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:The Intercontinental Chemical Transport Experiment B (INTEX-B) aircraft campaign over the North Pacific and western coast of North America made measurements of OH
and HO2 (Mao et al., 2009; Singh et al., 2009).
Category: