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New particle formation (NPF) has been investigated intensively during the last 2 decades because of its influence on aerosol population and the possible contribution to cloud condensation nuclei. However, intensive measurements and modelling activities on this topic in urban metropolitan areas in China with frequent high-pollution episodes are still very limited. This study provides results from a comprehensive modelling study on the occurrence of NPF events in the western part of the Yangtze River Delta (YRD) region, China. The comprehensive modelling system, which combines the WRF-Chem (the Weather Research and Forecasting model coupled with Chemistry) regional chemical transport model and the MALTE-BOX sectional box model (the model to predict new aerosol formation in the lower troposphere), was shown to be capable of simulating atmospheric nucleation and subsequent growth. Here we present a detailed discussion of three typical NPF days, during which the measured air masses were notably influenced by either anthropogenic activities, biogenic emissions, or mixed ocean and continental sources. Overall, simulated NPF events were generally in good agreement with the corresponding measurements, enabling us to get further insights into NPF processes in the YRD region. Based on the simulations, we conclude that biogenic organic compounds, particularly monoterpenes, play an essential role in the initial condensational growth of newly formed clusters through their low-volatility oxidation products. Although some uncertainties remain in this modelling system, this method provides a possibility to better understand particle formation and growth processes.

The detailed mechanism of secondary new particle formation in the atmosphere is still under debate. It is proposed that particle formation happens via activation of 1–2 nm atmospheric neutral molecular clusters and/or large molecules. Since traditional instrumentation does not reach these sizes, the hypothesis has not yet been verified. By directly measuring particle size distributions down to mobility diameters of about 1.3 nm with a pulse-height CPC, we provide evidence of the nucleation mechanism in a coastal environment (Mace Head, Ireland) and in a boreal forest (Hyytiälä, Finland). In both places neutral sub-3 nm condensation nuclei (nano-CN) were continuously present, even when no new particle formation was detected. In Mace Head, however, the concentration of the nano-CN was far too low to account for the particle formation rates during particle bursts. Thus the results imply that at coastal sites new particle formation initiates, as previously proposed, via homogenous nucleation from biogenic iodine vapors. In contrast, activation of pre-existing nano-CN remains a possible explanation in the boreal forest, but the observed concentrations are not the limiting factor for the particle formation events.

Aerosol dynamical box model simulations of coastal new particle formation were performed in order to investigate the nucleation and growth mechanisms in this environment. In the simulations the nucleating vapor was assumed to be iodine dioxide (OIO). Both Eulerian and Lagrangian type simulations were made and compared with observations. We tested three nucleation mechanisms: kinetic nucleation of OIO (K x [OIO](2)), activation of clusters by OIO (A x [OIO]) and sulphuric acid-induced activation of clusters containing OIO (B x [OIO] x [H2SO4]). All the nucleation mechanisms provided reasonable results, although the growth of particles due to condensation is inadequate in kinetic nucleation cases as compared with experimental measurements. Growth of newly formed particles could be assisted by any low-volatility vapors should their concentration exceed 10(9) cm(-3). Using the obtained values of coefficients K, A, and B we found that nucleation driven by iodine compounds in remote marine areas is possible, but by OIO and H2SO4 alone, only a minor fraction of newly formed particles is likely to reach detectable sizes. Owing to the scavenging by coagulation with background aerosol particles, few of them will likely reach climatically relevant sizes by acting as seed particles for other low-volatility vapors. In order to elucidate the significance of our results, more detailed measurements of OIO source and photolysis rates, dimensions of precursor areas, and particle chemical composition are needed.