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With the GREAT receiver at the Stratospheric Observatory for InfraredAstronomy (SOFIA), nine massive molecular clumps have been observed in theammonia $3_{2+}- 2_{2-}$ line at 1.8~THz in a search for signatures of infall.The sources were selected from the ATLASGAL submillimeter dust continuum surveyof our Galaxy. Clumps with high masses covering a range of evolutionary stagesbased on their infrared properties were chosen. The ammonia line was detectedin all sources, leading to five new detections and one confirmation of aprevious detection of redshifted absorption in front of their strong THzcontinuum as a probe of infall in the clumps. These detections include twoclumps embedded in infrared dark clouds. The measured velocity shifts of theabsorptions compared to optically thin \CSEO\ (3--2) emission are0.3--2.8~km/s, corresponding to fractions of 3\%\ to 30\% of the free-fallvelocities of the clumps. The ammonia infall signature is compared withcomplementary data of different transitions of HCN, HNC, CS, and HCO$^+$, whichare often used to probe infall via their blue-skewed line profiles. The bestagreement with the ammonia results is found for the HCO$^+$ (4--3) transitions,but the latter is still strongly blended with emission from associatedoutflows. This outflow signature is far less prominent in the THz ammonialines, which confirms it as a powerful probe of infall in molecular clumps.Infall rates in the range from 0.3 to 16~$10^{-3}\,M_\odot/$yr were derivedwith a tentative correlation with the virial parameters of the clumps. The newobservations show that infall on clump scales is ubiquitous through a widerange of evolutionary stages, from $L/M$ covering about ten to severalhundreds.

Infall through the evolution of high-mass star-forming clumps