The role of highly oxidized multifunctional organic molecules for the
growth of new particles over the boreal forest region
Emilie Öström1,2, Pontus Roldin1,3, Guy Schurgers4, Mikhail Mishurov5, Zhou Putian3, Niku Kivekäs6, Heikki Lihavainen6, Mikael Ehn3, Matti P. Rissanen3, Theo Kurtén7, Michael Boy3, and Erik Swietlicki11Division of Nuclear Physics, Lund University, Lund, P.O. Box 118, 221 00, Sweden 2Centre for Environmental and Climate Research, Lund University, Lund, P.O. Box 118, 221 00, Sweden 3Department of Physics, University of Helsinki, Helsinki, Box 64, 00014, Finland 4Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, 1350, Denmark 5Department of Physical Geography and Ecosystem Science, Lund University, Lund, 223 62, Sweden 6Finnish Meteorological Institute, Helsinki, Box 503, 00101, Finland 7Department of Chemistry, University of Helsinki, Helsinki, P.O. Box 55, 00014, Finland
Received: 10 Oct 2016 – Accepted for review: 05 Dec 2016 – Discussion started: 09 Dec 2016
Abstract. Secondary organic aerosol particles (SOA) are important climate forcers, especially in otherwise clean environments such as the boreal forest. There are, however, major uncertainties in the mechanisms behind the formation of SOA, and in order to predict the growth and abundance of SOA at different conditions, process-based understanding is needed. In this study, the processes behind new particle formation (NPF) events and subsequent growth of these particles in the northern Europe sub-Arctic forest region are explored with the one-dimensional column trajectory model ADCHEM. The results from the model are compared with particle number size distribution measurements from Pallas Atmosphere-Ecosystem Supersite in Northern Finland. The model was able to reproduce the observed growth of the newly formed particles if a small fraction of the emitted monoterpenes that are oxidized by O3 and OH undergo autoxidation and form highly oxidized multifunctional organic molecules (HOMs) with low or extremely low volatility. The modeled particles originating from the NPF events (diameter < 100 nm) are composed predominantly of HOMs. While the model seems to capture the growth of the newly formed particles between 1.5 and ~ 20 nm in diameter, it underestimated the particle growth between ~ 20 and 80 nm in diameter. Due to the high fraction of HOMs in the particle phase, the oxygen-to-carbon (O : C) atomic ratio of the SOA was nearly 1. This unusually high O : C and the discrepancy between the modeled and observed particle growth might be explained by the fact that the model did not consider any particle-phase reactions involving semi-volatile organic compounds with relatively low O : C. According to the model the phase state of the SOA (assumed either liquid or amorphous solid) had an insignificant effect on the evolution of the particle number size distribution during the NPF events. The results were sensitive to the method used to estimate the vapor pressures of the HOMs. If the HOMs were assumed to be extremely low volatile organic compounds (ELVOCs) or non-volatile the modeled particle growth was substantially higher than when the vapor pressures of the HOMs were estimated based on continuum solvent model calculations using quantum chemical data. Overall, the model was able to capture the main features of the observed formation and growth rates during the studied NPF-events if the HOM mechanism was included.
Öström, E., Roldin, P., Schurgers, G., Mishurov, M., Putian, Z., Kivekäs, N., Lihavainen, H., Ehn, M., Rissanen, M. P., Kurtén, T., Boy, M., and Swietlicki, E.: The role of highly oxidized multifunctional organic molecules for the
growth of new particles over the boreal forest region, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-912, in review, 2016.