Description: 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.

Description: Eddy covariance fluxes of formic acid, HCOOH, were measured over a boreal forest canopy in spring/summer 2014. The HCOOH fluxes were bidirectional, but mostly upward during daytime, in contrast to studies elsewhere that reported mostly downward fluxes. Downward flux episodes were explained well by modeled dry deposition rates. The sum of net observed flux and modeled dry deposition yields an upward “gross flux” of HCOOH, which could not be quantitatively explained by literature estimates of direct vegetative/soil emissions nor by efficient chemical production from other volatile organic compounds (VOC), suggesting missing or greatly underestimated HCOOH sources in the boreal ecosystem. We implemented a vegetative HCOOH source into the GEOS-Chem chemical transport model to match our derived gross flux, and evaluated the updated model against air- and space-borne observations. Model biases in the boundary layer were substantially reduced based on this revised treatment, but biases in the free troposphere remain unexplained.