## 3 Advanced Multidimensional Techniques for Aerosol WSOM Analysis
### Application of Off-Line Multidimensional NMR Spectroscopy for Water-Soluble OA Analysis  
NMR spectroscopy belongs to the set of offline analytical techniques that provides a thorough knowledge resolution on the functional groups and a detailed description and structural identification of the main substructures present in the OA. This technique has gradually become one of the most predominant tools in the study of OA, namely of their water-soluble fraction [24]. However, deep insight into the sample complexity provided by this technique comes at the cost of temporal resolution since NMR techniques require large amounts of sample and, therefore, the sample to be analyzed comes from several sampling hours or even days. The most widely used NMR method in the analysis of aerosol WSOM is the one-dimensional (1D) liquid-state \({}^{1}\)H NMR [21; 47; 48; 49; 50; 51; 52]. As reviewed by [PERSON] and [PERSON] [24], the potential of 1D liquid-state \({}^{1}\)H NMR has been exploited for the identification of functional groups, molecular modeling, and determination of source, seasonal and temporal characteristic profiles of the water-soluble fraction of OA. However, a 1D liquid-state \({}^{1}\)H NMR spectrum of aerosol WSOM is often characterized by the presence of strong overlapping signals. On this occurrence, it is only possible to identify some general structural features, thus making it not advisable to attempt the identification of specific molecular structures using this 1D NMR technique alone. Furthermore, \({}^{1}\)H NMR exhibits a very low sensitivity for detecting functional groups that do not carry protons (e.g., substituted aromatic compounds) or containing groups with rapidly exchangeable protons (e.g., carboxyl and hydroxyl groups) [24]. Despite these constraints, it has been shown that protons in aliphatic structures (H-C) are the dominant moieties in aerosol WSOM samples (30-60% of the total \({}^{1}\)H content), followed by oxygenated aliphatic compounds (H-C-O) and unsaturated aliphatic (H-C-C=) groups (5-27%), and only a minor contribution from aromatic groups (typically, below 15%) [24]. The \({}^{1}\)H NMR data has also been used to deduce the sources of aerosol WSOM in different areas by making use of a source apportionment model, developed by [PERSON] et al. [32], where the ratios of calculated carbonyl and aliphatic carboxylic groups (H-C-C=O)-to-total aliphatics (H-C-C=O/\(\Sigma\)Aliphatics) and oxygenated aliphatic groups (H-C-O)-to-total aliphatics (H-C-O/\(\Sigma\)Aliphatics) are plotted in a 2D coordinate system. For this model, the concentrations of organic hydrogen determined in the different spectral regions of the \({}^{1}\)H NMR spectra were converted into carbon concentrations using hydrogen-to-carbon (H/C) ratios, whereas the contribution of H-C-C=O groups is indi  
Figure 3: 3D coordinate system developed by [PERSON] et al. [64], showing the distribution of common organic components of air particles in the 3D space. The dimensions sort the organic matter in aerosol by volatility, polarity, water solubility, and reactivity. Reprinted from _Atmospheric Environment_, Vol. 62, [PERSON], [PERSON], [PERSON], The chemical evolution and physical properties of organic aerosol: A molecular structure-based approach, 199–207, Copyright (2012), with permission from Elsevier. rectly estimated by subtraction of benzylic groups (assumed to be 16.7% of the measured H-Ar signal) from the measured H-C-C= signal. In their study, [PERSON] et al. [32] used different sets of water-soluble OA samples to identify and establish fingerprint boundaries regions for three major aerosol sources, namely SOA, aged BBOA, and marine OA (as shown in Figure 4). Since this seminal work, the \({}^{1}\)H NMR source apportionment model has been applied by other researchers to water-soluble OA samples collected under different emission scenarios [38; 39; 41; 65].