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Using measurements of the aerosol charging state in determination of the particle growth rate and the proportion of ion-induced nucleation

by J. Lepp??, S. Gagn??, L. Laakso, H. E. Manninen, K. E J Lehtinen, M. Kulmala, V. M. Kerminen
Atmospheric Chemistry and Physics ()
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The fraction of charged nucleation mode particles as a function of\nparticle diameter depends on the particle growth rate and the proportion\nof particles formed via ion-induced nucleation. In this study we have\ntested the applicability of recent data analysis methods to determine\nthe growth rate and the proportion of ion-induced nucleation from the\nmeasured charged fractions. For this purpose we have conducted a series\nof aerosol dynamic simulations covering a wide range of atmospheric\nconditions. The growth rate and initial fraction of charged particles\nwere estimated from simulated data using these methods and compared with\nthe values obtained directly from the simulations. We found that the\ndata analysis methods used in this study should not be used when the\nnuclei growth rate is less than similar to 3 nm h(-1), or when charged\nparticles grow much more rapidly than neutral ones. Furthermore, we\nfound that the difference in removal rates of neutral and charged\nparticles should be taken into account when estimating the proportion of\nion-induced nucleation. Neglecting the higher removal rate of charged\nparticles compared with that of neutral ones could result in an\nunderestimation of the proportion of ion-induced nucleation by up to a\nfactor of 2. This underestimation is further increased if charged\nparticles grow more rapidly than neutral ones. We also provided a simple\nway of assessing whether these methods are suitable for analyzing data\nmeasured under specific conditions. The assessment procedure was\nillustrated using a few examples of actual measurement sites with a more\ndetailed examination of the typical conditions observed at the SMEAR II\nstation in Hyytiala, Finland.

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