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tokens: also the channel maps by using the 13 CO data cube of the Galactic Ring Survey (GRS; Jackson et al. 2006 ). The RRLs corresponding to discrete regions are then used in the analysis of resolving the KDA. 3.1 Bubbles with detections of formaldehyde absorption The formaldehyde (H 2 CO) traces cold and dense ( n ∼ 10 4 cm −3 ) molecular gas (e.g. Young et al. 2004 ). By its absorption features of six fine structure transitions of (1 10 –1 11 ), H 2 CO has been widely detected in our Galaxy against radio continuum sources or even cosmic microwave background (CMB; Rodríguez et al. 2006 ). The transitions of H 2 CO (4829.6412–4829.6710 MHz) 5 close to the rest frequency of H110α (4874.157 MHz) have long been used to resolve the KDA for regions in the inner Galaxy (e.g. Downes et al. 1980 ; Watson et al. 2003 ; Sewilo et al. 2004 ; Du et al. 2011 ). Our observation covered the H 2 CO (1 10 –1 11 ) transition lines towards each of the targeted Spitzer infrared bubbles. The TianMa beam size for observing H 2 CO lines is about 3.9 arcmin. The typical on-source time is 2.5 min. Data processing is the same as for a single hydrogen RRL as described in Section 2 . H 2 CO absorptions along the line of sight of 27 out of the 75 infrared bubbles were detected. The absorption feature(s) is fitted by Gaussian(s) after removing a polynomial baseline (typically third order; Fig. 3 ). For the bubbles with H 2 CO detections, the spectra of RRL and H 2 CO are given in Appendix B (Supporting Information) . The fitted LSR velocity, the FWHM line width, the velocity-integrated line intensity, and their errors are listed in Table 3 . The rms noise for each spectrum estimated from the line-free channels is also incorporated. Figure 3. Examples of Spitzer infrared bubbles with detections of both RRLs ( lower panels , stacked spectrum) and formaldehyde lines near 4829.66 MHz ( upper panels ). From left to right, we show the KDA solutions of three bubbles assigned at near kinematic distance, far kinematic distance, and at the tangent point, respectively, according to the H 2 CO absorption method (see Section 3.2 ). The red vertical lines indicate the tangent point velocities derived by using the rotation curve of Russeil et al. ( 2017 ) with Galaxy fundamental parameters of R 0 = 8.34 kpc and (Reid et al. 2014 ). Table 3. Spitzer infrared bubbles with detections of H 2 CO (1 10 –1 11 ) transitions near 4829.66 MHz by the TianMa 65-m radio telescope. Column 1 is the source name; column 2 lists the measured LSR velocity from the H 2 CO spectrum, and its 1σ uncertainty is given in column 3; columns 4 and 5 are the measured FWHM line width and its 1σ uncertainty; columns 6 and 7 are the velocity-integrated intensity and its uncertainty, respectively; column 8 is the rms noise of the H 2 CO spectrum, estimated from the line-free channels. 3.2 Solution to the kinematic distance ambiguity Distance is a key parameter for Galactic sources. The trigonometric parallax (e.g. Reid et al.