The physical and chemical properties of cold and dense molecular clouds are key to understanding how stars form. Using the IRAM 30 m and NRO 45 m telescopes, we carried out a Multiwavelength line-Imaging survey of the 70 μm-dArk and bright clOuds (MIAO). At a linear resolution of 0.1-0.5 pc, this work presents a detailed study of parsec-scale CO depletion and HCO deuterium (D-) fractionation toward four sources (G11.38 0.81, G15.22-0.43, G14.49-0.13, and G34.74-0.12) included in our full sample. In each source with T deuterium (D-) fractionation toward four sources (G11.38 0.81, G15.22-0.43, G14.49-0.13, and G34.74-0.12) included in our full sample. In each source with T H ∼ 104-105 cm-3, we compared pairs of neighboring 70 μm bright and dark clumps and found that (1) the H2 column density and dust temperature of each source show strong spatial anticorrelation; (2) the spatial distribution of CO isotopologue lines and dense gas tracers, such as 1-0 lines of H13CO and DCO, are anticorrelated; (3) the abundance ratio between C18O and DCO shows a strong correlation with the source temperature; (4) both the C18O depletion factor and D-fraction of HCO show a robust decrease from younger clumps to more evolved clumps by a factor of more than 3; and (5) preliminary chemical modeling indicates that chemical ages of our sources are ∼8 × 104 yr, which is comparable to their free-fall timescales and smaller than their contraction timescales, indicating that our sources are likely dynamically and chemically young.
