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Author: Dusek, Ulrike × Author: Wiedensohler, Alfred × Type: article × Subject: size distribution ×

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    Aerosol number to volume ratios in Southwest Portugal during ACE-2
    (Milton Park : Taylor & Francis, 2017) Dusek, Ulrike; Covert, David S.; Wiedensohler, Alfred; Neusúss, Christian; Weise, Diana
    Past studies have indicated that long-term averages of the aerosol number to volume ratios (defined as the number of particles larger than a certain diameter divided by the particle volume over some range less than 1 μm) show little variability over the Atlantic. This work presents number to volume ratios (R) measured during the ACE-2 experiment on the land-based Sagres field site located in Southwest Portugal. The values of R measured in Sagres compare reasonably well with previous measurements over the Atlantic. The main emphasis of this work is therefore to investigate more closely possible reasons for the observed stability of the number to volume ratio. Aerosol number size distributions measured in Sagres are parametrized by the sum of two log-normal distributions fitted to the accumulation and to the Aitken mode. The main factor that limits the variability of R is that the parameters of these log-normal distributions are not always independent but show some covariance. In polluted air mass types correlations between parameters of the Aitken and accumulation mode are mostly responsible for stabilizing R. In marine air mass types the variability of R is reduced by an inverse relationship between the accumulation-mode mean diameter and standard deviation, consistent with condensational processes and cloud processing working on the aerosol. However, despite this reduction, the variability of R in marine air mass types is still considerable and R is linearly dependent on the number concentration of particles larger than 90 nm. This partly due to a mil of Aitken-mode particles extending to sizes larger than 90 nm.
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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.