## 3 Advanced Multidimensional Techniques for Aerosol WSOM Analysis  
### Online vs. Offline Techniques: Where Does Multidimensionality Stand? Ideally, the analysis of atmospheric organic matter should provide, in a continuous way, a thorough chemical profile of the sample, simultaneously ensuring a continuous size and time resolution. Yet, nowadays, there is no technique capable of providing this sort of information. Therefore, analytical techniques can be broadly divided into two major groups: online and offline techniques. The online methodologies mainly ensure high-time resolution for insight into brief events and diurnal changes [44], whereas offline methodologies are commonly applied to attain high chemical resolution [45; 43]. Online or in situ methodologies are typically used in field observations to provide information on the chemical and physical properties of OA on short timescales. When using these methodologies, the analysis is performed almost at the same time that the particles are sampled and with very fast acquisition rates. These online measurements avoid potential artifacts associated with offline analysis methods, such as evaporation and chemical reactions during long aerosol sample collection and prolonged time of analysis periods [43; 44; 45]. Online aerosol mass spectrometry (AMS) analysis alongside positive matrix factorization (PMF) and high-resolution time-of-flight AMS (HR-ToF-AMS) analyzers have allowed a more in-depth investigation into the sources and composition of OA and its evolution in the atmosphere (e.g., [6; 37]). Using this online approach, organics have been deconvoluted into several categories based on source, volatility, and distribution of oxygen-containing functionalities, namely: HOA, low-volatility oxidized OA (LV-OOA), semi-volatile OOA (SV-OOA), and BBOA. In the case of aerosol WSOM, [PERSON] et al. [46] compared these AMS factors for OOA with the NMR features of aerosol WSOM samples collected in filters. The authors concluded that the very oxidized LV-OOAfrom AMS correlates with the NMR factor showing the so-called \"humic-like substances\" (HULIS) features. The question now lies on where multidimensionality stands in these online MS methodologies for the analysis of aerosol WSOM. The concept of multidimensional MS has been highlighted in the review work of [PERSON] et al. [47] on lipidomic analyses. According to these authors, a two-dimensional (2D) mass spectrum, showing cross-peaks (i.e., the individual fragments) of a given primary molecule ion in the first dimension with the second dimension representing the fragments of this given molecule ion, readily produce an informative 2D map of the structure of the given molecule as well as its isomers and isobars [47]. This 2D MS concept evolves into a multidimensional MS model when conducting comprehensive MS analyses by changing an array of instrumental variables (e.g., different ionization conditions and/or the effects of collision conditions on fragmentation processes) to acquire critical information on the molecular species present in a given sample and their abundance, in what collectively comprise a multidimensional MS spectrum [47]. It should also be mentioned that a detailed interpretation of this huge amount of MS data requires advanced computational technology capable of handling all this information. At this moment, the multidimensional MS spectrum is decomposed into multiple 2D MS, which are displayed by varying only one variable at a time while keeping the others constant under fixed experimental conditions [47]. This concept of multidimensional MS can also be applied in online aerosol WSOM analysis. For example, [PERSON] et al. [26] employed HR-ToF-AMS for the characterization of fog and fine aerosol samples, using the relative intensity of peak _m_/z 60 (f\({}_{60}\)) and peak _m_/z 44 (f\({}_{44}\)) as markers of fresh biomass burning influence and oxygenated and processed dissolved organic molecules, respectively.