3.2. Chemical composition of PM2.5 samples3.2.1. Inorganic ionsIonic analysis of the aqueous PM2.5 extracts confirmed the presence of Na+, K+, Mg2+, Ca2+, F−, Cl−, NO3−, and SO4 2 − in all samples. It was found that the ionic components made up only 47% of the total mass of PM2.5. Among these ionic species, SO4 2 − had the highest concentration at each of the three stations, in the range of 0.71–7.53 μg/m3 (mean ± SD of 3.58 ± 2.14 μg/m3), 0.88–10.88 μg/m3 (3.69 ± 2.91 μg/m3), and 0.43–3.35 μg/m3 (1.87 ± 1.05 μg/m3) for traffic, urban, and suburban stations, respectively (data not shown). The contribution of the ionic components to the total mass of PM2.5 is illustrated in Fig. 5. The ioniccomponents in PM2.5 for traffic stations were (in descending order) SO4 2− N NO3− N Cl− N Ca2+ N Na+ N K+ N Mg2+ N F−, whereas those for urban and suburban stations were SO4 2− N NO3− N Ca2+ N Na+ N Cl− N K+ N Mg2+ N F− (Fig. 6). A statistically significant difference (t-test, p < 0.01) was found for the concentrations of both sulfates and nitrates compared with the other ions for all sampling sites. It should be noted that SO4 2 − and NO3− were the most frequently found ionic components of PM2.5 at all three stations. Higher concentrations of SO4 2 − could be due to the enhanced photochemical oxidation of SO2 during warm periods, which is in accordance with the results of other studies (Samara et al., 2016; Szigeti et al., 2015). Such increased SO4 2 − and NO3− concentrations for PM2.5 may also be due to the combustion of fossil fuels in motor vehicles. In 2013, the annual emissions of SOx and NOx in Tehran were 37.4 and 85.5 kt, respectively. Notably, mobile sources produce nearly 6.22% of SOx and 46.1% of NOx, whereas stationary sources are responsible for the remained of the emissions.