## 1 Introduction  
Atmospheric aerosols or particulate matter are liquid or solid airborne particles that are ubiquitous in the air that we breathe. They affect our lives by reducing visibility and influencing climate due to their reflection/absorption of incoming solar radiation and role in cloud formation [1]. Most importantly, they have serious adverse health effects including mortality and morbidity [2, 3, 4]. Organic aerosol (OA) is a major component of the sub-micrometer atmospheric particulate matter [5, 6]. OA is either emitted directly into the atmosphere as particulate matter (primary OA, POA) or is formed by gas-to-particle conversion of volatile, intermediate-volatility and semi-volatile organic compounds (secondary OA, SOA). The corresponding oxidation reactions of organic compounds lead to thousands of mostly unknown oxygenated products. Our understanding of SOA formation mechanisms and physical properties remains incomplete. Terrestrial vegetation emits a variety of biogenic volatile organic compounds into the atmosphere [7], some of which are subsequently oxidized, forming, among other products, secondary organic aerosols. Boreal forests produce a large amount of SOA mainly due to emitted monoterepenes and sesquiterpenes [8]. The biogenic SOA fraction has been found to represent 10 to 65% of the measured OA in or near forested regions [9; 10; 11; 12]. The use of lumped species allows the computationally efficient representation of OA in atmospheric chemical transport models (CTMs) [13]. The volatility basis set (VBS) framework [14] lumps organic compounds into surrogates along an axis of volatility. This approach typically employs species with effective saturation concentrations at 298 K separated by one order of magnitude, with values ranging from, say, 0.01 to \(10^{6}\)\(\mu\)g m\({}^{-3}\). By quantifying the volatility distributions of primary and secondary OA, a physically reasonable, yet suitable for large-scale CTMs, description of semi-volatile organics can be obtained [15]. The VBS framework was extended [16; 17] by adding a second dimension, the oxygen content (expressed as the O:C ratio), for the description of the OA chemical aging reactions [18; 19] using a one-dimensional Lagrangian CTM (PMCAMx-Trj) as the host model for the simulations. PMCAMx-Trj simulates the chemical evolution of a column of air as it travels towards a user-selected receptor site. Using this 2D-VBS framework, various parameterizations for the OA processing and aging can be developed in an effort to describe the various functionalization and fragmentation chemical pathways of the reactions of the various organic compounds and the OH radicals or other oxidants. [PERSON] et al. [19] tested three alternative parameterizations for the functionalization processes. Two were relatively simple, assuming a net reduction of volatility by one or two orders of magnitude (corresponding to 1-bin and 2-bin shifts) during each aging reaction step. These reactions were accompanied by an increase of one or two atoms of oxygen with an equal probability. The third parameterization was more complex and was based on a detailed functionalization scheme introduced by [PERSON] et al. [16]. In this scheme, there is a 30%, 50% and 20% probability of adding 1, 2 or 3 oxygen atoms to the parent molecule, respectively. At the same time, each addition of oxygen atoms results in a different volatility reduction distribution, averaged to \(-1.75\) in log\({}_{10}\)C* per oxygen group added. On the other hand, fragmentation is another important process that takes place during the chemical reactions of organic molecules with OH radicals, leading to products with lower carbon numbers than the precursors. [PERSON] et al. [19] assumed that this bond cleavage happens randomly and is uniformly distributed throughout the carbon backbone. [PERSON] et al. [20] used the same Lagrangian CTM with the 2D-VBS framework and evaluated hundreds of different parameterizations of OA formation and chemical aging for the Po Valley in Italy during the PEGASOS 2012 campaign.