## 3 Results and Discussion
### Classification of Wines by Geographical Origin Based on Volatile Compounds  
Volatile compounds only account for 2% of the wine composition but make a major contribution to its flavor; they are an important indicator of the flavor characteristics of a wine. The number of volatile compounds detected in wine ranged from 400 (1969) to more than 1300 (1983), and tens of thousands of chemical signals in recent vintages [(29)]. In this study, 47 volatile compounds were identified (Table S2). In total, 19 volatile compounds were quantified by the standard concentrations, including esters (isoamyl acetate, ethyl hexanoate, ethyl caprylate, ethyl caprate, diethyl succinate); alcohols (isobutanol, isoamyl alcohol, 1-hexanol, benzyl alcohol, 1-decanol, methionol, and phenylethyl alcohol); terpenoids (p-cymene, and citronellol); aldehydes and ketones (acetoin, nonanal, furfural, decanal, and benzaldehyde). After ANOVA, compounds with significant differences were used as variables in PCA. Principal components analysis (PCA) was used to display similarities and differences in volatile composition with significance between samples. The vectors in the variables plot (Figure 3a,c) show the relative loadings of the most variable volatile compounds. The individual plot (Figure 3b,d) shows the similarities and differences between the wine samples. There were fewer volatile compounds with significant differences in concentration among the 6 Ningxia sub-regions (10 volatile compounds) than between Ningxia, California, and Bordeaux (12 volatile compounds). The first (PC1, 33.3%) and second (PC2, 15.5%) principal components (Figure 3a,b), explained 48.8% of the total variance between the six Ningxia sub-regions. The wine samples from five of the sub-regions (Helan, Qingtongxia, Yinchuan, Yongning, and Shizuishan) were grouped together, indicating that their volatile compound profiles were similar, although those from Qingtongxia were much more scattered, i.e., more variable, than those of the other regions (Figure 3b). However, wines from the Hongsipu sub-region were moderately separated from the other five sub-regions (Figure 3a); the 10 volatile compounds and Hongsipu wines had opposite trend vectors in PC1, indicating that the volatile compound contents were negatively correlated with the Hongsipu wines. Overall, however, the Hongsipu wines had more similarities than differences, compared with those of the other sub-regions, and the differences may result from Hongsipu being the furthest south, with the warmest and driest climate (Figure 2). Differences in volatile compounds between wines from Ningxia, California, and Bordeaux were compared by PCA (Figure 3c,d), to investigate regional effects on the aroma profile. The first (PC1, 32.6%) and second (PC2, 14.7%) components explained 47.3% of the total variance. The wines were divided into three regional groups in the individual plots, with the Ningxia group on the upper left side and the California and Bordeaux groups on the bottom right side (Figure 3d). The Ningxia wines were relatively closely grouped. Except for nonanal, 1-hexanol, and benzaldehyde, all the volatile compounds in quadrants one, two, and three of the variables plots (Figure 3c) were all present in Ningxia wines, and quadrant two contained only compounds in Ningxia wines. Citronellol and acetoin in quadrant two are the characteristic compounds that distinguish Ningxia wines from those of Bordeaux and California. Bordeaux wines were mainly distributed  
Figure 2: Average temperature (**a**) and precipitation (**b**) records from March to September 2013–2018 in the six sub-regions of Ningxia. in quadrant one and were characterized by phenylethyll alcohol, ethyl caprylate, isoamyl alcohol, butanedioloic acid, diethyl ester, ethyl lexanoate, and decanal. California wines were relatively scattered and characterized by benzaldehyde, nonanal, and 1-hexanol in quadrant four. In summary, the aroma profiles of wines from the six Ningxia sub-regions were similar but clearly distinct from those of California and Bordeaux.