Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Some previous studies reported temporal and spatial variations of gaseous and particulate matter pollutants in China (Chai et al., 2014; Chen and Xie, 2014; Han et al., 2009; Johansson et al., 2007; Karar et al., 2006; Lewne et al., 2004; Li et al., 2014; Martini et al., 2015; Meng et al., 2009; Sun et al., 2013; Wang et al., 2008, 2014d; Zhang et al., 2014; Zheng et al., 2013), but detailedly observational data were scarce.
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