Modelling study on HOMs and their contribution to NPF at SORPES (2018)


Highly oxygenated multifunctional compounds (HOMs) play a key role in new particle formation (NPF), but their quantitative roles in different environments of the globe have not been well studied yet. Qi et al. (2018) utilized a combination of observations and modeling to explore the role of various condensable vapors to aerosol formation and growth. Frequent NPF events were observed at two 'flagship' stations under different environmental conditions, i.e. a remote boreal forest site (SMEAR II) in Finland and a suburban site (SORPES) in polluted eastern China. The averaged formation rate of 6 nm particles and the growth rate of 6–30 nm particles were 0.3 cm-3 s-1 and 4.5 nm h-1 at SMEAR II compared to 2.3 cm-3 s-1 and 8.7 nm h-1 at SORPES, respectively.

Figure 1 The relative contributions of precursor vapours to the growth of sub-100 nm particles at (a) SMEAR II and (b) SORPES.

To explore the differences of NPF at the two stations, the HOM concentrations and NPF events at two sites were simulated with the MALTE-BOX model, and their roles in NPF and particle growth in the two distinctly different environments are discussed. The model provides an acceptable agreement between the simulated and measured concentrations of sulfuric acid and HOMs at SMEAR II. The sulfuric acid and HOM organonitrate concentrations are significantly higher but other HOM monomers and dimers from monoterpene oxidation are lower at SORPES compared to SMEAR II. The model simulates the NPF events at SMEAR II with a good agreement but underestimates the growth of new particles at SORPES, indicating a dominant role of anthropogenic processes in the polluted environment. HOMs from monoterpene oxidation dominate the growth of ultrafine particles at SMEAR II while sulfuric acid and HOMs from aromatics oxidation play a more important role in particle growth. This study highlights the distinct roles of sulfuric acid and HOMs in NPF and particle growth in different environmental conditions and suggests the need for molecular-scale measurements in improving the understanding of NPF mechanisms in polluted areas like eastern China.

Qi, X. M., Ding, A. J., Roldin, P., Xu, Z. N., Zhou, P. T., Sarnela, N., Nie, W., Huang, X., Rusanen, A., Ehn, M., Rissanen, M., Petäjä, T., Kulmala, M., and Michael, B.: Modelling studies of HOMs and their contributions to new particle formation and growth: comparison of boreal forest in Finland and a polluted environment in China, Atmos. Chem. Phys., 18, 11779-11791, 2018