurements and illustrates the significance of characterizing cloud morphology in resolving radiometric measurements. This study demonstrates that aerosol-cloud-albedo interactions can be directly observed by simultaneous observations below, in, and above the clouds.