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Author: Wiedensohler, Alfred × Type: Text × Subject: atmospheric pollution × Subject: size distribution ×

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    Variation of CCN activity during new particle formation events in the North China Plain
    (München : European Geopyhsical Union, 2016) Ma, Nan; Zhao, Chunsheng; Tao, Jiangchuan; Wu, Zhijun; Kecorius, Simonas; Wang, Zhibin; Größ, Johannes; Liu, Hongjian; Bian, Yuxuan; Kuang, Ye; Teich, Monique; Spindler, Gerald; Müller, Konrad; van Pinxteren, Dominik; Herrmann, Hartmut; Hu, Min; Wiedensohler, Alfred
    The aim of this investigation was to obtain a better understanding of the variability of the cloud condensation nuclei (CCN) activity during new particle formation (NPF) events in an anthropogenically polluted atmosphere of the North China Plain (NCP). We investigated the size-resolved activation ratio as well as particle number size distribution, hygroscopicity, and volatility during a 4-week intensive field experiment in summertime at a regional atmospheric observatory in Xianghe. Interestingly, based on a case study, two types of NPF events were found, in which the newly formed particles exhibited either a higher or a lower hygroscopicity. Therefore, the CCN activity of newly formed particles in different NPF events was largely different, indicating that a simple parameterization of particle CCN activity during NPF events over the NCP might lead to poor estimates of CCN number concentration (NCCN). For a more accurate estimation of the potential NCCN during NPF events, the variation of CCN activity has to be taken into account. Considering that a fixed activation ratio curve or critical diameter are usually used to calculate NCCN, the influence of the variation of particle CCN activity on the calculation of NCCN during NPF events was evaluated based on the two parameterizations. It was found that NCCN might be underestimated by up to 30 % if a single activation ratio curve (representative of the region and season) were to be used in the calculation; and might be underestimated by up to 50 % if a fixed critical diameter (representative of the region and season) were used. Therefore, we suggest not using a fixed critical diameter in the prediction of NCCN in NPF. If real-time CCN activity data are not available, using a proper fixed activation ratio curve can be an alternative but compromised choice.
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    Cloud condensation nuclei spectra derived from size distributions and hygroscopic properties of the aerosol in coastal south-west Portugal during ACE-2
    (Milton Park : Taylor & Francis, 2016) Dusek, Ulrike; Covert, David S.; Wiedensohler, Alfred; Neusüss, Christian; Weise, Diana; Cantrell, Will
    In this work we propose and test a method to calculate cloud condensation nuclei (CCN) spectra basedon aerosol number size distributions and hygroscopic growth factors. Sensitivity studies show thatthis method can be used in a wide variety of conditions except when the aerosol consist mainly oforganic compounds. One crucial step in the calculations, estimating soluble ions in an aerosol particlebased on hygroscopic growth factors, is tested in an internal hygroscopic consistency study. The resultsshow that during the second Aerosol Characterization Experiment (ACE-2) the number concentrationof inorganic ions analyzed in impactor samples could be reproduced from measured growth factorswithin the measurement uncertainties at the measurement site in Sagres, Portugal. CCN spectra were calculated based on data from the ACE-2 field experiment at the Sagres site.The calculations overestimate measured CCN spectra on average by approximately 30%, which iscomparable to the uncertainties in measurements and calculations at supersaturations below 0.5%. Thecalculated CCN spectra were averaged over time periods when Sagres received clean air masses and airmasses influenced by aged and recent pollution. Pollution outbreaks enhance the CCN concentrationsat supersaturations near 0.2% by a factor of 3 (aged pollution) to 5 (recent pollution) compared to theclean marine background concentrations. In polluted air masses, the shape of the CCN spectra changes.The clean spectra can be approximated by a power function, whereas the polluted spectra are betterapproximated by an error function.