Charging state of the atmospheric nucleation mode: Implications for separating neutral and ion‐induced nucleation

Abstract

[1] One way of getting information about the relative roles of neutral and ion-induced nucleation in the atmosphere is to measure the charging state of aerosol particles, i.e., the extent by which the charged fraction of aerosol particles differs from the corresponding equilibrium charged fraction. By using a theoretical approach, we investigated how the charging state of a growing nucleation mode behaves under different atmospheric conditions. We found that the time evolution of the nuclei charging state is governed by two parameters: the initial nuclei charging state determined by the nucleation mechanism and the parameter K that is directly proportional to the cluster ion concentration and inversely proportional to the nuclei growth rate. We demonstrated that if the value K is larger than a certain threshold value (2-4 nm À1 ), any information about the initial charging state of the nuclei, and thereby about the nucleation mechanism, will be lost by the time the nuclei grow into the measurement size range (>3 nm). By making a few simplifying assumptions, we derived an analytical expression for the functional dependence of the nuclei charging state as a function of the nuclei size. We demonstrated that the derived expression can usually be fitted into experimental data on nuclei charging states. When the value of K is small enough, the obtained fitting can be extrapolated successfully down to sizes where the nucleation has taken place to obtain information about the relative importance of neutral and ion-induced nucleation.Citation: Kerminen, V.-M., T. Anttila, T. Petäjä, L. Laakso, S. Gagné, K. E. J. Lehtinen, and M. , Charging state of the atmospheric nucleation mode: Implications for separating neutral and ion-induced nucleation,

General overview: European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) – integrating aerosol research from nano to global scales

“…The field measurements brought plenty of new insight into the role of ions in atmospheric nucleation Kerminen et al, 2007;Gagné et al, 2008Gagné et al, , 2010Manninen et al, 2010;Mirme et al, 2010;Boulon et al, 2010). The contribution of charged particles to the total formation rate of 2-nm particles was usually found to be well below 10 %, but it showed substantial temporal variability both during a nucleation event and between the different event days.…”

New insights into nocturnal nucleation

“…Possible reasons for the absence of charged nucleation-mode particles was that the nucleation took place mostly via a neutral pathway, and the nucleated particles grew so rapidly that they did not get detectable amount of charge until reaching larger sizes (e.g. Kerminen et al, 2007;Leppä et al, 2009). This is very important from the point of view of field event classification: if based on sole ion size distribution measurements, events classified as Aitken events could, in fact, be typical bananas for neutral particles.…”

The role of atmospheric ions in aerosol nucleation – a review

“…Since particles nucleate below the limit of detection the charged fraction may change from the time of nucleation and until detection. Kerminen et al (2007) described how the charged fraction of a particle population evolves with growth and found that it is dependent on two factors: the cluster ion concentration and the growth rate of the population (corresponding to the amount of condensable gases) such that at high ion concentrations and/or low growth rates the information on the original charging state may be lost when the particles are detected. If the electron can be ejected from a cluster due to the release of e.g.…”