## 4 Water-Soluble Indoor Organic Aerosols: Challenges and Opportunities  
### What Is Known about the WSOM of Indoor Air Particles? The study of the WSOM fraction in indoor air particles is at a very early stage of development compared to its outdoor counterpart. The body of knowledge produced thus far concerns only the amount of WSOC (or WSOM) present in indoor PM, and these data have mostly been used as an indicator of indoor SOA formation. This implies that nothing is known about the structural and molecular composition of the indoor aerosol WSOM. Table 2 summarizes the available data on the concentrations of WSOC in PM\({}_{2.5}\) collected under different indoor scenarios. To the best of the authors' knowledge, the first study reporting the quantification of the WSOC in indoor air particles was performed in the Refectory of Santa Maria Delle Grazie Church, which holds [PERSON]'s \"Last Supper\" [83]. The WSOM, estimated using a WSOM-to-WSOC conversion factor of 1.7, was identified as the second most abundant component of PM\({}_{2.5}\), after the water-insoluble organic matter, although it only contributed to a minor fraction, ca. 17.8 to 19.9%, of the fine aerosol organic matter (OM) [83]. Nevertheless, these WSOM/OC values should be viewed with caution due to the uncertainties associated with the use of a WSOM-to-WSOC conversion factor of 1.7. This conversion factor was selected based on the recommendations of [PERSON] and [PERSON] [84], [PERSON] [85], and [PERSON] et al. [86] for outdoor air particles, and it was considered a reasonable compromise between the values of 1.6 for urban aerosols and 2.1 for aged (nonurban) aerosols [84]. The empirical WSOM-to-WSOC factor of 2, proposed by [PERSON] and [PERSON] [84], has also been used by [PERSON] et al. [87] and [PERSON] et al. [88] for estimating the contribution of WSOM to PM\({}_{2.5}\) from personal exposure in Chinese rural locations. In both studies, the WSOM was reported as the highest component out of all bulk PM\({}_{2.5}\) chemical constituents, with higher levels in winter than in the summer season. These findings were associated with the use of biomass residues for cooking activities and space heating, with the latter source being more important during the colder periods [87; 88]. Additional measurements of WSOC in indoor PM\({}_{2.5}\) was performed by [PERSON] et al. [89], who reported that WSOC can account for a highly variable fraction (1% to 13%) of PM\({}_{2.5}\) collected in three retirement communities of the Los Angeles Basin. The variability in the WSOM-to-WSOC conversion factor values recommended for outdoor OA studies should be noted, which highlights the importance of measuring the WSOM-to-WSOC ratios to reduce the uncertainties associated with the chemical mass closure assessment of atmospheric OA [13; 33; 38]. The use of such empirical conversion factor values is even more challenging in chemical mass balance studies of indoor air particles, and the reason is twofold: (1) not only do the WSOM-to-WSOC ratios depend on the class of organic compounds present in the WSOC component [85], and there is a clear lack of knowledge on the chemical composition of indoor particulate WSOC; (2) but also, the OC in indoor PM seems to be particularly unaffected by outdoor sources (e.g., [90; 91]). Measurements and chemical characterization of the indoor WSOC fraction are, therefore, highly recommended to better ascertain the bulk chemical composition of indoor PM. Nowadays, the importance of water-soluble OA in climate and human health is widely recognized by the outdoor research community. Nonetheless, understanding the dynamic processes and climate and health impacts involving aerosol WSOM depends on how well one can identify its organic constituents. In a similar vein, one can anticipate that the composition of water-soluble OA can influence the health effects and fate of OA in indoor environments.