Contribution of ion-induced nucleation to new particle formation: Methodology and its application to atmospheric observations in Boulder, Colorado

This paper investigates the role of ion-induced nucleation (IIN) in new particle formation events observed near ground level at a sampling site located near Boulder, Colorado (14 March 2004 to 27 October 2005). Measurements of mobility distributions of small and intermediate ions (0.4-6.3 nm), size distributions of total particles (3 nm to 5 mm), and charged fractions (2.5-25 nm) were carried out. The relative contributions of neutral nucleation and IIN were inferred using both qualitative and quantitative analyses. First, a simple theoretical analysis is performed to show what can be learned about the initial charge state of the nucleated particles from charged fractions measured after they had grown to 3.0-5.5 nm. We found that for much of our data the charge fractions of freshly nucleated particles below 5 nm were significantly below stationary-state values, and that this tendency increased with decreasing size, indicating that neutral nucleation was dominant. However, the data also show that there were occasionally asymmetries between negative and positive charge fractions that we could not explain unless positive or negative IIN occurred to some extent. A quantitative analysis is then performed to estimate the fractional contribution of positive and negative IIN to new particle formation rates for each nucleation event observed during this period. The results show the average contribution of IIN is about 0.5% for both polarities indicating that IIN was a relatively insignificant contributor to new particle formation in this study. This result is consistent with the direct mass spectrometric measurements of sulfuric acid ion cluster compositions and concentrations performed at the same measurement site.