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Events of increased particle number concentrations around trade wind cumuli near Barbados
B. Wehner1, F. Ditas1, A. Wiedensohler1 and H. Siebert1 1Leibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, Germany Keywords: new particle formation, clouds, ultrafine particles. Presenting author email: [email protected] Beside numerous measurements at ground-based stations, new particle formation has been occasionally observed in higher altitudes of the troposphere. However, such measurements are rare and thus the relevant processes and meteorological conditions under which particle formation and growth occur are still not completely understood. In November 2010 and April 2011 the CARRIBA (Cloud, Aerosol, Radiation, and TuRbulence in the trade
wInd regime over BArbados) campaign (Siebert et al.,
2012) has been performed to observe the role of aerosol
particles within the life cycle of trade wind cumuli.
During these campaigns more than 30 research flights
using the helicopter-borne platform ACTOS (Airborne
Cloud and Turbulence Observation System) have been
done. For measuring aerosol particles a new FastCPC Figure 1. Time series of specific humidity r, total particle (Wehner et al., 2011) has been implemented on ACTOS number concentration N, liquid water content LWC, and to measure the total particle number concentration N with a time resolution of 10 Hz. The aerosol particle number size distribution from 6 nm to 2.5 m was measured using an SMPS (designed by TROPOS) and an OPC (Model 1.129, Grimm Aerosol Technik, Ainring, Germany). Furthermore, ACTOS contains various instruments thermodynamic parameters, such as temperature,
humidity and wind vector.
Results & Discussion
The flight pattern of the individual flights started Figure 2. Length of events of increased N. usually with a vertical profile over the sea east of the island of Barbados. After that ACTOS was usually After a quality check, 95 events of increased carried to a field of shallow cumuli with typical cloud particle number concentrations have been detected. Most of them occur close to cloud edges (~95%) others During individual flights, N was nearly constant without any cloud nearby. More than 50% of them lasted around clouds about a few hundred cm-3. During between 1 and 5 s corresponding to a horizontal approximately 70 % of all flights one or more events extension between 20 and 100 m. The detection of such with significantly increased (more than a factor of 5) small regions was possible due to the combination of the values of N have been observed. These events were often low true airspeed of 20 m/s and the fast measuring connected to clouds, i.e. the number concentration instrumentation. The implication of such particle bursts increased close to cloud edges. However, in a few cases for further cloud development is not yet fully understood. In conclusion, the cloud system does not only act as a Figure 1 shows an example for increased particle particle sink for CCN but also as a particle source. The number concentrations near cumulus clouds. The liquid relevance of new particle formation for the particle water content (LWC) is used as cloud indicator. Here, balance in general will be part of an on-going discussion. the background value of N is around 300 cm-3, while some peaks reach values above 4000 cm-3 near a cloud. Anthropogenic pollution as particle source can be Forschungsgemeinschaft (SI 1534/3-1 and WE 1900/18-1). Siebert, H., et al. (2012), The fine-scale structure of the trade wind regime over Barbados - An introduction to the CARRIBA project -, Figure 1 shows no obvious correlation between Atmos. Chem. Phys. Discuss., 12, 28609 - 28660. specific humidity r or vertical wind speed w and Wehner, B., et al.: Characterization of a new fast mixing type CPC and observed increased particle number concentration. This its application for atmospheric particle measurements, Atmos. Meas. is similar to most other cases and a more detailed

Source: http://eac2013.cz/EAC%20abstracta/Atmospheric%20Aerosols/Wehner-Birgit_Atmospheric_Aerosol_20130228-122848_5625853405.pdf