## 2 Characteristics and Sources of Aerosol WSOM in Outdoor Environments  
As mentioned earlier, the water-soluble OA fraction, generally measured as water-soluble organic carbon (WSOC), can contribute substantially to the aerosol carbon mass. Table 1 shows examples of ambient concentrations of organic carbon (OC), elemental carbon (EC), and WSOC in locations with different levels of pollution around the globe. The lowest WSOC concentrations are typically found in pristine locations, far from anthropogenic sources. Nevertheless, the WSOC is the most abundant carbon fraction at these locations; for example, for the Canadian High Arctic, it has been reported that the WSOC can account to 40-89% of the OC [28], and in the Colorado Rocky Mountains, more than 90% of the OC was found to be water soluble [29]. The origin of WSOC at these pristine locations include primary biological aerosol particles [28], long-range transported aerosols, and SOA formation via the photooxidation of anthropogenic and biogenic volatile organic compounds (VOCs) originating from either short- or long-term transport [28; 29; 30]. The Antarctic Peninsula is another a pristine location, where the analysis of WSOC in aerosol samples revealed fingerprints of three different sources: primary marine organics (i.e., sea spray), primary biological particles emitted from land biota and land vegetation (grasses, mosses, lichens), and SOA originating from VOCs emitted from algal communities colonizing the sea ice [31]. On average, outdoor studies worldwide have shown a decreasing gradient for the WSOC/OC ratios in the order rural/agricultural--suburban--urban--industrial, which reflects a less aged aerosol OC at the urban and industrial sites, although the contribution of secondary organics to the WSOC fraction cannot be disregarded (e.g., [13; 15; 32]). In fact, it has been reported that the WSOC/OC is higher in summer than in other seasons at a variety of locations, both urban and rural, with this feature associated with the strong photochemical processes due to higher sun radiation (e.g., [33; 34]). Nevertheless, SOA as sociated with anthropogenic precursors (e.g., nitro-monoaromatic hydrocarbons (NMAHs), nitrated and oxygenated derivatives of polycyclic aromatic hydrocarbons (PAHs), and phthalic acids) can also contribute to the water-soluble OA load in urban locations, even in colder seasons [13; 15; 32; 35]. This contribution can also change between urban locations; for example, [PERSON] et al. [35] reported that NMAH contribution to the so-called humic-like substances (HULIS) mass was higher in Thessaloniki (\(\approx\)1.8%) than in Mainz (\(\approx\)0.4%). Less oxidized OA originating from fossil fuel combustion emissions can also dominate in urban and industrial areas, which explains the low WSOC/OC ratios (0.3 to 0.4) and demonstrates that these anthropogenic emissions are not the main source of WSOC at these locations [15; 34; 36]. On the other hand, the temporal variations reported for the WSOC concentrations at both urban and industrial sites, with the highest values occurring at colder periods, have been associated with fresh and aged biomass combustion emissions. These results confirm the dominant contribution of these sources for the aerosol WSOC load in low temperature conditions [13; 32; 37].