## 2 Coping with Water-Soluble OA Complexity  
As mentioned above, the water-soluble OA fraction covers a huge variety of molecular structures with different physicochemical properties and sources. Nevertheless, according to [PERSON] et al. [22], not all atmospheric issues require the identification of all organic compounds present in OA samples. Depending on the purpose of the investigation, different levels of organic compositional information can be distinguished [22; 24]: (i) functional group analysis is typically employed when interested in understanding specific properties of OA (e.g., chemistry, optical properties or structural average parameters of OAs); (ii) resolve the chemical composition of OA into different organic components (e.g., hydrocarbon-like OA (HOA), oxidized OA (OOA), and biomass-burning OA (BBOA)) in real time is usually chosen for capturing the characteristic chemical and physical properties of OA in a rapidly changing environment; (iii) target analysis of molecular markers is typically used when monitoring known OA formation processes or sources; and (iv) identification of up to three specific organic compounds when studying unknown OA formation processes or sources. For over two decades, these different levels of OA analysis have been dedicated to the quantification and characterization of the aerosol WSOM component. These efforts have been accompanied by numerous laboratory and field studies focused on the characterization of WSOM sources, as well as its mechanisms of formation and rates of its atmospheric transformations. Currently, these issues are still poorly known, although the major sources of these compounds are considered to be biomass burning and secondary formation (involving both anthropogenic and biogenic volatile organic compounds). To further enhance the diversity and complexity of this OA fraction, the atmospheric WSOM and their precursor gases can also be modified in the atmosphere by ox idative processes and, thereby, becoming less volatile, more hygroscopic and, consequently, more water-soluble [25; 26]. Several procedures and methodologies have been developed to study the chemical composition of the aerosol WSOM [8; 9; 10; 23; 27; 28; 29; 30; 31; 32; 33; 34]. These studies are usually carried out using a combination of total organic carbon analysis, isolation and fractionation procedures, and characterization of molecular fragments and intermolecular bonds by different analytical techniques. Such studies have demonstrated that this fraction consists of a highly diverse suite of oxygenated compounds, including dicarboxylic acids, keto-carboxylic acids, aliphatic aldehydes and alcohols, saccharides, saccharide anhydrides, aromatic acids, phenols, but also amines, amino acids, organic nitrates, and organic sulfates [11; 28; 30; 33; 35; 36; 37; 38; 39; 40; 41; 42]. Despite the advances in understanding the atmospheric importance of the aerosol WSOM component, there are still several fundamental questions related to the complexity of its chemical composition, mechanisms of formation, atmospheric fate, and reactivity [15; 22]. Determining the molecular composition and structure of aerosol WSOM is warranted to further increase the current understanding of its role in various atmospheric processes. Furthermore, due to its dynamic nature, the establishment of general models for the structure of the water-soluble fraction of OA still is far from being fully accomplished [23], and further studies are needed to understand their actual impact on a regional and global scale. As reviewed by [PERSON] et al. [22] and [PERSON] [43], a universal technique for OA analysis does not exist. Several different offline and online analytical methodologies have been developed and applied to mitigate the complexity of OA, as well as to unravel the structure and composition of this aerosol component. The following Section 3 addresses the state-of-the-art multidimensional analytical strategies applied thus far in the characterization of OA.