## 4 Discussion
### Sources of Pb and Cd between Needles and Twigs  
Principal component analysis and factor analysis were performed to reveal the relationship of Cd and Pb with other trace elements and potential sources between needles and twigs in the eastern Tibetan Plateau. The initial component matrix displayed vague results. The matrix was rotated to eliminate ambiguities. The results suggested that the concentrations of heavy metals can be divided into three components, which account for almost 71% of the total variance in the data (Table 4). V, Cr, Co, Ni, Cu, and As have more than 34% factor loadings in the first principal component (PC1). V, Cr, Co, Ni, Cu, and As primarily originate from the crust in background organic soils [39]. Therefore, we assumed that PC1 represents natural sources. PC2 consisted of As, Sb, Pb, Cd, and Hg, all of which belong to the group of five toxic elements with severe toxicity. PC2 had up to 0.713 and 0.465 factor loadings for Pb and Cd, respectively. Pb and Hg are elements of the long-range transport family [18]. Cd can be transported over long distances via the atmosphere [40]. We speculated that PC2 represented climatic factors. Figure 2 shows a tendency to decline for elements in needles and twigs from Yunnan Province in the northern direction as well as from Gongga Mountain to western areas. The regular distribution patterns implied that climatic factors are more dominant compared with mining activities. Combining PCA results and distribution patterns, a definite conclusion can be drawn: climatic factors overtly affect the distribution of Cd and Pb in needles and twigs. The hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) model of 72 h backward trajectories was performed to explore the possible sources of aerosol and transport routes of air masses. The trajectories were computed for an altitude of 3800 m in 2012. Figure 4 presents the results for each month. From January to May and October to December, the investigated region was affected by the atmosphere, mainly originating from northern and northeastern India. In June, the air mass mostly came from northwestern Yunnan. The atmosphere from South Asia and developed cities located in eastern China had an impact on the Tibetan Plateau. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameters** & **Mann–Whitney U** & **Wilcoxon W** & **Z** & \(p\) & \(n\) & **Twigs/Needles** \\ \hline Cd & 144 & 495 & \(-\)3.511 & \(p\) \textless{} 0.001 & 26 & 1.9 \\ Pb & 50 & 199 & \(-\)2.544 & \(p\) \textless{} 0.05 & 26 & 2.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The results of statistical analysis (Mann–Whitney U test). Severe air pollution was reported in South Asia [41; 42]. The atmospheric mass mostly came from developed cities in eastern China and southwestern Yunnan Province during summer [18]. Figure 2 shows that pronounced concentrations of Cd and Pb were found in the south and east directions, which were influenced by southwesterly and southeasterly monsoon. Massive amounts of Pb and Cd particles produced by mining activities, steel works, and metallurgical industry were delivered to the Tibetan Plateau through long-distance transportation via wide ranges [43] as well as dry and wet deposition [18; 44].