The complete figure set (70 images) is available in the online journal.

Outflow wing selection: using spectra of ¹³CO (gray solid line) and C¹⁸O (gray dashed line) for the ATLASGAL clumps. Blue and red wing identification process: (a) scaling the C¹⁸O spectrum to the ¹³CO peak, shown as a red dashed–dotted line; (b) fitting a Gaussian to the scaled C¹⁸O, shown by a blue dotted line; (c) obtaining ¹³CO residual spectra (black solid line) by subtracting the Gaussian fit to the scaled C¹⁸O (red dashed–dotted) from ¹³CO (gray solid line); (d) blue and red wings, shown as red crosses, can be determined from where the ¹³CO residuals are larger than the 3σ line, where σ is the noise level of the emission-free spectrum. The outflow wing selection toward ATLASGAL clumps in the ¹³CO profile shows clear evidence of self-absorption as follows. First, a Gaussian is fit to the shoulders of the ¹³CO spectra, and the fitted Gaussian is used as the true peak temperature of the ¹³CO. Then, the above procedures (a)–(d) are followed.