## 4 Water-Soluble Indoor Organic Aerosols: Challenges and Opportunities
### Why Should Aerosol WSOC Be Considered in Indoor Air Chemistry Studies? Almost two decades of WSOC measurements and characterization in outdoor air particles worldwide have highlighted the compositionally specific signatures associated with either chemical processing of OA or seasonal and emission source differences [50]. These compositional data have contributed greatly to our understanding of outdoor water-soluble OA and their effects on climate and human health. The indoor air chemistry community also has a long history with respect to the specific chemical species to be measured (mostly in the gas-phase) and their rationale, the analytical techniques to be employed in such measurements, and the most interesting indoor spaces to be studied (e.g., [98, 99]). As recently reviewed by [PERSON] and [PERSON] [99], indoor chemistry differs greatly from the outdoor chemistry, with the former being mostly driven by non-reactive partitioning and reactive processes that occur between gases, aerosol particles, and surfaces. The current understanding of indoor air chemistry and the nature of organic compounds in the indoor environment seems to be inevitably driven by the characterization according to volatility. Nonetheless, aerosol particles are also a dynamic player in indoor air chemistry [99, 100], which means that the characterization of indoor aerosol composition, evolution, sources, and potential to affect human health is likewise highly needed. The question brought about in this viewpoint (i.e., \"why should aerosol WSOC be considered in indoor air chemistry studies?\") suggests an alternative untargeted, and yet complementary, screening approach toward an in-depth knowledge on indoor air particle composition. Residential indoor PM simulations have recently explored the assumption that hygroscopic constituents in indoor air particles may promote uptake of aerosol water, depending on relative humidity, which can constrain the phase state of OA [101]. Simulations suggest that residential OA is likely to be liquid in hot and humid climates, or when the indoor OA loading is high so that OA is less viscous [101]. A recent study in an occupied office further demonstrated that indoor \"dampness\" (i.e., high indoor humidity conditions) promotes the occurrence of high indoor SOA concentrations, thus increasing indoor organics exposure [102]. Using an Aerodyne Time-of-Flight Aerosol Chemical Speciation Monitor under these \"dampness\" conditions, the SOA was shown to be the most significant contributor (up to 99%) to total OA, with the more-oxidized oxygenated OA component becoming dominant (54%) indoors [102]. The presence of more-oxidized OA with high polarity tend to absorb water more rapidly, which might have implications on the aqueous-phase chemistry [103] and the uptake of semi-volatile or WSOGs indoors [27]. Considering that in outdoor OA studies a significant fraction (up to 88%) of the more-oxidized OA is water-soluble [60, 94, 104], one might expect a similar water-solubility behavior for the more-oxidized OA indoors. Furthermore, it has been also shown that the WSOC alters the hygroscopicity of outdoor air particles [105, 106, 107], with the chemical composition of WSOC being one of the main drivers of the particles' hygroscopic growth behavior. These outdoor evidences suggest that the WSOC may also be a prime contributor to indoor OA and its hygroscopicity, and it might be helpful to use water-solubility to gain further insights into the chemical composition of OA indoors. This new challenge can be accomplished by bringing the WSOC measurement and characterization methods to the indoor environment by using the knowledge gained from outdoor studies. The use of an untargeted approach for studying the WSOC in indoor air particles is the most appropriate pathway to maximize the level of chemical information withdrawn from the complex WSOC pool and, therefore, provide the opportunity to detect unexpected compositional changes.