## 5 Conclusions  
In recent years, the study of water-soluble organic compounds in atmospheric aerosols has become one of the areas of intense focus and interest in atmospheric chemistry research. The water-soluble fraction of OA plays an important role in climate, environment, and public health. However, due to the limited knowledge on the composition, sources, formation mechanisms, and transformation processes, their climatic and health impact are still far from being completely understood. Due to the inherent heterogeneity and presence in relatively low atmospheric concentrations, an important fraction of atmospheric WSOM still resides outside of the current analytical window. Nevertheless, during the past two decades, the rapidly evolving field of analytical instrumentation has produced sophisticated tools capable of answering this challenge. Within this context, this review provided an overview of the state-of-the-art multidimensional analytical strategies that have been used in the structural and molecular characterization of aerosol WSOM (Figure 7). The most relevant methodological issues that need to be addressed were also highlighted in this review. Ideally, multidimensional analytical strategies employed in the study of aerosol WSOM should provide detailed molecular and structural speciation with high-time resolution. Although the existent analytical approaches are not able to achieve these two requirements simultaneously, the synergistic information derived from both online and offline techniques should be able to fulfill this goal. Online AMS is probably the technique of choice for attaining high-time resolution in terms of bulk chemical information, with concomitant insight into brief events and daily variations of OA [44]. However, these online techniques fail to provide detailed molecular speciation and consequently can hardly replace multidimensional offline strategies. One of the most important steps for the implementation of offline multidimensional analytical strategies is the implementation of robust sampling, extraction and isolation procedures of field atmospheric WSOM samples. These requirements are of utmost importance, for example, for HR-MS studies of aerosol WSOM, which require the isolation of the organic analytes from the water-soluble inorganic constituents. The lack of standard procedures for sample collection and preparation is nowadays one of the biggest drawbacks for an extensive comparison of WSOM samples from different locations with different degrees of pollution (urban vs. rural vs. remote areas in different regions of the world). Therefore, future refinements may be directed toward analytical isolation procedures that allow the recovery of the majority of the WSOM without the artifacts associated with solid sorbent methods [43]. The use of offline MS methodologies, in particular with the advent of HR-MS detectors and their hyphenation with 2D chromatographic systems, or combination with EEM-PARAFAC methods, has allowed a huge improvement in the selectivity and molecular discrimination of the WSOM constituents, as well as the identification of specific chemical markers and tracers for OA source apportionment. Significant progress has also been achieved with the use of 1D NMR spectroscopy, particularly with liquid-state \({}^{1}\)H NMR, for structural characterization and source apportionment of aerosol WSOM. In this field, future  
Figure 7: Schematic representation of the range of multidimensional analytical strategies currently employed in the characterization of aerosol WSOM as a function of the level of speciation and quantification achieved. Definition of the acronyms is provided in the text. work should explore the potential of using multidimensional liquid-state (2D or higher [91]) NMR (e.g., \({}^{1}\)H-\({}^{1}\)H, \({}^{1}\)H-\({}^{13}\)C, and/or \({}^{1}\)H-\({}^{15}\) N correlation experiments) approaches.