## 1 Introduction  
Particulate matter (PM) of different sizes has been one of the most studied outdoor air pollutants. This interest in atmospheric PM is fueled by the realization of its negative impacts on air quality and human health, and large, but uncertain, effects on radiative climate forcing and atmospheric chemistry [1, 2]. Both size and chemical composition of atmospheric PM influences the climate and human health, with the PM\({}_{2.5}\) (aerodynamic diameter \(\leq\) 2.5 \(\upmu\)m) and PM\({}_{10}\) (aerodynamic diameter \(\leq\) 10 \(\upmu\)m) being the aerosol metrics measured systematically in national and international air pollution monitoring networks [2, 3]. The chemical composition and sources of atmospheric PM, either PM\({}_{2.5}\), PM\({}_{10}\), or size-segregated PM have also been the focus of extensive research over the past 20 years. While the analysis of the inorganic component of atmospheric PM is reasonably straightforward, the characterization of their organic counterpart is a much more challenging task [4]. This organic component constitutes an important and highly variable fraction (\(\sim\)20-50%) of PM\({}_{2.5}\) mass at continental mid-latitudes [3, 5, 6, 7], exhibiting a multitude of molecular forms, sources, and physical properties. While the task of determining the detailed molecular composition of the atmospheric organic component is daunting, it offers unparalleled rewards for a better understanding of its origin, physicochemical transformation in the atmosphere, and role on the climatic system [8] as well as its effects on air quality and health [2, 9, 10]. Over the past two decades, it has become also clear that an important fraction of airborne particulate organic matter (also known as organic aerosol, OA) is water-soluble. Observations made in Northern Hemisphere midlatitudes showed that water-soluble organic matter (WSOM) accounts for 10 to 80% of OA [11, 12, 13, 14], whereas lower percentage values (up to 13%) have been reported for Southern Hemisphere locations [15]. The WSOM is ubiquitous in rural, urban, and marine aerosols, playing a key role in cloud formation and properties [16; 17], Earth's radiative balance [5; 18], and atmospheric chemistry [5; 19]. Dry and wet deposition of aerosol WSOM can also affect carbon and nitrogen biogeochemical cycles in aquatic ecosystems [20; 21; 22]. The WSOM in fine air particles may also exert adverse health effects by generating reactive oxygen and nitrogen species (e.g., [23; 24]) or by promoting a moderate pro-inflammatory status [11]. Therefore, the structural and molecular identification of aerosol WSOM constituents in outdoor air and how they influence the climatic and health effects of fine OA has become an active area of research. In contrast to outdoor environments, the contribution, and chemical features of the WSOM of fine particles in indoor air remain poorly characterized. However, considering that secondary organic aerosols (SOA) contribute significantly to aerosol WSOM burden in outdoor PM\({}_{2.5}\) (e.g., [14]), and that indoor SOA can be a substantial fraction of indoor aerosols (e.g., [25]), one can hypothesize that the WSOM is an important component of indoor PM\({}_{2.5}\). Moreover, it has been suggested that indoor SOA composition is dominated by highly oxygenated organic molecules [26], whose characteristics may resemble those of WSOM. Furthermore, recent measurements showed that total polar water-soluble organic gases (WSOGs) concentrations are much higher indoors than directly outdoors (on average 15 times higher, on a carbon-mass basis) [27]. The aqueous processing of these WSOGs can alter the composition of residential indoor air in ways that are not yet understood [27]. This current lack of knowledge in indoor aerosol WSOM warrants further attention as well as ascertaining whether this indoor OA fraction could plausibly induce adverse health effects. The outdoor studies can provide guidance on issues such as the characterization and cytotoxicity of WSOM in indoor environments.