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

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(-1)), 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.