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:, 2009), particularly at altitudes less than 2 km, but found the overprediction to correlate with HCHO concentrations, indicating missing reactive VOCs in the model (Mao et al., 2009).
Category: