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:However, some short-term or long-term air pollution events have been reported frequently in many Chinese cities (Chai et al., 2014; Chan and Yao, 2008; Chen and Xie, 2014; Cheng et al., 2013; Li et al., 2014; Qu et al., 2010; Wang et al., 2008, 2014a–e) and especially long-lasting haze-fog episodes…
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