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: The applicability, methods and limitations of constrained peak-fitting on mass spectra of low mass resolving power (m/dm50 ~ 500) recorded with a time-of-flight aerosol chemical speciation monitor (ToF-ACSM) are explored. Calibration measurements as well as ambient data are used to exemplify the methods that should be applied to maximise data quality and assess confidence in peak-fitting results. Sensitivity analyses and basic peak fit metrics such as normalised ion separation are employed to demonstrate which peak-fitting analyses commonly performed in high-resolution aerosol mass spectrometry are appropriate to perform on spectra of this resolving power. Information on aerosol sulphate, nitrate, sodium chloride, methanesulphonic acid as well as semi-volatile metal species retrieved from these methods is evaluated. The constants in a commonly used formula for the estimation of the mass concentration of hydrocarbon-like organic aerosol may be refined based on peak-fitting results. Finally, application of a recently-published parameterisation for the estimation of carbon oxidation state to ToF-ACSM spectra is validated for a range of organic standards and its use demonstrated for ambient urban data.