## 3 Advanced Multidimensional Techniques for Aerosol WSOM Analysis
### Application of Off-Line Multidimensional NMR Spectroscopy for Water-Soluble OA Analysis
Moreover, this 2D model has been updated (Figure 4) with new fingerprint boundary regions, namely a rural OA region [39], a fresh pollen region [41], and a fresh BBOA region [65]. The fingerprint region of urban OA was firstly established by [PERSON] et al. [38] with NMR data from samples collected in an urban industrial center and subsequently verified by [PERSON] et al. [8] and [PERSON] et al. [10; 11]. 1D solid-state cross-polarization with magic-angle spinning (CP-MAS) \({}^{13}\)C NMR spectroscopy is another NMR-based technique successfully applied in the last fifteen years for investigating the distribution of carbon functional groups in the water-soluble fraction of OA [9; 30; 31; 66; 67]. This technique allows a semi-quantitative measure of the relative contribution of the different carbon functional groups to the OC present in the sample and, consequently, it can be used for investigating the composition and the structural variability of the water-soluble organic fraction according to different aerosol sources, sampling sites, and meteorological conditions [24; 68]. Although intuitively, the analysis in liquid-state of the water-soluble OA could make more sense, 1D solid-state CP-MAS \({}^{13}\)C NMR presents some advantages for the analysis of this fraction, especially when compared to 1D liquid-state \({}^{1}\)H NMR spectroscopy or even to 1D liquid-state \({}^{13}\)C NMR spectroscopy. Some of these advantages have been reviewed by [PERSON] and [PERSON] [24; 69] and include: the solid-state technique is not affected by the solvent, which can cause changes in the chemical shifts of the sample, hide some sample chemical resonances by the solvent signals, or even loss of some peaks, namely those of the carboxylic acids, due to the presence of rapidly exchangeable protons; the liquid-state NMR spectra may present lower resolution and sensitivity due to the low solubility of some organic components in the selected solvent, and the solid-state technique is a non-destructive method, allowing the sample to be used for further complementary structural analysis. On the other hand, a major limitation of the solid-state CP-MAS \({}^{13}\)C NMR technique is the difficulty in obtaining high-quality NMR spectra due to the relatively low carbon content of ambient water-soluble OA samples. This fact requires combining several aerosol samples collected under a common feature (e.g., similar ambient conditions) in order to obtain a sufficient amount of sample for  
Figure 4: 2D schematic representation of functional group distribution for different atmospheric OA samples. Boxes represent boundaries for marine OA, SOA, and Aged BBOA, based on [PERSON] et al. [32]; rural OA [39]; aged urban OA obtained from [PERSON] et al. [38]; fresh pollen obtained from [PERSON] et al. [41]; and fresh emissions from BBOA obtained from [PERSON] et al. [65]. analysis (it needs \(\sim\)20-100 mg of sample, depending on the size of the NMR probe) [69]. Furthermore, to make the sample amenable to CP-MAS \({}^{13}\)C NMR analysis, the organic material must be isolated from the inorganic ions (e.g., through solid-phase extraction, ion-exchange chromatography, or size-exclusion chromatography) and prepared as a solid (e.g., freeze-drying) prior to analysis [69]. It must be emphasized, however, that under these circumstances, the acquisition of a high-quality \({}^{13}\)C NMR spectrum is made at the expense of pursuing studies on near real-time variations of the chemical composition of OA. Nonetheless, recently, and for the first time, [PERSON] et al. [23] applied advanced solid-state \({}^{13}\)C NMR techniques, including nearly quantitative \({}^{13}\)C multiple cross-polarization/magic angle spinning (multiCP/MAS), multiCP/MAS with dipolar dephasing, multiCP/MAS with \({}^{13}\)C chemical shift anisotropy filter, and 2D \({}^{1}\)H-\({}^{13}\)C heteronuclear correlation (2D HETCOR), to acquire an accurate quantitative structural description of aerosol WSOM collected in an urban atmosphere.