Evaluation of the accuracy of analysis tools for atmospheric new particle formation

Abstract

Several mathematical tools have been developed in recent years to analyze new particle formation rates and to estimate nucleation rates and mechanisms at sub-3 nm sizes from atmospheric aerosol data. Here we evaluate these analysis tools using 1239 numerical nucleation events for which the nucleation mechanism and formation rates were known exactly. The accuracy of the estimates of particle formation rate at 3 nm (<i>J</i>₃) showed significant sensitivity to the details of the analysis, i.e. form of equations used and assumptions made about the initial size of nucleating clusters, with the fraction of events within a factor-of-two accuracy ranging from 43–97%. In general, the estimates of the actual nucleation rate at 1.5 nm (<i>J</i>_1.5) were less accurate, and even the most accurate analysis set-up estimated only 59% of the events within a factor of two of the simulated mean nucleation rate. The <i>J</i>_1.5 estimates were deteriorated mainly by the size dependence of the cluster growth rate below 3 nm, which the analysis tools do not take into account, but also by possible erroneous assumptions about the initial cluster size. The poor estimates of <i>J</i>_1.5 can lead to large uncertainties in the nucleation prefactors (i.e. constant <i>P</i> in nucleation equation <i>J</i>_1.5 = <i>P</i> &times; [H₂SO₄]^k). Large uncertainties were found also in the procedures that are used to determine the nucleation mechanism. When applied to individual events, the analysis tools clearly overestimated the number of H₂SO₄ molecules in a critical cluster for most events, and thus associated them with a wrong nucleation mechanism. However, in some conditions the number of H₂SO₄ molecules in a critical cluster was underestimated. This indicates that analysis of field data that implies a maximum of 2 H₂SO₄ molecules in a cluster does not automatically rule out a higher number of molecules in the actual nucleating cluster. Our analysis also suggests that combining data from several new particle formation events to scatter plots of H₂SO₄ vs formation rates (<i>J</i>_1.5 or <i>J</i>₃) and determining the slope of the regression line may not give reliable information about the nucleation mechanism. Overall, while the analysis tools for new particle formation are useful for getting order-of-magnitude estimates of parameters related to atmospheric nucleation, one should be very cautious in interpreting the results. It is, for example, possible that the tools may have misdirected our theoretical understanding of the nucleation mechanism.