## 4 Water-Soluble Indoor Organic Aerosols: Challenges and Opportunities
### What Is Known about the WSOM of Indoor Air Particles?
However, unravelling the structural features of WSOM in indoor air particles is a true analytical challenge, mostly due to the low levels of PM found inside closed spaces. In the quest to find the best analytical approach for particulate WSOC analysis, the outdoor atmospheric chemistry community has developed multiple targeted and non-targeted analytical strategies for both offline and real-time water-soluble OA composition measurements [4; 50]. As recently reviewed by [PERSON] et al. [50], offline methodologies are commonly applied to attain high chemical resolution, while online methodologies mainly ensure high-time resolution for insights into the effects of short-term events on the WSOC features. Considering that a complete characterization of atmospheric WSOC is hardly possible on a routine basis in outdoor environments, a similar approach \begin{table}
\begin{tabular}{c c c} \hline \hline
**Indoor Scenario** & \begin{tabular}{c} **WSOC** \\ **(\(\uppi\) C m\({}^{-3}\))** \\ \end{tabular} & **Reference** \\ \hline \begin{tabular}{c} Refectory of Santa Maria delle Grazie Church \\ \end{tabular} & 0.31 \(\pm\) 0.02 & [83] \\ \begin{tabular}{c} Retirement homes \\ _Warm season_ \\ _Cold season_ \\ \end{tabular} & 0.8–1.4 & [89] \\ \begin{tabular}{c} Personal exposure during cooking or space \\ heating, rural sites \\ \end{tabular} & 0.7–1.0 & \\ \begin{tabular}{c} _Meizi, Summer_ \\ _Meizi, Winter_ \\ _Anshang, Summer_ \\ _Anshang, Winter_ \\ _Inner Mongolia, Fall/Winter_ \\ _Sichuan, Fall/Winter_ \\ \end{tabular} & \begin{tabular}{c} 15.8 \(\pm\) 9.9 \\ \(39.3\pm 22.7\) \\ \(16.8\pm 11.9\) \\ \(23.0\pm 11.9\) \\ \(5.11\)–\(263\)1 \\ \(11.5\)–\(17.7\)1 \\ \end{tabular} & [87] \\ \
\begin{tabular}{c} Personal exposure, rural Sichuan 2 \\ _Summer_ \\ _Winter_ \\ \end{tabular} &
\begin{tabular}{c} 11.5–17.7 1 \\ \(24.8\pm 19.0\)1 \\ \(126\pm 131\)1 \\ \(44.4\pm 56.7\)1 \\ \(143\pm 150\)1 \\ \end{tabular} & [88] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average indoor concentrations (either range or average values) of WSOC in PM\({}_{2.5}\) collected under different indoor scenarios. seems to be even more logistically complicated for investigations of particulate WSOC in indoor environments. However, once the structural composition and fate of indoor WSOC are understood, recommendations can be provided regarding source control of indoor OA. The indoor air chemistry community has a good understanding of the sophisticated instrumentation as well as extensive quality control, quality assurance, and data treatment based on chemometric tools that are needed to understand the chemical features and dynamics of organic PM [93]. Most of these powerful tools, combining multidimensional hyphenated technologies and data mining methods, stem from the extensive and intensive field and laboratory studies produced by the outdoor air chemistry community [50], being as relevant to interpreting indoor OA as they are to outdoor OA. For example, online monitoring instruments such as the aerosol mass spectrometer (AMS) coupled with multivariate factor analysis tools (e.g., positive matrix factorization, PMF) and high-resolution time-of-flight AMS (HR-ToF-AMS) analyzers have been used to study the sources and composition of water-soluble OA and their evolution in the atmosphere (for additional details, see the recent review work of [PERSON] et al. [50] and references therein). Another example of an analytical multidimensional approach applied to the online chemical characterization of water-soluble OA has been described in the work of [PERSON] et al. [94]. In this work, a particle-into-liquid sampler (PILS) was coupled upstream of an HR-ToF-AMS for the online characterization of the water-solubility of OA in contrasting urban and rural environments.