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. Additionally, most of the studies were only limited in one single city or air pollutant. For example, based on dailymean concentrations of PM10, NO2 and SO2 for five fixed-site air monitoring stations in urban area of Guangzhou from 1 January 2001 to 31 December 2011, Li et al. (2014) analyzed timescale-dependent relationships between air pollution index (API) and a variety of meteorological factors by the wavelet analysis, and found temperature, RH (relative humidity), precipitation and wind speed were negatively correlated with API.
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