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Based on measurements with the Effelsberg 100-m telescope, a multi-line studyof molecular species is presented toward the south-western source of thegravitational lens system PKS 1830-211, which is by far the best known targetto study molecular gas in absorption at intermediate redshift. Determining lineparameters and optical depths and performing Large Velocity Gradient radiativetransfer calculations, the aims of this study are (1) to evaluate physicalparameters of the absorbing foreground gas at z~0.89, in particular itshomogeneity, and (2) to monitor the spectroscopic time variability caused byfluctuations of the z~2.5 background continuum source. We find, that the gas isquite inhomogeneous with n(H2)~2 x 10^3 cm^-3 for most molecular species butwith higher values for H2CO and lower ones for SO. Measuring the CS J=1-0transition during a time interval of more than a decade, from 2001 to 2012, thepeak absorption depth of the line remains approximately constant, while theline shape undergoes notable variations. Covering the time between 1996 and2013, CS, HCO+, and CH3OH data indicate maximal integrated optical depths in~2001 and 2011/2012. This is compatible with a ~10 yr periodicity, which,however, needs confirmation by substantially longer time monitoring. Comparingmolecular abundances with those of different types of Galactic and nearbyextragalactic clouds we find that the observed cloud complex does notcorrespond to one particular type but to a variety of cloud types with morediffuse and denser components as can be expected for an observed region with atransverse linear scale of several parsec and a likely larger depth along theline-of-sight. A tentative detection of Galactic absorption in the c-C3H21(10)-1(01) line at 18.343 GHz is also reported.

The inhomogeneous ISM toward PKS1830-211 SW - A detailed view on molecular gas at a look-back time of 7.5 Gyr