## 2 Characteristics and Sources of Aerosol WSOM in Outdoor Environments
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sampling Site** & **OC** & **EC** & **WSOC** & **Reference** \\  & **(ug C m\({}^{-3}\))** & **(ug C m\({}^{-3}\))** & **(ug C m\({}^{-3}\))** & **Reference** \\ \hline \multicolumn{5}{c}{_Pristine_} \\ _Jungfranjoch, CH_ & 1.2 \(\pm\) 0.4 & 0.19 \(\pm\) 0.08 & 0.63 \(\pm\) 0.23 & [38] \\ _Alert, Canadian High Arctic_ & 0.07–0.39 & & 0.04–0.30 & [28] \\ _Antarctic Peninsula_ & & & 0.07–0.14 & [31] \\ _Colorado Rocky Mountains, US_ & 0.43 \(\pm\) 0.24 & 0.04 \(\pm\) 0.07 & 0.41 \(\pm\) 0.27 & [29] \\ _Chichijima Island, JP_ & 0.60–1.13 & 0.04 – 0.28 & 0.20–0.59 & [30] \\ _Rural/Agricultural_ & & & 2.2–39.6 & [39] \\ _Rondonia, pasture site, BR_ & & & 2.56–11.6 & 0.45–1.30 & 1.11–4.62 & [33] \\ _Moitinho, agricultural, PT_ & & & 0.03–0.10 & [15] \\ _Porto Velho, forest, MBB\({}^{1}\), BR_ & & & & 0.01–0.43 & [15] \\ _Porto Velho, forest, IBB\({}^{1}\), BR_ & & & & 0.01–0.43 & [40] \\ _Thompson Farm, US_ & 4.49 & & & 3.23 & [40] \\ _Sainte-Anne-de-Belleve, Montreal, CA_ & & & & 0.96 & [41] \\ _Cruha, Summer, ES_ & & & & & \\ _Coruha, Winter, ES_ & & & & 0.06–0.15 & [14] \\ _Coruha, Winter, ES_ & & & 0.13–0.69 & [14] \\ _Saitama, JP_ & 5.18–12.1 & 0.90–1.44 & 2.74–7.64 & [42] \\ _Drummond, Montreal, CA_ & & & 1.19 & [41] \\ \multicolumn{5}{c}{_Urban_} \\ _Aveiro, PT_ & 4.99–8.22 & 0.13–1.10 & 1.81–3.12 & [13] \\ _Lisbon, Summer, PT_ & & & 0.95–5.7 & [15] \\ _Mainz, DE_ & & & 2.07 & [35] \\ _Thessaloniki, GR_ & & & 4.20 & [37] \\ _Helsinki, FI_ & 0.67–7.5 & & 0.26–4.6 & [37] \\ _Rio de Janeiro, Summer, BR_ & & & 0.83–1.3 & \\ _Sao Paulo, BR_ & & & 0.07–0.45 & \\ _Medellin, Dry season, CO_ & & & 0.05–0.34 & \\ _Bogotia, Wet season, CO_ & & & 0.04–0.13 & [15] \\ _Lima, Winter, PE_ & & & 0.27–0.38 & \\ _Buenos Aires, Winter, AR_ & & & 0.07–4.7 & \\ _Atlanta, Summer, US_ & 4.30 & & 3.12 & [43] \\ _Little Rock, US_ & & & 0.7–5.9 & [32] \\ _Anjou, Montreal, CA_ & & & 1.13 & [41] \\ _Gwangju, Winter, KR_ & 2.5–17.7 & 0.7–8.5 & 1.2–6.7 & [44] \\ _Beijing, CN_ & 3.4–36.7 & 1.6–10.6 & 1.3–14.7 & [36] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A few examples of ambient concentrations (either range or average values) of OC, EC, and WSOC in areas with different levels of pollution. As for the chemical features of aerosol WSOM in outdoor environments, this fraction covers a huge variety of molecular structures with different physicochemical properties, which reflects its different sources. To further enhance the diversity and structural complexity of this aerosol component, the chemical aging of primary OA during its transport can also be an important pathway for the formation of WSOM [45; 46]. Therefore, it has become important to assess the structural and molecular characteristics of the WSOC to discriminate between primary OA and SOA because their quantitative contributions are a key aspect in evaluating air pollution control strategies. With the purpose of unravelling the complex structural composition of this aerosol component, several different offline and online analytical methodologies have been developed and applied by the outdoor research community [12; 13; 14; 43; 47; 48; 49; 50; 51; 52; 53; 54]. Figure 1 summarizes the different state-of-the-art multidimensional analytical strategies currently available to quantify and characterize the atmospheric WSOM constituents and their various levels of identification. The studies published thus far have demonstrated that the aerosol WSOM fraction consists of a highly diverse suite of oxygenated compounds including dicarboxylic acids, keto-carboxylic acids, aliphatic aldehydes and alcohols, saccharides, saccharide anhydrides,