## 3 Advanced Multidimensional Techniques for Aerosol WSOM Analysis
### Application of Off-Line Multidimensional NMR Spectroscopy for Water-Soluble OA Analysis
Two urban aerosol WSOM samples collected in two short periods of time (one week each) under different wintry weather conditions were investigated. NMR data successfully pinpointed the variability of whole aerosol WSOM composition, allowing to suggest source-specific structural characteristics for each sample in those two short periods of time [23]. A new structural model of urban aerosol WSOM was build based on the acquired 2D compositional data (Figure 5), showing the presence of three independent classes of compounds that vary both in content and molecular diversity within those two short periods of time: heteroatom-rich aliphatics (either chain or branched), carbohydrate-like moieties, and highly substituted aromatic units [23]. In another study of [PERSON] et al. [9], a chemometric tool, called generalized perturbation-based 2-D correlation spectroscopy (2-D correlation spectroscopy), was used to highlight subtle changes in compound class distributions within urban water-soluble OA samples from different seasons, using the traditional solid-state CP-MAS \({}^{13}\)C NMR and Fourier-transform infrared (FTIR) spectroscopy data as chemical probes [59]. This chemometric tool generates a synchronous 2D correlation map involving CP-MAS \({}^{13}\)C NMR and/or FTIR spectra (Figure 6), which provide 2D information on how \({}^{13}\)C NMR and/or FTIR bands correlate along a seasonal gradient (e.g., median of the air temperature within each season). Using this chemometric approach, [PERSON] et al. [9] demonstrated that the water-soluble compounds isolated from urban aerosol samples (Aveiro, Portugal) consist of at least two classes of compounds: one is rich in both carboxylic and hydroxyl functional groups, and it has an aliphatic character, and the other entails lignin-derived structures. Figure 5: 2D \({}^{1}\)H-\({}^{13}\)C HETCOR NMR spectrum (**a**) and associated \({}^{1}\)H slices (**b**) of the WSOM extracted from fine urban air particulate matter. Reprinted from _Atmospheric Environment_, 230, [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], Exploring water-soluble organic aerosols structures in the urban atmosphere using advanced solid-state \({}^{13}\)C NMR spectroscopy, 117503, Copyright (2020), with permission from Elsevier. Another set of NMR-based multidimensional methodologies that have emerged for the structural analysis of water-soluble OA are the 2D liquid-state NMR techniques [33; 70; 71; 72; 73]. These techniques have proven to be very useful to solve the spectral information overlaid in 1D NMR spectra by separating the data into a second frequency dimension. The combination of the data from some of these 2D liquid-state NMR techniques, namely \({}^{1}\)H-\({}^{1}\)H homonuclear (COSY and total correlation spectroscopy (TOCSY)) and \({}^{1}\)H-\({}^{13}\)C heteronuclear (HSQC and HMBC) connectivity, improve the spectral resolution and provide a deeper insight into the C-H backbone of the organic compounds from the atmospheric aerosols [69]. Although the use of liquid-state 2D NMR techniques to water-soluble OA analysis seems to be still very limited, the existing studies [70; 71; 72; 73; 70] have demonstrated the potential of these high-resolution NMR techniques for a comprehensive description of the substructures present in aerosol WSOM, as well as to identify molecular fingerprints of the different aerosol sources and formation mechanisms. Conceivably, one of the most important drawbacks of these techniques is the time required for signal acquisition, which can reach several days, thus limiting its application, at least on a routine basis. Nevertheless, as reviewed by [PERSON] and [PERSON] [43; 69], the keys to developing accurate knowledge about the structural features of water-soluble OA also lies in the development of analytical procedures taking advantage of the synergistic application of all of the aforementioned NMR techniques (i.e., 1D solid-state NMR, 1D and 2D liquid-state NMR), as well as their association with other advanced analytical techniques, namely HR-MS.