Chemical composition of indoor and outdoor PM_2.5 in the eastern Arabian Peninsula

Water-soluble and trace metal species in fine particulate matter (PM_2.5) were determined for indoor and outdoor environments in Doha, Qatar. During the study period, PM_2.5 concentrations showed significant variability across several indoor locations ranging from 7.1 to 75.8 μg m^-3, while the outdoor mass concentration range was 34.7-154.4 µg m^-3. The indoor and outdoor PM_2.5 levels did not exhibit statistically significant correlation, suggesting efficient building envelope protection against outdoor PM_2.5 pollution. Rather than outdoor sources, human activities such as cooking, cleaning, and smoking were the most significant influence on chemical composition of indoor PM_2.5. NH₄⁺ concentration was insufficient to neutralize SO₄^2- indoors and outdoors, indicating the predominant presence of NH₄HSO₄. The enrichment factors indicated that outdoor Fe, Mn, Co, Cr, and Ni in PM_2.5 mostly originated from crustal sources. In contrast, the remaining outdoor trace metals (Cu, Zn, As, Cd, Pb, and V) were mainly derived from anthropogenic sources. The indoor/outdoor concentration ratios revealed significant indoor sources for NH₄⁺ and Cu. The crustal matter, water-soluble ions, and sea salt explained 42%, 21%, and 1% of the indoor PM_2.5 mass, respectively. The same groups sequentially constituted 41%, 16%, and 1% of the outdoor PM_2.5 mass.