Negative atmospheric ions and their potential role in ion-induced nucleation

Mass identified ion cluster distributions were measured under ambient atmospheric conditions and compared with model predictions based on laboratory ion cluster thermodynamics data. The results are shown from several days where atmospheric sulfur concentrations were high and thus ion-induced cluster growth was anticipated. Atmospheric gas phase sulfuric acid, temperature, relative humidity, SO2, mobility distributions of ions and small charged particles, and aerosol size distributions were also measured in support of the model calculations. The relative agreement of measurement and model for the first and second sulfuric acid clusters (HSO4- (H2SO4)(m)) for m = 1 and 2 is quite good but suggests that sulfuric acid clustering may not occur at the collision rate. Clusters for higher m values were not observed, which is also consistent with model predictions for the conditions under which measurements were performed. The lack of both observed and predicted large ion clusters is also consistent with the independent measurements of ion mobility distributions and particle size distributions, which showed similar numbers of positively and negatively charged ultrafine particles, suggesting that neither positive nor negative ion-induced nucleation processes were likely to have contributed significantly to observed new particle formation rates during this study. The relatively low observed concentrations of the bisulfate ion also suggest that the processes leading to the first sulfuric acid/bisulfate cluster (HSO4- H2SO4) may be more complicated than simple sulfuric acid clustering or exchange reactions. While nucleation was observed on some days, measurements suggest that ion-induced nucleation did not contribute significantly to new particle production or growth during these events. This does not rule out the possibility that ion-induced nucleation could contribute significantly to atmospheric new particle formation under very different atmosphere conditions such as in areas with much lower temperatures and higher ion concentrations.