3.1. Organic tracer compound concentrations
Primary saccharides have been used as tracers for organic soil dust as they are tracer compounds for vegetal debris and fungi (Fu et al., 2016; Medeiros et al., 2006; Wan and Yu, 2007). Sucrose and glucose are predominantly derived from plant materials (Pacini, 2000; Puxbaum and Tenze-kunit, 2003). Amongst sugar alcohols, mannitol is a tracer of airborne fungi (Bauer et al., 2008; Fu et al., 2016; Samaké et al., 2019; Verma et al., 2018). In this study glucose, mannitol, sucrose and fructose are the compounds found in higher concentrations (see Table 3), and directly related to the soil dust in combination with microorganism. Higher indoor concentrations were possibly affected by sugar-holding food consumption due to high concentrations of sucrose and glucose inside schools. The spearman rank correlation coefficients calculated between the levels of pollutants in school dust show correlations between statistically significant glucose, mannitol and sucrose (p˂0.001).
In this study, the maximum concentration of the DEHP in dust collected indoor schools was 373 µg/g (Santa Filomena School), while outdoor was 473 µg/g (Ingles Quintero School) (see Table 3). These values are lower than the concentrations reported by Kim et al., (2022) (1310.00 µg/g), but in the range of those observed by Blanchard et al., (2014) in samples of settled dust. In this study, schools with higher DEHP likely reflect the presence of plastic toys in the classrooms, use of plastics in classroom material and of PVC flooring (major sources of phthalates in indoor dust) (Besis et al., 2023; Jeon et al., 2016).
The hopanes are molecular tracer compounds of mineral oil contributions and vehicle exhaust (Schauer et al., 1999) and their presence at the schools can be related to penetrated outdoor air to the indoor air through ventilation (Tian et al., 2021). The two hopanes analyzed 7a(H)21b(H)-29-norhopane (Norhop) and 17α(H),21β(H)-Hopane (Hop), were the most abundant and their concentrations showed a very strong correlation (R2 = 0.97; p < 0.001). During the winter, the school with the highest concentration of these two hopanes (indoor), was the Basica Chocota school (847ng/g). These school concentrations are similar to those found in the studies of van Drooge et al., (2020), and can result from the fact that this school is situated in the high traffic area. On other hand, the lowest hopanes concentrations were measured outdoor Santa Filomena School (74,8 ng/g).
Levoglucosan is a molecular tracer compound for biomass burning (Deshmukh et al., 2019; Simoneit, 2002). Previous studies have reported levoglucosan in aerosols (Barbaro et al., 2015; van Drooge et al. 2020; Zangrando et al., 2016), and soils (Otto et al., 2006). Here, the highest concentrations were measured during the winter season indoor the Greda school (1.207 ng/g) (see Table 3). Equally the levels of levoglucosan were are three times higher indoor than outdoor of the schools.
PAHs are common byproducts of incomplete combustion and are considered ubiquitous indoor pollutants owing to their widespread sources, with the possibility after the penetration of outdoor air to the indoor air through ventilation (Ali, 2019; van Drooge et al., 2018; van Drooge et al., 2020; Blanchard et al., 2014; Yang et al., 2015). In the studied schools, combustion of fossil fuels from motorized vehicles is the major source for PAHs in urban areas (van Drooge et al. 2020). Concentrations and profiles of PAHs in indoor/outdoor dust samples were summarized in Table 4, 1S, Fig. 1.
The most frequent PAH species identified in indoor dust samples from schools were PYR and PHE (16% and 14%, respectively), followed by CHR and FLT (12%).
Similarly, in the case of dust samples collected outside the schools, the predominant PAHs were PYR (17%), followed by CHR and PHE respectively (14%). Therefore, according to the results obtained the PAHs tend to accumulate in indoor dust, which is an important indicator of indoor pollution.
Highest ΣPAH concentrations (summer) were measured indoor the schools from Puchuncavì; Greda (Alerces) (556 ng/g), Greda (464 ng/g) and Santa Filomena (352 ng/g) the latter is located in Quintero. Equally, in winter the highest concentrations were reported in Greda (Alerces) School (746 ng/g), Santa Filomena (507 ng/g). In the case of outdoor conditions in schools the highest ΣPAH concentrations were measured in the Greda School (summer) 537 ng/g and (winter) 497 ng/g. Equally, the lowest ΣPAH concentrations were measured in the summer (outdoor) at Santa Filomena school (43 ng/g) and in the Basica la Chocota school (76 ng/g). In general, the relative composition of individual PAH was dominated by phenanthrene, pyrene, and chrysene, followed by benzo[ghi]perylene and the other PAHs in minor concentrations. In dust samples collected indoor the schools during the summer in Puchuncavi the higher abundances of benzo[b + j + k] fluoranthene were observed, moreover in winter the higher abundances of pyrene, followed by chrysene and fluoranthene were observed. Equally, in the samples collected indoor the schools from Quintero (winter and summer), the higher abundances of benzo[a]pyrene, followed by chrysene and pyrene were observed. In the complete data set this correlation is also significant suggesting that the combustion is the major source for PAHs.
3.2. Source apportionment
The concentrations of the analyzed compounds were evaluated with PCA, and the resolved components are shown in Table 2S. In the present study, loadings were considered significant for values of 0.7 or greater (Comrey and Lee, 1992).
A four-component solution explains 81,065% of the variance in the data set (Fig. 2). In this model the contribution related to combustion (PAHs; CP-1) is most relevant in the Santa Filomena school, whereas a source represented to traffic (norhopane; hopane; PAHs; CP-2) predominate in the Santa Filomena school and the Greda school.
Equally, the possible contribution of a soil + plastic (saccharides; DEHP; CP-3) source is stronger in the Inglès Quintero school, Basica Chocota school and Santa Filomena school, mainly. Finally, another possible source is the biomass burning (levoglucosan; CP-4) present in the Inglès Quintero school and the Santa Filomena school (see Fig. 3).
The schools ubicated in Puchuncavi that were sampled throughout the winter were identified by the presence of indoor pollution from outdoor sources of combustion (Greda los Alerces school), traffic (Basica Horcon school), and indoor soil + plastic (Basica Chocota school and the Greda school). On the other hand, there was a distinct trend toward higher combustion sources indoors of school Greda los Alerces. In the Greda school and Basica la Chocota school, the traffic contribution was higher outside than inside these sampling sites. When comparing indoor and outdoor samples it is possible to observe that the traffic and combustion sources predominate outdoors of the respective schools (Fig. 3).
In relation to the schools ubicated in Quintero, the source that predominated in the indoor was traffic (Santa Filomena and Politecnica de Quintero schools). The biomass burning was relevant indoor the schools El Faro and Santa Filomena (Fig. 3).