Characterization of PM2.5-bound polycyclic aromatic hydrocarbons and its deposition in Populus tomentosa leaves in Beijing

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous constituents of air particulate matter and can be taken up by plants from the atmosphere. However, the purification of particulate-bound PAHs in the atmosphere by greening tree species has not been reported. In this study, we assess the concentrations, distribution, and sources of PM₂.₅-bound PAHs at three representative sites of Beijing in April, July, and November (non-heating period) and analyze the correlation between PAHs in Populus tomentosa leaves and in atmospheric PM₂.₅. The total PAH concentrations in PM₂.₅ were in the range of 19.85 ± 13.59–42.01 ± 37.17 ng/m³ with mean value of 31.35 ng/m³ at the three sites, and the PM₂.₅-bound PAHs concentrations in the two suburban sites (YF and YQ) were significantly higher than that in urban site (XZM) in November (autumn). At the three sites, the high molecular weight (HMW) PAHs in PM₂.₅ were dominant, accounting for 54.09–64.90% of total PAHs and the concentration of HMW PAHs was, on average, 9.1 times higher than that of low molecular weight (LWM) PAHs. Principal component analysis combined with diagnostic ratio analysis indicated that vehicle emission, wood combustion, and industrial processes were the main sources for PM₂.₅-bound PAHs in the non-heating period of Beijing. However, the LMW PAHs were dominant in P. tomentosa leaves. The concentrations of HMW PAHs (BbF, BkF, BaP, IcdP, and BghiP) in P. tomentosa leaves reached 26.11 ± 2.39, 41.42 ± 7.77, and 55.70 ± 12.33 ng/g at YQ, XZM, and YF in autumn, respectively, and were, on average, 2.1 times higher than those in April (spring) at the three sites. The ∑5PAHs concentration in P. tomentosa leaves accumulatively increased from spring to autumn, which was not related to the temporal variation of PM₂.₅-bound PAHs. Nevertheless, the ∑5PAHs mean concentrations followed the order of YF > XZM > YQ. This trend was consistent with spatial distribution of atmosphere PM₂.₅, indicating that HMW PAHs in leaves increased with the increase of atmosphere PM₂.₅ concentration. Our results indicated that P. tomentosa may be used as a useful species for removing PAHs from the air and biomonitoring PAHs in atmosphere.