Abstract
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 (J3) 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 (J1.5) 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 J1.5 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 J1.5 can lead to large uncertainties in the nucleation prefactors (i.e. constant P in nucleation equation J1.5 = P × [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 (J1.5 or J3) 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.
Highlights
Recent ion cluster measurements have indicated that atmospheric new particle formation via nucleation initiates at a cluster size of ∼1.5 nm in diameter (Manninen et al, 2009)
The cluster growth rate (Eq 7) was calculated from the time delay between N3−6 and [H2SO4]b profiles. This approach assumes that N3−6 follows [H2SO4]b with a time shift tN3−6, which is the case if the growth from initial nucleation size to 3 nm were dominated by condensation with a constant growth rate and if the coagulation sink of the clusters remained fairly constant for the duration of the event
We have evaluated the accuracy of the mathematical tools commonly used to analyse atmospheric new particle formation events in 1239 cases in which the nucleation mechanism and rate as well as the particle formation rate at 3 nm were known
Summary
Recent ion cluster measurements have indicated that atmospheric new particle formation via nucleation initiates at a cluster size of ∼1.5 nm in diameter (Manninen et al, 2009). Previous studies have developed a set of analysis tools to estimate the actual nucleation rate (J1.5) based on the measured size distribution and gas phase data The foundation of these tools, originally presented in Fiedler et al (2005) and Sihto et al (2006), lies in the observation that the diurnal profiles of sulphuric acid (H2SO4). Since H2SO4 is currently thought to be the key nucleating vapour, this time delay has been assumed as the time it takes for a cluster formed at 1–1.5 nm to grow to the detectable size of 3 nm This assumption makes it possible to estimate the cluster growth rate below 3 nm and, together with information about the coagulation scavenging of the clusters to background particles, it can be used to estimate the fraction of formed clusters that survive to the detectable sizes (Kerminen and Kulmala, 2002; Lehtinen et al, 2007). This information is in turn used to extrapolate the actual nucleation rate at 1.5 nm (J1.5) from the measured particle formation rate at 3 nm (J3) (Kerminen and Kulmala, 2002)
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