## 3 Advanced Multidimensional Techniques for Aerosol WSOM Analysis
### Online vs. Offline Techniques: Where Does Multidimensionality Stand?
The relative intensity of f\({}_{44}\) and f\({}_{60}\) can then be used to build a 2D map to distinguish between fresh and aged biomass-burning OA [48] or between fresh and aged WSOM from fog samples [26] (Figure 1). Following a similar approach, [PERSON] et al. [37] analyzed 43 AMS datasets from multiple field studies of OA in the Northern Hemisphere. The authors focused their attention on the OOA components, which were deconvolved into LV-OOA and SV-OOA. Differences in the mass spectra of these two components were characterized in terms of two main ions: _m_/z 44 (CO\({}_{2}\)+, f\({}_{44}\)) and _m_/z 43 (mostly C\({}_{2}\)H\({}_{3}\)O+, f\({}_{43}\)). The relative intensities of f\({}_{44}\) and f\({}_{43}\) were used to develop a 2D mass spectral profile for assessing the aging of the LV-OOA and SV-OOA components in the atmosphere: the LV-OOA component has higher f\({}_{44}\) and lower f\({}_{43}\) than SV-OOA. The work of [PERSON] et al. [49] is another example of the multidimensionality of data acquired from online HR-ToF-AMS analysis. The authors coupled a particle-into-liquid sampler (PILS) upstream of an HR-TOF-AMS for the direct and online characterization of water-solubility of OA in contrasting urban and rural environments in the Southeastern United States. A 2D map of the mass defect spectrum (calculated by subtracting the ion's nominal mass from its exact mass) vs. _m_/z of HR-ToF-AMS organic ions (C\({}_{\rm x}\)+, C\({}_{\rm x}\)H\({}_{\rm y}\)N+, C\({}_{\rm x}\)H\({}_{\rm y}\)+, C\({}_{\rm x}\)H\({}_{\rm y}\)O+, and C\({}_{\rm x}\)H\({}_{\rm y}\)O \(>\) 1+) allowed us to visualize how the water-solubility of OA factors (resolved by PMF analysis) varies with time and location. Nevertheless, this online PILS- HR-ToF-AMS methodology has large uncertainties associated with estimating the ambient concentration of HOA or OA from sources with low water-solubility [49]. Although the multidimensional nature of the online HR-ToF-AMS approach has not been fully explored, this methodology was also employed for assessing the size-resolved nonrefractory chemical composition of submicron aerosol particles across ocean waters next to the Antarctic Peninsula [50]. Notwithstanding the high-time resolution in terms of bulk chemical information attained by the online MS strategies, these approaches provide limited information and an oversimplification of the structural complexity of organic molecules present in atmospheric WSOM, particularly when compared with offline analytical assemblies. In fact, the use of online MS strategies is still far from replacing the offline techniques for an in-depth structural characterization of water-soluble OA, being mostly employed to spot marked differences in terms of different sources and atmospheric aging of sample's organic content. The multidimensionality concept is more prone to be applied when using offline methodologies for studying the composition of aerosol WSOM. These offline methodologies are typically based on highly complex spectroscopic and mass spectrometry systems that can be associated with a separation method, being used to achieve detailed information on the chemical features and functional groups composition of the WSOM of atmospheric particles collected by filter-based techniques [43; 51]. Despite the high spectral and chemical resolution attained on WSOM, these analytical strategies still have some drawbacks; namely, they usually require large amounts of sample (aerosol samples collected from several days to weeks), resulting in low temporal resolution, which may also lead to some problems of sample representativeness [51]. Furthermore, the use of long sampling times increases the risk of sampling artifacts during aerosol collection, which are translated by volatilization/condensation processes that may occur on the filter, or particles surface, for semi-volatile organics, as well as oxidation of filter deposited organics by strong oxidants, such as ozone.