## 3 Results  
### Differences in Aroma Composition of Lingtou Dancong Oalong Tea among Different Altitudes  
We detected 59 volatile compounds common to the Lingtou Dancong oolong teas produced from plants grown at three different altitudes (Table 1). These compounds were classified into nine categories: aldehydes, alcohols, esters, ketones, terpenoids, oxides, nitrogen heterocyclic compounds, sulfur-containing compounds, and others (Table 1). Themost abundant compounds were ketones (11), followed by aldehydes (10), alcohols (9), and esters (9) (Figure 1). Esters accounted for the largest proportion of the total volatiles content, and alcohols, aldehydes, ketones, and nitrogen heterocyclic compounds also accounted for relatively high proportions (Figure 2). The tea plants grown at different altitudes were ranked, from highest total aroma compound content to lowest, as follows: high altitude (26,227.13 ug/kg), low altitude (23,431.05 ug/kg), and then medium altitude (15,557.94 ug/kg) (Figure 2). Among the esters, those with the highest concentrations were _cis_-jasmine lactone (1974-8196 ug/kg) and \(\delta\)-dodecalactone (2437-3410 ug/kg). Among the alcohols, those with relatively high concentrations were: hotrienol with a honey fragrance (1100-2664 ug/kg), linaloal with a floral fragrance (421.8-701.3 ug/kg), and geraniol with a rose fragrance (733.4-910.7 ug/kg) [1; 5]. The aldehyde butanal, 2-methyl- was present at relatively high concentrations (803.4-1441 ug/kg), as was the ketone (Z)-jasmone (392.9-1139 ug/kg). Among the nitrogenous compounds, indole (873.4-3882 ug/kg) showed the highest concentration. Figure 1: Categories of aroma compounds and specific compounds in Lingtou Dancong oolong tea product. The number in brackets indicates the number of compounds in each category. Compounds with an odor activity value (OAV) higher than 1 are highlighted in red. Aroma components are important indicators for evaluating tea quality, and multivariate chemometric approaches are often used to analyze the diversity of sensory characteristics and chemical components of tea samples. Raw data were subjected to unsupervised PCA analysis and supervised OPLA-DA analysis (Figure 3). In the PCA, the first principal component (PC1, \(X\)-axis) explained 34.3% of the variation in total aroma, and the second principal component (PC2, \(Y\)-axis) explained 22.8%. The samples from three different altitudes formed different clusters on the plot. The OPLS-DA model had R\({}^{2}\)X of 0.784, R\({}^{2}\)Y of 0.99, Q\({}^{2}\) of 0.963. Cross validation and permutation tests showed that the intercept between the blue regression line and the \(Y\) axis was less than 0, and all values of R\({}^{2}\) and Q\({}^{2}\) were lower than the original values, indicating that the model was not over-fitted and had good applicability and predictability (Figure S1). The compounds in the loading plots are referred to using serial numbers corresponding to the compounds in Table 1 and Figure 3C. The loading plot showed that hotrienol (23), nerol oxide (26), \(\alpha\)-ionone (46), (\(Z\))-jasmone (44) were present at higher levels in high-altitude tea than in medium- and low-altitude teas (Figure 3C). Compounds such as 5-ethyl-6-methyl-3\(E\)-hepten-2-one (25), (\(Z\))-4-hepten (6), 1-octen-3-ol (10) were present at higher levels in the medium-altitude tea than in the high- and low-altitude teas. The contents of isoeigenol (47) and 1,2-dihydro-1,1,6-trimethyl naphthalene (41) were higher in low-altitude tea than in medium- and high-altitude teas. These compounds had relatively important effects on the aroma quality of tea made from plants grown at different altitudes. GC-O/MS Analysis to Determine Contribution of Odorants to Aroma of Lingtou Dancong Oolong Tea Made from Plants Grown at Different Altitudes  
The contribution of volatiles to the aroma profile not only depends on their content but also on their threshold. GC-O/MS analysis was performed to further identify key contributors to the overall aroma profile of the various tea materials.