The APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) is an unbiased 870 µm submillimetre survey of the inner Galactic plane (| | < 60 • with |b| < 1.5 • ). It is the largest and most sensitive ground-based submillimetre wavelength Galactic survey to date and has provided a large and systematic inventory of all massive, dense clumps in the Galaxy (≥1000 M at a heliocentric distance of 20 kpc) and includes representative samples of all of the earliest embedded stages of high-mass star formation. Here we present the first detailed census of the properties (velocities, distances, luminosities and masses) and spatial distribution of a complete sample of ∼8000 dense clumps located in the Galactic disk (5 • < | | < 60 • ). We derive highly reliable velocities and distances to ∼97 per cent of the sample and use midand far-infrared survey data to develop an evolutionary classification scheme that we apply to the whole sample. Comparing the evolutionary subsamples reveals trends for increasing dust temperatures, luminosities and line-widths as a function of evolution indicating that the feedback from the embedded proto-clusters is having a significant impact on the structure and dynamics of their natal clumps. We find that the vast majority of the detected clumps are capable of forming a massive star and 88 per cent are already associated with star formation at some level. We find the clump mass to be independent of evolution suggesting that the clumps form with the majority of their mass in-situ. We estimate the statistical lifetime of the quiescent stage to be ∼5×10 4 yr for clump masses ∼1000 M decreasing to ∼1×10 4 yr for clump masses >10000 M . We find a strong correlation between the fraction of clumps associated with massive stars and peak column density. The fraction is initially small at low column densities but reaching 100 per cent for column densities above 10 23 cm −2 ; there are no clumps with column density clumps above this value that are not already associated with massive star formation. All of the evidence is consistent with a dynamic view of star formation wherein the clumps form rapidly and are initially very unstable so that star formation quickly ensues.

Context. In the low-mass regime, molecular cores have spatially resolved temperature and density profiles allowing a detailed study of their chemical properties. It is found that the gas-phase abundances of C-bearing molecules in cold starless cores rapidly decrease with increasing density. Here the molecules tend to stick to the grains, forming ice mantles. Aims. We study CO depletion in a large sample of massive clumps, and investigate its correlation with evolutionary stage and with the physical parameters of the sources. Moreover, we study the gradients in Methods. From the ATLASGAL 870 µm survey we selected 102 clumps, which have masses in the range ∼10 2 −3 × 10 4 M , sampling different evolutionary stages. We use low-J emission lines of CO isotopologues and the dust continuum emission to infer the depletion factor f D . RATRAN one-dimensional models were also used to determine f D and to investigate the presence of depletion above a density threshold. The isotopic ratios and optical depth were derived with a Bayesian approach. Results. We find a significant number of clumps with a high degree of CO depletion, up to ∼20. Larger values are found for colder clumps, thus for earlier evolutionary phases. For massive clumps in the earliest stages of evolution we estimate the radius of the region where CO depletion is important to be a few tenths of a pc. The value of the [ 12 C]/[ 13 C] ratio is found to increase with distance from the Galactic centre, with a value of ∼66 ± 12 for the solar neighbourhood. The [ 18 O]/[ 17 O] ratio is approximately constant (∼4) across the inner Galaxy between 2 kpc and 8 kpc, albeit with a large range (∼2−6). Clumps are found with total masses derived from dust continuum emission up to ∼20 times higher than M vir , especially among the less evolved sources. These large values may in part be explained by the presence of depletion: if the CO emission comes mainly from the low-density outer layers, the molecules may be subthermally excited, leading to an overestimate of the dust masses. Conclusions. CO depletion in high-mass clumps seems to behave as in the low-mass regime, with less evolved clumps showing larger values for the depletion than their more evolved counterparts, and increasing for denser sources.